Guile-hoot/doc/hoot.texi

\input texinfo    @c -*- texinfo -*-
@c %**start of header
@setfilename hoot.info
@settitle Guile Hoot
@documentencoding UTF-8
@documentlanguage en
@syncodeindex pg cp
@c %**end of header

@dircategory The Algorithmic Language Scheme
@direntry
* Hoot: (hoot).   Scheme to Wasm compiler backend for Guile and Wasm toolchain.
@end direntry

@finalout
@titlepage
@title Guile Hoot
@author David Thompson (@email{dave@@spritely.institute})
@author The Spritely Institute
@end titlepage

@contents

@ifnottex
@node Top
@top Guile Hoot

This is the manual for Guile Hoot, a Scheme to WebAssembly compiler
backend for @url{https://gnu.org/software/guile,GNU Guile} and general
purpose Wasm toolchain.

Both this manual and Guile Hoot itself are released under Apache v2.
See @ref{License} for more information.
@end ifnottex

@menu
* Introduction::                  What's the deal with Wasm, anyway?
* Compiling to Wasm::             Using the compiler and development tools.
* Web deployment::                Scheme in the browser!
* Scheme reference::              Hoot-specific Scheme extensions.
* Toolchain reference::           General purpose Wasm tools.
* Contributing::                  Lend a hand!
* License::                       Copying, distributing, and using this text.
* Index::
@end menu

@node Introduction
@chapter Introduction

Guile Hoot is a Scheme to WebAssembly (henceforth referred to as
@emph{Wasm}) compiler backend for
@url{https://gnu.org/software/guile,GNU Guile} and a general purpose
Wasm toolchain.  Wasm is an abstract but low-level binary compilation
target that can run on all major web browsers, and increasingly in
other, more ``native'' contexts, as well.  For over two decades,
JavaScript has been @emph{the} official language of the web, and while
the language has improved a lot over the years, its design leaves much
to be desired. Thus web developers looked for ways to bring their
preferred language to the browser to use instead.  In the past, the
only option available was to @emph{compile that language to
JavaScript!}  This approach worked in some sense but it was unideal
because many languages do not map cleanly to JavaScript.  In the case
of Scheme, for example, the lack of a tail call facility makes
compiling tail-recursive Scheme code unpleasant.  At long last, thanks
to Wasm, it has become possible to use an alternative language with
fewer compromises and better performance.  Hoot aims to bring Guile's
``whole self'' to the web, as well as to other Wasm GC runtimes.

Hoot is being developed by the
@url{https://spritely.institute,Spritely Institute} in collaboration
with @url{https://www.igalia.com/,Igalia} to advance Spritely's
mission to build the infrastructure for a completely decentralized
social Internet.  And for that mission, what better platform to target
than the web browser?
@url{https://spritely.institute/goblins/,Goblins}, Spritely's
distributed object programming environment, is primarily written in
Guile.  So, to meet users where they are at @emph{and} not use
JavaScript at the same time, Spritely needs a Guile to Wasm compiler!

A secondary goal of Hoot is to advocate for all dynamic programming
languages' (Python, Ruby, etc.) rightful place on the client-side web.
The Wasm 1.0 specification was not a habitable environment for
languages that require a garbage collector.  The Wasm GC proposal,
among others, has made it possible for dynamic languages to target
Wasm in a real way.  However, such advances are not without their
detractors.  Without the necessary support, a useful proposal will
never make it into the core specification.  For example, strings are a
particularly controversial subject in the WebAssembly Community Group
and proposals that would greatly benefit Hoot and other languages have
not reached consensus.  Implementing and targeting emergent and useful
Wasm proposals helps those proposals find their way into the core
specification.  A rising tide lifts all boats, as they say, and while
we may be little schemers, we want our work to help advance the Wasm
standard for all dynamic languages.

@menu
* Status::                        What works.  What doesn't.
* Installation::                  Setting up Hoot.
* Tutorial::                      Compiling your first Scheme program to Wasm.
@end menu

@node Status
@section Status

Hoot's Wasm output is compatible with Google Chrome starting with
version 119 and Mozilla Firefox starting with version 121.  As of
writing, WebKit/Apple Safari is not yet compatible.

Hoot is still in an early phase of active development and its API
should be considered unstable and subject to change in future
releases.  Hoot currently supports a subset of the R7RS-small Scheme
specification, along with a small set of Guile-specific functionality
such as @inlinefmtifelse{html,
@url{https://www.gnu.org/software/guile/manual/html_node/Prompts.html,
prompts}, @ref{Prompts,,,Guile Reference}}.

Missing R7RS-small features include:

@enumerate
@item Complex numbers (constants are supported, math procedures are not)
@item Environments and evaluation (@code{environment}, @code{eval}, etc.)
@end enumerate

Future releases will add support for all of R7RS-small and eventually
full Guile-flavored Scheme.

To compile Scheme to Wasm, Hoot takes advantage of several new Wasm
proposals.  The most important of these new features are tail calls
and GC reference types.  The @code{return_call} family of instructions
has made the implementation of Scheme's tail recursive procedure call
semantics relatively straightforward.  GC reference type instructions
allow for heap allocated objects (and immediates via the @code{i31}
type) that are managed by the Wasm runtime.  This allows Hoot to take
advantage of production garbage collectors already present in web
browsers, obviating the need to implement and ship our own which would
be both inferior to the host's and a major source of binary bloat.

There's an additional Wasm proposal that Hoot has been built on that
has, unfortunately, not found its way into the core Wasm
specification: stringref.  We still emit stringref, but it is reduced
to being an intermediate form.  A lowering pass replaces stringref
instructions with something resembling the JS String Builtins
proposal.

@node Installation
@section Installation

The @emph{easiest} way to get up and running with Hoot is by using the
@url{https://guix.gnu.org,GNU Guix} package manager for which we
provide a @file{guix.scm} file ready for use with @command{guix
shell}:

@example
cd guile-hoot/
guix shell
@end example

@command{guix shell} will download/compile all required dependencies
and start an interactive shell that is ready to use for building Hoot.

To use Hoot without Guix requires building Guile from source.  Hoot is
currently undergoing a lot of development and requires a bleeding-edge
Guile built against the @code{main} branch.  Eventually Hoot will just
require a stable release of Guile.

With a sufficiently fresh Guile, via Guix or otherwise, the build can
begin.  If you are building from a Git checkout rather than an
official release tarball, the first step is to bootstrap the build
system:

@example
./bootstrap.sh
@end example

Release tarballs have a pre-bootstrapped build system and do not
require the above step.

Now, build Hoot:

@example
./configure
make
@end example

If you'd like to install Hoot onto your system, run:

@example
sudo make install
@end example

The GNU build system defaults to @file{/usr/local} as the installation
prefix.  This can be changed by re-running the configure script:

@example
./configure --prefix=/some/where/else
sudo make install
@end example

To try out Hoot without installing it, use the @file{pre-inst-env}
wrapper to launch Guile in the context of the Hoot build directory:

@example
./pre-inst-env guile
@end example

If you installed Guile to your system, simply run @command{guile}.

If everything went well, you will be greeted with a Guile REPL prompt.
Regardless of installation status, to verify that Guile can find the
Hoot modules, run:

@lisp
scheme@@(guile-user)> ,use (hoot compile)
@end lisp

If there is no error then congratulations!  Your setup is correct.
Proceed to the tutorial for a crash course in how to use Hoot, or see
later chapters for an API reference.

@subsection Running the test suite

This is entirely optional, but if you'd like further verification that
your build is good (or perhaps you're packaging Hoot for
distribution), the test suite can be run via @command{make check}.  By
default, the tests are run against two Wasm runtimes: Hoot's own Wasm
interpreter and @url{https://v8.dev/,V8} via the @command{d8} tool.

Getting V8 can be tricky, and will most likely require you to
@url{https://v8.dev/docs/build,compile it from source.}  It's a pain!
To skip all of that trouble and just run the tests against the
built-in interpreter, run:

@example
make check Wasm_HOST=hoot
@end example

@node Tutorial
@section Tutorial

Let's compile some simple Scheme programs and learn how to work with
their compiled Wasm forms.

As we all know, the answer to everything is simply 42.  So, we should
make sure that we can compile 42 to Wasm.  To do so, import the
@code{(hoot compile)} module and call the @code{compile} procedure.

@lisp
@verbatim
scheme@(guile-user)> ,use (hoot compile)
scheme@(guile-user)> (define the-answer (compile 42))
@end verbatim
@end lisp

The result is a Wasm module.  There is a lot of stuff inside, but
we're not going to focus on that right now.  We should load and run
the module to verify that it outputs 42 like we expect.  We can do so
from the comfort of our Guile REPL because Hoot includes a Wasm
interpreter.  There's no need to use a web browser or other Wasm
runtime to try out small programs.

First, import the @code{(hoot reflect)} module.  Then, instantiate
@code{the-answer} to load it into the Wasm interpreter:

@lisp
@verbatim
scheme@(guile-user)> ,use (hoot reflect)
scheme@(guile-user)> (define instance (hoot-instantiate the-answer))
@end verbatim
@end lisp

All that's left to do now is execute the program with
@code{hoot-load}:

@lisp
@verbatim
scheme@(guile-user)> (hoot-load instance)
$5 = 42
@end verbatim
@end lisp

Ta-da!  It feels kind of funny to compile a Scheme program to Wasm
only to load it back into Scheme, but it's a quick and easy way to
test things out.

For cases when you simply want to compile an expression and see the
result immediately, there is a faster method. Just use the
@code{compile-value} procedure instead:

@lisp
@verbatim
scheme@(guile-user)> (compile-value '(list 1 2 3))
$6 = #<hoot (1 2 3)>
@end verbatim
@end lisp

With @code{compile-value}, the compiled Wasm module is thrown away,
which is just fine for testing throwaway code.

Lists are cool and 42 is ultimately the answer to everything, but it
would be a shame if we didn't talk about compiling something a little
more complicated.  Let's compile a simple, tail-recursive procedure!
How about good ol' factorial?

@lisp
@verbatim
scheme@(guile-user)> (define hoot-factorial
                       (compile-value
                        '(let ()
                           (define (factorial x result)
                             (if (= x 1)
                                 result
                                 (factorial (- x 1)
                                            (* result x))))
                           factorial)))
@end verbatim
@end lisp

A Hoot procedure can be called just like a regular procedure:

@lisp
@verbatim
scheme@(guile-user)> (hoot-factorial 5 1)
$7 = 120
@end verbatim
@end lisp

The Hoot reflection in Guile is great for quickly iterating on code,
but what we really want is to get our programs running in a web
browser.  We've compiled a couple of things to Wasm now, but the
resulting modules have stayed within the confines of the Guile
process.  To make something that can be loaded by a web browser, we
need to use the assembler to create a Wasm binary:

@lisp
@verbatim
scheme@(guile-user)> (define hello (compile "Hello, world!"))
scheme@(guile-user)> ,use (wasm assemble)
scheme@(guile-user)> (define bin (assemble-wasm hello))
@end verbatim
@end lisp

Now, create a new directory for this tutorial:

@example
mkdir hoot-tutorial
@end example

Write the binary to disk in that directory:

@lisp
@verbatim
scheme@(guile-user)> ,use (ice-9 binary-ports)
scheme@(guile-user)> (call-with-output-file "/path/to/hoot-tutorial/hello.wasm"
                       (lambda (port)
                         (put-bytevector port bin)))
@end verbatim
@end lisp

To inspect Scheme values from JavaScript, Hoot provides the
@file{js-runtime/reflect.js} library.  Copy that file and its
associated Wasm helper modules, @file{js-runtime/reflect.wasm} and
@file{js-runtime/wtf8.wasm}, to the @file{hoot-tutorial} directory:

@example
cd /path/to/hoot-tutorial
cp /path/to/guile-hoot/js-runtime/reflect.js .
mkdir js-runtime
cp /path/to/guile-hoot/js-runtime/reflect.wasm js-runtime/
cp /path/to/guile-hoot/js-runtime/wtf8.wasm js-runtime/
@end example

To run @file{hello.wasm}, we need a little JavaScript glue code.
Let's call this @file{hello.js}:

@example
@verbatim
async function load() {
  const [message] = await Scheme.load_main("hello.wasm", {});
  console.log(message);
}
window.addEventListener("load", load);
@end verbatim
@end example

We also need a minimal @file{index.html} web page to bring it all
together:

@example
<!DOCTYPE html>
<html>
  <head>
    <script type="text/javascript" src="reflect.js"></script>
    <script type="text/javascript" src="hello.js"></script>
  </head>
  <body>
    Guile is a hoot!
  </body>
</html>
@end example

The file tree in @file{hoot-tutorial} should look like this:

@example
./js-runtime
./js-runtime/wtf8.wasm
./js-runtime/reflect.wasm
./reflect.js
./hello.js
./index.html
./hello.wasm
@end example

Finally, we need a local web server to serve the files.  Fortunately,
Guile includes all the building blocks we need to make a minimal one
for the purposes of this tutorial.  Save the following to
@file{web-server.scm}:

@lisp
(use-modules (ice-9 binary-ports) (ice-9 format) (ice-9 match)
             (web server) (web request) (web response) (web uri))

(define (extension file)
  (match (string-split file #\.)
    (() #f)
    ((_ ... ext) ext)))

(define (mime-type file-name)
  (or (assoc-ref '(("js" . application/javascript)
                   ("html" . text/html)
                   ("wasm" . application/wasm))
                 (extension file-name))
      'text/plain))

(define (render-file file-name)
  (values `((content-type . (,(mime-type file-name))))
          (call-with-input-file file-name get-bytevector-all)))

(define (not-found path)
  (values (build-response #:code 404) (string-append "Not found: " path)))

(define (directory? file-name)
  (eq? (stat:type (stat file-name)) 'directory))

(define (serve-file path)
  (let ((f (string-append (getcwd) (uri-decode path))))
    (if (and (file-exists? f) (not (directory? f)))
        (render-file f)
        (not-found path))))

(define (handle-request request body)
  (let ((method (request-method request))
        (path (uri-path (request-uri request))))
    (format #t "~a ~a\n" method path)
    (serve-file path)))

(run-server handle-request 'http '(#:port 8080))
@end lisp

Start the web server like so:

@example
guile web-server.scm
@end example

Visit @url{http://localhost:8080/index.html} in your web browser, and
if it is new enough to have Wasm GC enabled, you should see the text
``Hello, world!'' printed in the developer console.

We hope this tutorial has helped you get started with Hoot!  Read on
for full API documentation.

@node Compiling to Wasm
@chapter Compiling to Wasm

@menu
* Invoking the compiler::         Compiling a Hoot module.
* High-level development tools::  Using Hoot modules from the host environment.
* Low-level development tools::   Inspecting and debugging Wasm.
@end menu

@node Invoking the compiler
@section Invoking the compiler

In Guile's compiler tower, Scheme code goes through several
transformations before being compiled to VM bytecode.  Scheme is
lowered to @ref{Tree-IL,,,Guile Reference}, which is then lowered to
@ref{Continuation Passing Style,,,Guile Reference}(CPS), and then
finally to @ref{Bytecode,,,Guile Reference}.  Hoot adds an additional
backend that compiles CPS to Wasm.

Currently, Hoot does not use Guile's module system and instead peforms
whole program compilation using a prelude that provides R7RS-small
features (and also some Guile ones, like prompts.)  Support for
modules will be added in a future release.

For hooking the Hoot compiler up to a build system such as GNU Make,
invoke the @command{guild compile-wasm} tool:

@example
guild compile-wasm -o foo.wasm foo.scm
@end example

When writing Scheme intended to be compiled to the Wasm target, a
special form called @code{%inline-wasm} is available for implementing
a section of code in WAT rather than Scheme.  For example, here's how
@code{port?} is implemented:

@lisp
(define (port? x)
  (%inline-wasm '(func (param $obj (ref eq))
                       (result (ref eq))
                       (if (ref eq)
                           (ref.test $port (local.get $obj))
                           (then (ref.i31 (i32.const 17)))
                           (else (ref.i31 (i32.const 1)))))
                x))
@end lisp

An inline Wasm form specifies a single function using WAT expressions
whose parameters and result types are all @code{(ref eq)}, as well as
the Scheme variables that map to the parameters.  The compiler then
transforms and splices the function body into the procedure.

Like Guile's built-in compiler, the Hoot compiler can also be invoked
within Scheme.  The @code{(hoot compile)} module provides the
interface to the Wasm compiler backend.

@deffn {Procedure} compile exp [#:import-abi? #f] [#:export-abi? #t] @
                   [#:from (current-language)] @
                   [#:imports %default-program-imports] @
                   [#:optimization-level (default-optimization-level)] @
                   [#:warning-level (default-warning-level)] @
                   [#:dump-cps? #f] [#:dump-wasm? #f] [#:emit-names? #f] @
                   [#:opts '()]

Compile the Scheme expression @var{exp} to Wasm and return a Wasm
module.

The environment in which @var{exp} is evaluated is defined by
@var{imports}, a list of module names such as @code{(scheme time)} or
@code{(hoot ffi)}.

When @var{import-abi?} is @code{#t}, the Wasm module will be built
such that it needs to import its ABI from another module.  When
@var{export-abi?} is @code{#t}, the Wasm module will be built such
that it exports its ABI functions.  A typical use of these flags is to
export the ABI from one ``main'' module and then import that ABI into
any additional modules that are being used.

When @var{emit-names?} is @code{#t} then human-readable names will be
embedded in the resulting Wasm object.  By default, this is turned off
as it greatly increases binary size.

@xref{Compiling Scheme Code,,,Guile Reference} for more information
about invoking Guile's compiler.
@end deffn

@deffn {Procedure} read-and-compile port [#:import-abi? #f] [#:export-abi? #t] @
                   [#:from (current-language)] @
                   [#:optimization-level (default-optimization-level)] @
                   [#:warning-level (default-warning-level)] @
                   [#:dump-cps? #f] [#:dump-wasm? #f] [#:emit-names? #f] @
                   [#:opts '()]

Like @code{compile}, but read Scheme expressions from @var{port}.

If the first expression is an @code{import} form, then only the
bindings from those modules will be imported into the compilation
unit.  If the @code{import} form is omitted, a default set of modules
will be imported.  It is highly recommended to be explicit and use
@code{import}.

@end deffn

@deffn {Procedure} compile-file input-file [#:import-abi? #f] @
                   [#:export-abi? #t] @
                   [#:from (current-language)] @
                   [#:optimization-level (default-optimization-level)] @
                   [#:warning-level (default-warning-level)] @
                   [#:dump-cps? #f] [#:dump-wasm? #f] [#:emit-names? #f] @
                   [#:opts '()]

Like @code{read-and-compile}, but read the Scheme expression from
@var{input-file}.
@end deffn

@node High-level development tools
@section High-level development tools

The @code{(hoot reflect)} module provides an interface for inspecting
and manipulating Scheme values that live within Wasm modules.  This is
the primary interface for testing compiler output directly from Guile.

@deffn {Procedure} hoot-instantiate scheme-wasm [imports '()] [reflector]
Instantiate and return a new Hoot module using the compiled Scheme
Wasm module @var{scheme-wasm} and the reflection module
@var{reflector}.  If @var{reflector} is not specified, a new reflector
instance will be created.

Optionally, @var{imports} may contain a 2-tier association list
structure of imported functions, globals, tables, and memories:

@lisp
`(("math" . (("random" . ,(lambda (x) (random x))))))
@end lisp

@end deffn

@deffn {Procedure} hoot-load module
Invoke the load thunk of @var{module} and return the reflected
result values.
@end deffn

@deffn {Procedure} compile-value exp [imports '()]
Compile @var{exp} and return the result.

Optionally, @var{imports} may contain a 2-tier association list
structure of imported functions, globals, tables, and memories.  See
@code{hoot-instantiate} for an example of such a structure.
@end deffn

@deffn {Procedure} compile-call proc-exp arg-exps ...
Compile @var{proc-exp} and all @var{arg-exps}, call the procedure with
the arguments, then return the results.
@end deffn

@deffn {Procedure} hoot-module? obj
Return @code{#t} if @var{obj} is a Hoot module.
@end deffn

@deffn {Procedure} hoot-module-reflector module
Return the reflection module for @var{module}.
@end deffn

@deffn {Procedure} hoot-module-instance module
Return the Wasm instance for @var{module}.
@end deffn

@deffn {Procedure} reflector? obj
Return @code{#t} if @var{obj} is a reflector.
@end deffn

@deffn {Procedure} reflector-instance reflector
Return the Wasm instance of @var{reflector}.
@end deffn

@deffn {Procedure} reflector-abi reflector
Return the association list of ABI imports for @var{reflector}.
@end deffn

@deffn {Procedure} hoot-object? obj
Return @code{#t} if @var{obj} is a Hoot object.
@end deffn

@deffn {Procedure} hoot-complex? obj
Return @code{#t} if @var{obj} is a Hoot complex number.
@end deffn

@deffn {Procedure} hoot-complex-real complex
Return the real part of @var{complex}.
@end deffn

@deffn {Procedure} hoot-complex-imag complex
Return the imaginary part of @var{complex}.
@end deffn

@deffn {Procedure} hoot-fraction? obj
Return @code{#t} if @var{obj} is a Hoot fraction.
@end deffn

@deffn {Procedure} hoot-fraction-num fraction
Return the numerator of @var{fraction}
@end deffn

@deffn {Procedure} hoot-fraction-denom fraction
Return the denominator of @var{fraction}.
@end deffn

@deffn {Procedure} hoot-pair? obj
Return @code{#t} if @var{obj} is a Hoot pair.
@end deffn

@deffn {Procedure} mutable-hoot-pair? obj
Return @code{#t} if @var{obj} is a mutable Hoot pair.
@end deffn

@deffn {Procedure} hoot-pair-car pair
Return the first element of @var{pair}.
@end deffn

@deffn {Procedure} hoot-pair-cdr pair
Return the second element of @var{pair}.
@end deffn

@deffn {Procedure} hoot-vector? obj
Return @code{#t} if @var{obj} is a Hoot vector.
@end deffn

@deffn {Procedure} mutable-hoot-vector? obj
Return @code{#t} if @var{obj} is a mutable Hoot vector.
@end deffn

@deffn {Procedure} hoot-vector-length vec
Return the length of @var{vec}.
@end deffn

@deffn {Procedure} hoot-vector-ref vec i
Return the @var{i}th element of @var{vec}.
@end deffn

@deffn {Procedure} hoot-bytevector? obj
Return @code{#t} if @var{obj} is a Hoot bytevector.
@end deffn

@deffn {Procedure} mutable-hoot-bytevector? obj
Return @code{#t} if @var{obj} is a mutable Hoot bytevector.
@end deffn

@deffn {Procedure} hoot-bytevector-length bv
Return the length of @var{bv}.
@end deffn

@deffn {Procedure} hoot-bytevector-ref bv i
Return the @var{i}th byte of @var{bv}.
@end deffn

@deffn {Procedure} hoot-bitvector? obj
Return @code{#t} if @var{obj} is a Hoot bitvector.
@end deffn

@deffn {Procedure} mutable-hoot-bitvector? obj
Return @code{#t} if @var{obj} is a mutable Hoot bitvector.
@end deffn

@deffn {Procedure} hoot-bitvector-length bv
Return the length of @var{bv}.
@end deffn

@deffn {Procedure} hoot-bitvector-ref bv i
Return the @var{i}th bit of @var{bv}.
@end deffn

@deffn {Procedure} hoot-symbol? obj
Return @code{#t} if @var{obj} is a Hoot symbol.
@end deffn

@deffn {Procedure} hoot-symbol-name sym
Return the string name of @var{sym}.
@end deffn

@deffn {Procedure} hoot-keyword? obj
Return @code{#t} if @var{obj} is a Hoot keyword.
@end deffn

@deffn {Procedure} hoot-keyword-name keyword
Return the name string of @var{keyword}.
@end deffn

@deffn {Procedure} mutable-hoot-string? obj
Return @code{#t} if @var{obj} is a mutable Hoot string.
@end deffn

@deffn {Procedure} mutable-hoot-string->string str
Return the underlying string for @var{str}.
@end deffn

@deffn {Procedure} hoot-procedure? obj
Return @code{#t} if @var{obj} is a Hoot procedure.
@end deffn

@deffn {Procedure} hoot-variable? obj
Return @code{#t} if @var{obj} is a Hoot variable.
@end deffn

@deffn {Procedure} hoot-atomic-box? obj
Return @code{#t} if @var{obj} is a Hoot atomic box.
@end deffn

@deffn {Procedure} hoot-hash-table? obj
Return @code{#t} if @var{obj} is a Hoot hash table.
@end deffn

@deffn {Procedure} hoot-weak-table? obj
Return @code{#t} if @var{obj} is a Hoot weak table.
@end deffn

@deffn {Procedure} hoot-fluid? obj
Return @code{#t} if @var{obj} is a Hoot fluid.
@end deffn

@deffn {Procedure} hoot-dynamic-state? obj
Return @code{#t} if @var{obj} is a Hoot dynamic state.
@end deffn

@deffn {Procedure} hoot-syntax? obj
Return @code{#t} if @var{obj} is a Hoot syntax object.
@end deffn

@deffn {Procedure} hoot-port? obj
Return @code{#t} if @var{obj} is a Hoot port.
@end deffn

@deffn {Procedure} hoot-struct? obj
Return @code{#t} if @var{obj} is a Hoot struct.
@end deffn

@node Low-level development tools
@section Low-level development tools

The @code{(hoot repl)} module provides a set of REPL commands to
assist with inspecting and debugging Wasm modules.  As a matter of
course, Hoot's Scheme compiler @emph{should not} cause low-level Wasm
runtime errors, but when it does, or when working with the Wasm
toolchain directly, these REPL tools may provide some assistance.

To install the REPL commands, simply import the module:

@lisp
scheme@@(guile-user)> ,use (hoot repl)
@end lisp

To see a list of all the Wasm commands, run:

@lisp
scheme@@(guile-user)> ,help wasm
@end lisp

To demonstrate the debugging features, let's create a trivial module
with a buggy function:

@lisp
@verbatim
scheme@(guile-user)> (define src
                       '(module
                         (func (export "main") (param $x i32) (result i32)
                               (i32.add (local.get $x)
                                        (unreachable)))))
@end verbatim
@end lisp

When called, this function will hit the @code{unreachable} instruction
and throw a runtime error.  Let's compile the WAT source, load it into
the VM, and get a reference to the @code{main} function:

@lisp
@verbatim
scheme@(guile-user)> ,use (wasm resolve) (wasm vm) (wasm wat)
scheme@(guile-user)> (define wasm (validate-wasm (resolve-wasm (wat->wasm src))))
scheme@(guile-user)> (define instance (instantiate-wasm wasm))
scheme@(guile-user)> (define main (wasm-instance-export-ref instance "main"))
@end verbatim
@end lisp

To trap the Wasm runtime error and open a Wasm debugging REPL, the
@command{wasm-catch} REPL command can be prefixed before an
expression:

@lisp
@verbatim
scheme@(guile-user)> ,wasm-catch (main 7)
ice-9/boot-9.scm:1674:22: In procedure raise-exception:
ERROR:
  1. &wasm-runtime-error:
      instruction: (unreachable)
      position: (func 0 1)
      instance: #<wasm-instance 140506559041920>
      stack: #<<wasm-stack> items: (7)>
      blocks: ((wasm-block))
      locals: #(7)
  2. &message: "Wasm runtime error: unreachable"
  3. &irritants: ()

Entering Wasm debug prompt. Type `,help wasm' for info or `,q' to continue.
scheme@(guile-user) [1]>
@end verbatim
@end lisp

Once in a Wasm debug context, many of the other REPL commands become
usable.  To highlight the instruction where execution has paused, use
@command{wasm-pos}:

@lisp
@verbatim
scheme@(guile-user) [1]> ,wasm-pos
(func 0 (param $x i32) (result i32)
  (local.get 0)
  <<< (unreachable) >>>
  (i32.add))
@end verbatim
@end lisp

To print the contents of the values stack, use @command{wasm-stack}:

@lisp
@verbatim
scheme@(guile-user) [1]> ,wasm-stack
Value stack:
  0:	7
@end verbatim
@end lisp

To print the contents of the function locals, use @command{wasm-locals}:

@lisp
@verbatim
scheme@(guile-user) [1]> ,wasm-locals
Locals:
  0:	7
@end verbatim
@end lisp

To evaluate arbitary Wasm instructions in the current context, either
in an attempt to repair interpreter state or just for fun, use
@command{wasm-eval}:

@lisp
@verbatim
scheme@(guile-user) [1]> ,wasm-eval '(local.get 0)
scheme@(guile-user) [1]> ,wasm-stack
Value stack:
  0:	7
  1:	7
@end verbatim
@end lisp

There are now two i32 values on the stack.  If we were to proceed with
execution, the next instruction, @code{i32.add}, should add them
together and return a result of 14.  To resume execution, use
@command{wasm-continue}:

@lisp
@verbatim
scheme@(guile-user) [1]> ,wasm-continue
$5 = 14
@end verbatim
@end lisp

Evaluating arbitrary Wasm commands in a debugging context is very
helpful when trying to understand the nature of a bug, but bear in
mind that cursed things may happen during the process as there is no
validation applied. This goes especially for when you try to resume
execution.

See @ref{Interpreter} for detailed information on running Wasm within
Guile and @ref{Toolchain reference} in general for working with Wasm
directly.

@deffn {REPL Command} wasm-trace exp
Evaluate @var{exp} with verbose Wasm tracing enabled.  This will print
out every instruction along with the state of the value stack and
function locals at the time of evaluation.
@end deffn

@deffn {REPL Command} wasm-freq exp
Evaluate @var{exp} and print out a table showing how many times each
kind of Wasm instruction was executed as well as a total instruction
count.
@end deffn

@deffn {REPL Command} wasm-catch exp
Catch and debug Wasm runtime errors that are raised by evaluating
@var{exp}.
@end deffn

The following commands are usable only in the context of a Wasm debug
REPL:

@deffn {REPL Command} wasm-stack
Print the state of the Wasm stack.
@end deffn

@deffn {REPL Command} wasm-locals
Print the state of the Wasm function locals.
@end deffn

@deffn {REPL Command} wasm-pos
Print the current function disassembly and highlight the instruction
where Wasm execution has paused.
@end deffn

@deffn {REPL Command} wasm-eval instr
Evaluate the Wasm instruction @var{instr} in the current debug
context.  Use this when attempting to fix the state of the Wasm stack
or locals before attempting to resume with @code{,wasm-continue}.
@end deffn

The following commands behave differently depending on if they are run
within a Wasm debug REPL or not.

@deffn {REPL Command} wasm-dump [wasm]
Display information about @var{wasm}, or the current Wasm instance
when debugging.
@end deffn

@deffn {REPL Command} wasm-continue
When in a debugger, exit and resume Wasm execution.  In the event that
this is run after trapping a runtime error, your warranty is void and
all bets are off!  While it may be dangerous, this does allow one to
manually fix the Wasm interpreter state manually with
@code{,wasm-eval} and attempt to proceed, which can come in handy
sometimes.

When not in a debugger, set the Wasm execution mode to continue
without interruption.  In other words, deactive the instruction
stepper if it is active.
@end deffn

@deffn {REPL Command} wasm-step
When in a debugger, resume Wasm execution but pause before the next
instruction is evaluated.

When not in a debugger, set Wasm execution to pause before each
instruction is evaluated.
@end deffn

@node Web deployment
@chapter Web deployment

On the client-side web, JavaScript is the host environment for Wasm
modules and the
@url{https://developer.mozilla.org/en-US/docs/WebAssembly,WebAssembly}
API is used to load and run them.  Hoot includes a JavaScript library,
@file{reflect.js} that wraps the @code{WebAssembly} API and
furthermore can inspect Scheme values and call Scheme procedures.
This chapter documents deploying Hoot artifacts and using the
reflection API to run Scheme in the browser.

@menu
* Web server setup::            Prepare a server to run Hoot programs.
* JavaScript API reference::    JavaScript reflection interface.
@end menu

@node Web server setup
@section Web server setup

In order to run Hoot binaries in the browser, a web server needs to
host a copy of the Hoot JavaScript runtime.

The runtime files can be found in the
@file{$prefix/share/guile-hoot/js-runtime} directory, where
@code{$prefix} is the directory where Hoot was installed on your
system.  This is typically @file{/usr} or @file{/usr/local} on Linux
distributions such as Debian, Ubuntu, Fedora, etc.

Don't forget to upload the Wasm files for the Scheme programs, too!

A bit of JavaScript code is needed to bootstrap a Scheme program using
the @file{js-runtime/reflect.js} library.  For example, here's an
example @file{boot.js} file that runs the Scheme program
@file{hello.wasm} and prints the return values:

@example
@verbatim
window.addEventListener("load", async () => {
  const results = await Scheme.load_main("/hello.wasm", {});
  console.log(results);
});
@end verbatim
@end example

The @code{Scheme} namespace is defined in @file{reflect.js}.
@xref{JavaScript API reference} for more information.

To run @file{boot.js} on a web page, add @code{<script>} tags for it
and @file{reflect.js}:

@example
<!DOCTYPE html>
<html>
  <head>
    <script type="text/javascript" src="/js-runtime/reflect.js"></script>
    <script type="text/javascript" src="/boot.js"></script>
  </head>
  <body>
    <h1>Hello, Hoot!</h1>
  </body>
</html>
@end example

@node JavaScript API reference
@section JavaScript API reference

The @code{Scheme} class is used to load a Hoot binary, start the
program, and initialize reflection.

@deftp {Class} Scheme
A Scheme runtime environment.
@end deftp

@defop {Static method} Scheme load_main path abi [user_imports @code{@{@}}]
Fetch and execute the Hoot Wasm binary at the URL @var{path} and
return an array of Scheme values produced by the program.

The @var{abi} parameter is for more advanced usage where multiple Hoot
binaries share a single application binary interface (ABI).  This
should be set to @code{@{@}} when loading the first Scheme binary.  It
is better to use the @code{load_extension} method for subsequent
binaries, though.

The @var{user_imports} parameter is for providing concrete
implementations of functions declared using the @ref{Foreign function
interface}.  It uses a two-tier nested object structure to map import
names to the functions that implement them.

For example, this Scheme code:

@lisp
(define-foreign make-text-node
  "document" "createTextNode"
  (ref string) -> (ref null extern))
@end lisp

Could be instantiated like so:

@example
@verbatim
Scheme.load_main("hello.wasm", {}, {
  document: {
    createTextNode: Document.prototype.createTextNode.bind(document)
  }
});
@end verbatim
@end example

@end defop

@defmethod Scheme load_extension path [user_imports @code{@{@}}]
Fetch and load an additional Hoot binary at the URL @var{path} that
shares the ABI of @code{this}.  Optionally, a set of user-defined
imported functions can be specified with the @var{user_imports}
parameter.
@end defmethod

All of the fundamental Scheme types have an associated JavaScript
class that can reflect their values.  Calling the @code{repr} function
on an instance of a reflected Scheme object will return a Scheme-like
printing of the object.

@example
repr(pair) // => "(1 . 2)"
@end example

@deftp {Class} Char
A Unicode character.
@end deftp

@deftp {Class} Eof
End-of-file object.
@end deftp

@deftp {Class} Null
The empty list.
@end deftp

@deftp {Class} Unspecified
The unspecified value.
@end deftp

@deftp {Class} Complex real imag
Complex number with real part @var{real} and imaginary part
@var{imag}.
@end deftp

@deftp {Class} Fraction num denom
An exact fraction with numerator @var{num} and denominator
@var{denom}.
@end deftp

The @code{HeapObject} class is the parent class of all of the
remaining Scheme types.

@deftp {Class} HeapObject
A Scheme heap object.
@end deftp

@defivar HeapObject reflector
The reflector for @code{this}, an instance of the @code{Scheme} class.
@end defivar

The @code{reflector} property can be used in conjuction with the
@code{load_extension} method to load additional Hoot binaries that
share the same ABI.

@example
heapObject.reflector.load_extension("/helper.wasm")
@end example

@deftp {Class} Procedure
A Scheme procedure.
@end deftp

Procedure instances can be invoked with the @code{call} method to
perform a Javascript to Scheme function call.

@defmethod Procedure call args@dots{}
Call procedure with @var{args} and return an array of result values.
@end defmethod

@deftp {Class} Pair
An immutable cons cell.
@end deftp

@deftp {Class} MutablePair
A mutable cons cell.
@end deftp

@deftp {Class} Vector
An immutable vector.
@end deftp

@deftp {Class} MutableVector
A mutable vector.
@end deftp

@deftp {Class} Bytevector
An immutable bytevector.
@end deftp

@deftp {Class} MutableBytevector
A mutable bytevector.
@end deftp

@deftp {Class} Bitvector
An immutable bitvector.
@end deftp

@deftp {Class} MutableBitvector
A mutable bitvector.
@end deftp

@deftp {Class} MutableString
A mutable string.
@end deftp

@deftp {Class} Sym
A symbol.
@end deftp

@deftp {Class} Keyword
A keyword.
@end deftp

@deftp {Class} Variable
A mutable variable.
@end deftp

@deftp {Class} AtomicBox
A mutable box with atomic updates.
@end deftp

@deftp {Class} HashTable
A hash table.
@end deftp

@deftp {Class} WeakTable
A weak key hash table.
@end deftp

@deftp {Class} Fluid
A dynamic variable.
@end deftp

@deftp {Class} DynamicState
A set of fluids.
@end deftp

@deftp {Class} Syntax
A syntax object.
@end deftp

@deftp {Class} Port
An I/O port.
@end deftp

@deftp {Class} Struct
A user-defined structure.
@end deftp

@node Scheme reference
@chapter Scheme reference

In addition to supporting standard Scheme and Guile-specific features,
Hoot includes some of its own Scheme extensions.  This chapter
documents the APIs of these extensions.

@menu
* Hash tables::                   Mutable key/value data structures.
* Foreign function interface::    Call host functions from Scheme.
@end menu

@node Hash tables
@section Hash tables

There are many mutable hashtable APIs amongst all the various Scheme
implementations, standards, and SRFIs.  From our point of view, there
is no clear ``best'' hashtable API that has emerged, but we think the
R6RS interface is OK.  Guile's own hashtable API has design issues
that are best left in the past.  So, the @code{(hoot hashtables)}
module is R6RS-like.

Currently, this interface supports keys hashed by object identity
(@code{eq?}) only.  The object equivalence (@code{eqv?}) and deep
object equality (@code{equal?}) hashing strategies are not yet
implemented.

@deffn {Procedure} make-eq-hashtable
Return a new, empty hash table.
@end deffn

@deffn {Procedure} hashtable? obj
Return @code{#t} if @var{obj}
@end deffn

@deffn {Procedure} hashtable-size hashtable
Return the number of key/value pairs in @var{hashtable}.
@end deffn

@deffn {Procedure} hashtable-ref hashtable key default
Return the value associated with @var{key} in @var{hashtable} or
@var{default} if there is no such association.
@end deffn

@deffn {Procedure} hashtable-set! hashtable key value
Modify @var{hashtable} to associate @var{key} with @var{value},
overwriting any previous association that may have existed.
@end deffn

@deffn {Procedure} hashtable-delete! hashtable key
Remove the association with @var{key} in @var{hashtable}, if one
exists.
@end deffn

@deffn {Procedure} hashtable-clear! hashtable
Remove all of the key/value associations in @var{hashtable}.
@end deffn

@deffn {Procedure} hashtable-contains? hashtable key
Return @code{#t} if @var{key} has an associated value in
@var{hashtable}.
@end deffn

@deffn {Procedure} hashtable-copy hashtable
Return a copy of @var{hashtable}.
@end deffn

@deffn {Procedure} hashtable-keys hashtable
Return a vector of keys in @var{hashtable}.
@end deffn

@deffn {Procedure} hashtable-entries hashtable
Return a vector of values in @var{hashtable}.
@end deffn

@deffn {Procedure} hashtable-for-each proc hashtable
For each key/value pair in @var{hashtable}, call @var{proc} with two
arguments: the key and the value.
@end deffn

Hoot also includes weak key hash tables that wrap those of the Wasm
host platform, such as the @code{WeakMap} JavaScript class on the web.

@deffn {Procedure} make-weak-key-hashtable
Return a new weak key hashtable.
@end deffn

@deffn {Procedure} weak-key-hashtable? obj
Return @code{#t} if @var{obj} is a weak key hashtable.
@end deffn

@deffn {Procedure} weak-key-hashtable-ref hashtable key [default #f]
Return the value associated with @var{key} in @var{hashtable} or
@var{default} if there is no such association.
@end deffn

@deffn {Procedure} weak-key-hashtable-set! hashtable key value
Modify @var{hashtable} to associate @var{key} with @var{value},
overwriting any previous association that may have existed.
@end deffn

@deffn {Procedure} weak-key-hashtable-delete! hashtable key
Remove the association with @var{key} in @var{hashtable}, if one
exists.
@end deffn

@node Foreign function interface
@section Foreign function interface

WebAssembly follows the capability security model, which means that
modules cannot do much on their own.  Wasm modules are guests within a
host.  They must be given capabilities by the host in order to
interact with the outside world.  Modules request capabilities by
declaring imports, which the host then fills out with concrete
implementations at instantiation time.  Hoot provides a foreign
function interface (FFI) in the @code{(hoot ffi)} module to embed
these import declarations within Scheme code.

The @code{define-foreign} form declares an import with a given type
signature (Wasm is statically typed) and defines a procedure for
calling it.  The FFI takes care of converting Scheme values to Wasm
values and vice versa.  For example, declaring an import for creating
text nodes in a web browser could look like this:

@lisp
(define-foreign make-text-node
  "document" "createTextNode"
  (ref string) -> (ref null extern))
@end lisp

In the above example, the procedure is bound to the variable
@code{make-text-node}.  In the Wasm binary, this import is named
``createTextNode'' and resides in the ``document'' namespace of the
import table.  A Wasm host is expected to satisfy this import by
providing a function that accepts one argument, a string, and returns
an arbitary host value which may be null.

Note that declaring an import @emph{does not} do anything to bind that
import to an implementation on the host.  The Wasm guest cannot grant
capabilities unto itself.  Furthermore, the host could be any Wasm
runtime, so the actual implementation will vary.  In the context of a
web browser, the JavaScript code that instantiates a module with this
import could look like this:

@example
@verbatim
Scheme.load_main("hello.wasm", {}, {
  document: {
    createTextNode: Document.prototype.createTextNode.bind(document)
  }
});
@end verbatim
@end example

And here's what it might look like when using the Hoot interpreter:

@lisp
(use-modules (hoot reflect))
(hoot-instantiate (call-with-input-file "hello.wasm" parse-wasm)
                  `(("document" .
                     (("createTextNode" . ,(lambda (str) `(text ,str)))))))
@end lisp

Once defined, @code{make-text-node} can be called like any other
procedure:

@lisp
(define text-node (make-text-node "Hello, world!"))
@end lisp

Since the return type of @code{make-text-node} is @code{(ref null
extern}), the value of @code{text-node} is an @emph{external
reference}.  To check if a value is an external reference, use the
@code{external?} predicate:

@lisp
(external? text-node) ; => #t
@end lisp

External references may be null, which could indicate failure, a cache
miss, etc.  To check if an external value is null, use the
@code{external-null?} predicate:

@lisp
(if (external-null? text-node) 'yay 'uh-oh)
@end lisp

@deffn {Syntax} define-foreign scheme-name namespace import-name param-types ... -> result-type

Define @var{scheme-name}, a procedure wrapping the Wasm import
@var{import-name} in the namespace @var{namespace}.

The signature of the function is specified by @var{param-types} and
@var{result-type}, which are all Wasm types expressed in WAT form.

Valid parameter types are:

@itemize
@item i32: 32-bit integer
@item i64: 64-bit integer
@item f32: 32-bit float
@item f64: 64-bit float
@item (ref string): a string
@item (ref extern): a non-null external value
@item (ref null extern): a possible null external value
@item (ref eq): any Scheme value
@end itemize

Valid result types are:

@itemize
@item none: no return value
@item i32: 32-bit integer
@item i64: 64-bit integer
@item f32: 32-bit float
@item f64: 64-bit float
@item (ref string): a string
@item (ref extern): a non-null external value
@item (ref null extern): a possibly null external value
@item (ref eq): a Scheme value
@end itemize

@end deffn

@deffn {Procedure} external? obj
Return @code{#t} if @var{obj} is an external reference.
@end deffn

@deffn {Procedure} external-null? extern
Return @code{#t} if @var{extern} is null.
@end deffn

@deffn {Procedure} external-non-null? extern
Return @code{#t} if @var{extern} is not null.
@end deffn

@node Toolchain reference
@chapter Toolchain reference

Hoot is not just a Scheme to Wasm compiler.  It's also a
self-contained and general purpose Wasm toolchain.  Hoot does not use
binaryen, wabt, emscripten, etc. in order to assemble and disassemble
Wasm.  The entire toolchain is implemented as a set of Scheme modules
that can be used to automate other Wasm targeted build workflows.
Since everything is implemented in one place, in a single language,
and because Guile encourages a REPL-driven development workflow, Hoot
makes a great platform for learning Wasm in a hands-on, interactive
way!

@menu
* Data types::                    Core Wasm module data types.
* GWAT::                          Guile-flavored WebAssembly Text format.
* Resolver::                      Lower human-readable identifiers to index values.
* Linker::                        Add a standard library to a Wasm module.
* Assembler::                     Create Wasm binaries.
* Binary Parser::                 Parse Wasm binaries.
* Printer::                       Print the contents of a Wasm module.
* Interpreter::                   Execute Wasm within Guile.
@end menu

@node Data types
@section Data types

The @code{(wasm types)} module contains all the core data types that
comprise a Wasm module.

@subsection Modules

The Wasm module type is the top type, incorporating values of all the
types that are to follow.

@deffn {Procedure} wasm? obj
Return @code{#t} if @var{obj} is a Wasm module.
@end deffn

@deffn {Procedure} wasm-id wasm
Return the symbolic ID of @var{wasm}.
@end deffn

@deffn {Procedure} wasm-types wasm
Return the list of types in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-imports wasm
Return the list of imports in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-funcs wasm
Return the list of functions in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-tables wasm
Return the list of tables in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-memories wasm
Return the list of memories in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-globals wasm
Return the list of globals in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-exports wasm
Return the list of exports in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-elems wasm
Return the list of element segments in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-datas wasm
Return the list of data segments in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-tags wasm
Return the list of tags in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-strings wasm
Return the list of strings in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-custom wasm
Return the list of custom segments in @var{wasm}.
@end deffn

@deffn {Procedure} wasm-start wasm
Return the start function index for @var{wasm}.
@end deffn

@subsection Types

Wasm has four numeric types:

@enumerate
@item @code{i32}:
32-bit integer (signed or unsigned)
@item @code{i64}:
64-bit integer (signed or unsigned)
@item @code{f32}:
32-bit single precision IEEE floating point number.
@item @code{f64}:
64-bit double precision IEEE floating point number.
@end enumerate

There is also the @code{v128} vector type, but it is currently
unsupported.

Then there are a number of reference types that fall into 3
categories: function, external, and internal.

Function reference types:

@enumerate
@item @code{func}:
Function reference.
@item @code{nofunc}:
Bottom type for functions.  No function is of type @code{nofunc}.
@end enumerate

External reference types:

@enumerate
@item @code{extern}:
External reference introduced by the host.
@item @code{noextern}:
Bottom type for external references.  No external reference is of type
@code{noextern}.
@end enumerate

Internal reference types:

@enumerate
@item @code{any}:
The top type of all internal reference types.
@item @code{eq}:
Structural equivalence type.  Subtype of @code{all}.
@item @code{i31}:
Used for immediate references (such as the empty list or fixnums in
Scheme.)  Subtype of @code{eq}.
@item @code{array}:
Super type of all array types. Subtype of @code{eq}.
@item @code{struct}:
Super type of all struct types. Subtype of @code{eq}.
@item @code{none}:
The bottom type for internal references.  No internal reference is of
type @code{none}.
@end enumerate

Of course, modules may specify their own compound types assembled from
these primitives.

The type hierarchy looks like this:

@verbatim
              .-----.                         .-------.     .---------.
   .--------- | any | ------------.           | func  |     | extern  |
   |          `-----'             |           `-------'     `---------'
   ↓             ↓                ↓               ↓              ↓
 .-----.      .-----.        .---------.    .-----------.  .-----------.
 | i31 |      | eq  |        | struct  |    | all funcs |  | noextern  |
 `-----'      `-----'        `---------'    `-----------'  `-----------'
                 ↓                ↓               ↓
          .-------------.  .-------------.   .---------.
          | all arrays  |  | all structs |   | nofunc  |
          `-------------'  `-------------'   `---------'
                 ↓
              .-----.
              | any |
              `-----'
@end verbatim

A collection of type descriptor objects form a type table that
describes all non-primitive types used within a module.  Type objects
associate an identifier with a function signature or reference type
descriptor.

@deffn {Procedure} type? obj
Return @code{#t} if @var{obj} is a type.
@end deffn

@deffn {Procedure} type-id type
Return the symbolic ID of @var{type}.
@end deffn

@deffn {Procedure} type-val type
Return the type descriptor of @var{type}.
@end deffn

Types may also be nested within recursive type groups that allow for
circular and self references to the types within the group.  Types
@emph{not} within a group can be thought of as belonging to a group of
one.

@deffn {Procedure} rec-group? obj
Return @code{#t} if @var{obj} is a recursive type group.
@end deffn

@deffn {Procedure} rec-group-types rec-group
Return the types within @var{rec-group}.
@end deffn

Note that while each Wasm module contains a full inventory of its
types, structurally identical type groups across Wasm modules are
canonicalized at runtime and are considered to be identical
(@code{eq?} in Scheme terms.)  This allows for passing references
between modules.

Type uses refer to function signatures and are used for specifying the
type of a @code{block}, @code{loop}, or @code{if} expression.

@deffn {Procedure} type-use? obj
Return @code{#t} if @var{obj} is a type use.
@end deffn

@deffn {Procedure} type-use-idx type-use
Return the type index of @var{type-use}.
@end deffn

@deffn {Procedure} type-use-sig type-use
Return the function signature of @var{type-use}.
@end deffn


@deffn {Procedure} ref-type? obj
Return @code{#t} if @var{obj} is a reference type.
@end deffn

@deffn {Procedure} ref-type-nullable? ref-type
Return @var{#t} if @var{ref-type} is nullable.
@end deffn

@deffn {Procedure} ref-type-heap-type ref-type
Return the heap type of @var{ref-type}.
@end deffn

As mentioned above, reference types support structural subtyping.

@deffn {Procedure} sub-type? obj
Return @code{#t} if @var{obj} is a sub type.
@end deffn

@deffn {Procedure} sub-type-final? sub-type
Return @code{#t} if @var{sub-type} is marked as final.
@end deffn

@deffn {Procedure} sub-type-supers sub-type
Return super types of @var{sub-type}.
@end deffn

@deffn {Procedure} sub-type-type sub-type
Return the concrete type descriptor of @var{sub-type}.
@end deffn

Compound types take the form of arrays and structs.

@deffn {Procedure} array-type? obj
Return @code{#t} if @var{obj} is an array type.
@end deffn

@deffn {Procedure} array-type-mutable? array-type
Return @code{#t} if @var{array-type} is mutable.
@end deffn

@deffn {Procedure} array-type-type array-type
Retun the element type descriptor of @var{array-type}.
@end deffn

@deffn {Procedure} struct-type? obj
Return @code{#t} if @var{obj} is a struct type.
@end deffn

@deffn {Procedure} struct-type-fields struct-type
Return the field descriptors of @var{struct-type}.
@end deffn

Struct types are composed of several fields.

@deffn {Procedure} field? obj
Return @code{#t} if @var{obj} is a struct field.
@end deffn

@deffn {Procedure} field-id field
Return the symbolic ID of @var{field}.
@end deffn

@deffn {Procedure} field-mutable? field
Return @code{#t} if @var{field} is mutable.
@end deffn

@deffn {Procedure} field-type field
Return the type descriptor of @var{field}.
@end deffn

Both arrays and struct fields allow for packed data using the special
@code{i8} and @code{i16} data types.

@subsection Globals

Wasm supports both mutable and immutable global variables.

@deffn {Procedure} global? obj
Return @code{#t} if @var{obj} is a global.
@end deffn

@deffn {Procedure} global-id global
Return the symbloc ID of @var{global}.
@end deffn

@deffn {Procedure} global-type global
Return the type of @var{global}.
@end deffn

@deffn {Procedure} global-init global
Return the initialization instructions of @var{global}.  Only constant
instructions are allowed.
@end deffn

@deffn {Procedure} global-type? obj
Return @code{#t} if @var{obj} is a global type.
@end deffn

@deffn {Procedure} global-type-mutable? global-type
Return @code{#t} if @var{global-type} is mutable.
@end deffn

@deffn {Procedure} global-type-type global-type
Return the type descriptor of @var{global-type}.
@end deffn

@subsection Functions

@deffn {Procedure} func? obj
Return @code{#t} if @var{obj} is a function.
@end deffn

@deffn {Procedure} func-id func
Return the symbolic ID of @var{func}.
@end deffn

@deffn {Procedure} func-type func
Return the signature of @var{func}.
@end deffn

@deffn {Procedure} func-locals func
Return the locals of @var{func}.
@end deffn

@deffn {Procedure} func-body func
Return the body instructions of @var{func}.
@end deffn

The type of a function is its signature.  Notably, Wasm supports
multiple return values, just like Scheme.

@deffn {Procedure} func-sig? obj
Return @code{#t} if @var{obj} is a function signature.
@end deffn

@deffn {Procedure} func-sig-params func
Return the parameters of @var{func}.
@end deffn

@deffn {Procedure} func-sig-results func
Return the result types of @var{func}.
@end deffn

Function parameters pair a local identifier with its type.

@deffn {Procedure} param? obj
Return @code{#t} if @var{obj} is a param.
@end deffn

@deffn {Procedure} param-id param
Return the symbolic ID of @var{param}.
@end deffn

@deffn {Procedure} param-type param
Return the type descriptor of @var{param}.
@end deffn

Locals provide additional mutable variables scoped to the body of a
function.

@deffn {Procedure} local? obj
Return @code{#t} if @var{obj} is a function local.
@end deffn

@deffn {Procedure} local-id local
Return the symbolic ID of @var{local}.
@end deffn

@deffn {Procedure} local-type local
Return the type descriptor of @var{local}.
@end deffn

@subsection Imports/exports

Functions, globals, memories, and tables can be imported from the host
or another Wasm module.  They are organized into a two layer
hierarchy.  An import module groups many imports under an umbrella
name, and then the individual item names distinguish imported data
within a module.

@deffn {Procedure} import? obj
Return @code{#t} if @var{obj} is an import.
@end deffn

@deffn {Procedure} import-mod import
Return the module name string of @var{import}.
@end deffn

@deffn {Procedure} import-name import
Return the name string of @var{import}.
@end deffn

@deffn {Procedure} import-kind import
Return the kind of @var{import}.  Either @code{func}, @code{global},
@code{memory}, or @code{table}.
@end deffn

@deffn {Procedure} import-id import
Return the symbolic ID of @var{import}.
@end deffn

@deffn {Procedure} import-type import
Return the type descriptor of @var{import}.
@end deffn

Likewise, functions, globals, memories, and tables can be exported
from a module to be used by the host or by other modules.

@deffn {Procedure} export? obj
Return @code{#t} if @var{obj} is an export.
@end deffn

@deffn {Procedure} export-name export
Return the name string of @var{export}.
@end deffn

@deffn {Procedure} export-kind export
Return the kind of @var{export}.  Either @code{func}, @code{global},
@code{memory}, or @code{table}.
@end deffn

@deffn {Procedure} export-idx export
Return the index of @var{export}.
@end deffn

@subsection Linear memory

Memory objects specify linear chunks of bytes that a module can write
to/read from at runtime.  The size of a memory is specified in terms
of 64KiB pages.  While many memory objects coud be included in a
module, the Wasm specification currently only allows the use of a
single memory at index 0.

@deffn {Procedure} memory? obj
Return @code{#t} if @var{obj} is a memory.
@end deffn

@deffn {Procedure} memory-id memory
Return the symbolic ID of @var{memory}.
@end deffn

The type of a memory currently just specifies the size limitations.

@deffn {Procedure} memory-type memory
Return the type of @var{memory}.
@end deffn

@deffn {Procedure} mem-type? obj
Return @code{#t} if @var{obj} is a memory type.
@end deffn

@deffn {Procedure} mem-type-limits mem-type
Return the limits of @var{mem-type}.
@end deffn

Instructions that manipulate linear memory use the memory argument
type to point to a specific offset within a memory.

@deffn {Procedure} mem-arg? obj
Return @code{#t} if @var{obj} is a memory argument.
@end deffn

@deffn {Procedure} mem-arg-id mem-arg
Return the symbolic ID of @var{mem-arg}.
@end deffn

@deffn {Procedure} mem-arg-offset mem-arg
Return the offset of @var{mem-arg}.
@end deffn

@deffn {Procedure} mem-arg-align mem-arg
Return the alignment of @var{mem-arg}.
@end deffn

@subsection Data segments

Data segments are static chunks of data used to initialize regions of
memory.  They have two possible modes of use:

@enumerate
@item @strong{Active:}
The data segment is copied into memory during instantiation.
@item @strong{Passive:}
The data segment is copied into memory using the @code{memory.init}
instruction.
@end enumerate

@deffn {Procedure} data? obj
Return @code{#t} if @var{obj} is a data segment.
@end deffn

@deffn {Procedure} data-id data
Return the symbolic ID of @var{data}.
@end deffn

@deffn {Procedure} data-mode data
Return the mode of @var{data}.  Either @code{passive} or
@code{active}.
@end deffn

@deffn {Procedure} data-mem data
Return the memory associated with @var{data}.
@end deffn

@deffn {Procedure} data-offset data
Return the instructions that compute the offset of @var{data}.  Only
constant instructions are allowed.
@end deffn

@deffn {Procedure} data-init data
Return a bytevector containing the initialization data of @var{data}.
@end deffn

@subsection Tables

Tables specify a vector of heap object references of a particular
reference type.

@deffn {Procedure} table? obj
Return @code{#t} if @var{obj} is a reference table.
@end deffn

@deffn {Procedure} table-id table
Return the symbolic ID of @var{table}.
@end deffn

@deffn {Procedure} table-type table
Return the type of @var{table}.
@end deffn

Table types specify the reference type of the elements as well as the
size limitations.

@deffn {Procedure} table-type? obj
Return @code{#t} if @var{obj} is a table type.
@end deffn

@deffn {Procedure} table-type-limits table-type
Return the limts of @var{table-type}.
@end deffn

@deffn {Procedure} table-type-elem-type table-type
Return the element type of @var{table-type}.
@end deffn

@subsection Element segments

Element segments are static vectors of references used to initialize
regions of tables (well, mostly.)  They have three possible modes of
use:

@enumerate
@item @strong{Active:}
The element segment is copied into its associated table during
instantiation.
@item @strong{Passive:}
The element segment is copied into its associated table using the
@code{table.init} instruction.
@item @strong{Declarative:}
The element segment is unavailable at runtime and is instead used for
forward declarations of types that are used elsewhere in the code.
@end enumerate


@deffn {Procedure} elem? obj
Return @code{#t} if @var{obj} is an element segment.
@end deffn

@deffn {Procedure} elem-id elem
Return the symoblic ID of @var{elem}.
@end deffn

@deffn {Procedure} elem-mode elem
Return the mode of @var{elem}.
@end deffn

@deffn {Procedure} elem-table elem
Return the table associated with @var{elem}.
@end deffn

@deffn {Procedure} elem-type elem
Return the type of @var{elem}.
@end deffn

@deffn {Procedure} elem-offset elem
Return the instructions that compute the offset of @var{elem}.  Only
constant instructions are allowed.
@end deffn

@deffn {Procedure} elem-inits elem
Return a list of initializer instructions for the items of @var{elem}.
Only constant instructions are allowed.
@end deffn

@subsection Limits

Both memories and tables use limits to constrain their minimum and
maximum size.  A valid limit must have a minimum of at least 1, but
the maximum may be @code{#f} if unbounded growth is allowed.

@deffn {Procedure} limits? obj
Return @code{#t} if @var{obj} is a limits.
@end deffn

@deffn {Procedure} limits-min limits
Return the minimum value of @var{limits}.
@end deffn

@deffn {Procedure} limits-max limits
Return the maximum value of @var{limits} or @code{#f} if there is no
maximum.
@end deffn

@subsection Tags

Tag segments specify types of runtime errors that may be raised.

@deffn {Procedure} tag? obj
Return @code{#t} if @var{obj} is a tag.
@end deffn

@deffn {Procedure} tag-id tag
Return the symbolic ID of @var{tag}.
@end deffn

@deffn {Procedure} tag-type tag
Return the type of @var{tag}.
@end deffn

@subsection Custom sections

Custom sections specify arbitrary data that is not covered by the Wasm
specification.

@deffn {Procedure} custom? obj
Return @code{#t} if @var{obj} is a custom segment.
@end deffn

@deffn {Procedure} custom-name custom
Return the name of @var{custom}.
@end deffn

@deffn {Procedure} custom-bytes custom
Return the bytevector of @var{custom}.
@end deffn

There is, however, one custom section that @emph{is} specified: the
name section.  This section contains various ``name maps'' that can be
used to translate integer identifiers to (hopefully) human-readable
names for the purposes of debugging.

Hoot supports the name subsections described in the Wasm core
specification, the Wasm GC specification, and the extended names
proposal:

@enumerate
@item Module name
@item Function name map
@item Function local indirect name map
@item Block label indirect name map
@item Type name map
@item Table name map
@item Memory name map
@item Global name map
@item Element name map
@item Data name map
@item Struct field indirect name map
@item Tag name map
@end enumerate

Name maps are represented as association lists mapping integers to
strings.  Indirect name maps are represented as association lists
mapping integers to name maps.

@deffn {Procedure} names? obj
Return @code{#t} if @var{obj} is a name section object.
@end deffn

@deffn {Procedure} names-module names
Return the module name of @var{names}.
@end deffn

@deffn {Procedure} names-func names
Return the function name map of @var{names}.
@end deffn

@deffn {Procedure} names-local names
Return the function local indirect name map of @var{names}.
@end deffn

@deffn {Procedure} names-label names
Return the block label indirect name map of @var{names}.
@end deffn

@deffn {Procedure} names-type names
Return the type name map of @var{names}.
@end deffn

@deffn {Procedure} names-table names
Return the table name map of @var{names}.
@end deffn

@deffn {Procedure} names-memory names
Return the memory name map of @var{names}.
@end deffn

@deffn {Procedure} names-global names
Return the global name map of @var{names}.
@end deffn

@deffn {Procedure} names-elem names
Return the element name map of @var{names}.
@end deffn

@deffn {Procedure} names-data names
Return the data name map of @var{names}.
@end deffn

@deffn {Procedure} names-fields names
Return the struct field indirect name map of @var{names}.
@end deffn

@deffn {Procedure} names-tag names
Return the tag name map of @var{names}.
@end deffn

@node GWAT
@section GWAT

The @code{(wasm wat)} module provides a parser for a variant of
WebAssembly Text (WAT) format.  Since the WAT uses an s-expression
syntax that resembles but is distinct from Scheme syntax, Hoot opts to
represent WAT code as Scheme expressions.  This allows for embedding
WAT directly into Scheme code and programmatically generating WAT code
via quasiquote templating or other means.  We call this variant GWAT
where the ``G'' stands for ``Guile'', of course.

The GWAT form has some additional expressive power such as allowing
string constants, bytevectors for data segments, and i32/i64 constants
in either the signed or unsigned range.

WAT has two variants: unfolded and folded.  In the unfolded form,
instruction sequences are linear, as they would be in the resulting
binary:

@lisp
'(module
  (func (export "add") (param $a i32) (param $b i32) (result i32)
        (local.get $a)
        (local.get $b)
        (i32.add)))
@end lisp

The folded form allows instructions to be nested within each other:

@lisp
'(module
  (func (export "add") (param $a i32) (param $b i32) (result i32)
        (i32.add (local.get $a)
                 (local.get $b))))
@end lisp

This form looks more like Scheme procedure calls and is generally
easier to write and reason about.

@deffn {Procedure} wat->wasm expr
Parse @var{expr}, a Wasm module expressed as WAT code, and return a
Wasm module.

@lisp
(parse-wat
 '(module
   (func (export "add") (param $a i32) (param $b i32) (result i32)
         (i32.add (local.get $a)
                  (local.get $b)))))
@end lisp

The returned Wasm module preserves named references, among other
things, and is thus unsuitable as input to the assembler or
interpreter.  To lower the module into a usable form, see
@code{resolve-wasm} in @ref{Resolver}.

@end deffn

@deffn {Procedure} wasm->wat wasm
Disassemble @var{wasm} and return its symbolic WAT form.
@end deffn

@node Resolver
@section Resolver

The @code{(wasm resolve)} module provides the @code{resolve-wasm}
procedure which lowers Wasm modules into a form that can be used by
the assembler or interpreter.  The resolver replaces named references
with their respective integer identifiers, fills out the type table,
and adjusts i32 and i64 constants into their canonical form.

@deffn {Procedure} resolve-wasm mod [#:name-section? #f]
Lower the Wasm module @var{mod} into a form that can be assembled or
interpreted.  Returns a new Wasm module and does not modify @var{mod}.

When @var{name-section?} is @code{#t}, the returned Wasm module will
include a name map that maps the original, human-readable names to the
resolved integer identifiers.
@end deffn

@node Linker
@section Linker

The @code{(wasm link)} module provides a means for extending a Wasm
module with the standard library that it needs at runtime.  Hoot uses
the linker to add the Scheme runtime to the compiled form of user
code.  The linker uses a form of tree-shaking to remove anything that
is not used by the base module.

@deffn {Procedure} add-stdlib wasm stdlib
Return a new Wasm module that is the combination of the Wasm module
@var{wasm} with the Wasm module @var{stdlib}.
@end deffn

@node Assembler
@section Assembler

The @code{(wasm assemble)} module is used to lower Wasm modules into
the Wasm binary format.

@deffn {Procedure} assemble-wasm wasm
Return a bytevector containing the assembled binary form of the Wasm
module @var{wasm}.
@end deffn

@node Binary Parser
@section Binary Parser

The @code{(wasm parse)} module parses the Wasm binary format.

@deffn {Procedure} parse-wasm port
Parse the Wasm binary data from @var{port} and return a Wasm module.
@end deffn

@node Printer
@section Printer

The @code{(wasm dump)} module provides the @code{dump-wasm} procedure
for generating a detailed print-out of a Wasm module's contents.  See
also @ref{Low-level development tools} for the @code{wasm-dump} REPL
command.

@deffn {Procedure} dump-wasm mod [#:port] [#:dump-func-defs? #t]
Write a detailed inventory of the Wasm module @var{mod} to @var{port}
or the current output port if @var{port} is not specified.  If
@var{dump-func-defs?} is @code{#t}, which is the default, all function
definitions are printed, including the instructions in the body of
each.  Depending on the size of the module, this may be an
overwhelming amount of data, thus it is made optional.
@end deffn

@node Interpreter
@section Interpreter

The @code{(wasm vm)} module provides a virtual machine for
interpreting Wasm functions.  To use the interpreter, a Wasm module is
first validated for type safety (among other things) and then
instantiated, at which point exported functions become callable from
Scheme.

The interpreter only accepts validated Wasm.  The @code{validate-wasm}
procedure validates and wraps a Wasm module to indicate successful
validation:

@lisp
(use-modules (wasm vm) (wasm resolve))

(define validated-wasm
  (validate-wasm
   (wat->wasm
    '(module
      (func (export "main") (result i32)
            (i32.const 42))))))
@end lisp

When starting with a Wasm binary, the convenient
@code{load-and-validate-wasm} procedure parses the binary and then
performs validation:

@lisp
(call-with-input-file "hello.wasm" load-and-validate-wasm)
@end lisp

Once the Wasm module has been validated, the runtime data needed for
interpretation can be created by instantiating the module:

@lisp
(define instance (instantiate-wasm validated-wasm))
@end lisp

Exported Wasm functions then become usable as Scheme procedures:

@lisp
(define wasm-main (wasm-instance-export-ref instance "main"))
(wasm-main) ;; => 42
@end lisp

Wasm functions are statically typed, which means that calls from
Scheme to Wasm require runtime type checking for each call.

@subsection Validation

@deffn {Procedure} validate-wasm wasm
Validate the Wasm module @var{wasm} and return a validated Wasm
object.
@end deffn

@deffn {Procedure} load-and-validate-wasm obj
Load and validate the Wasm module within @var{obj} then return a
validated Wasm object.  @var{obj} may be a @code{<wasm>} record as
produced by @code{resolve-wasm}(@pxref{Resolver}), a bytevector
containing a Wasm binary, or an input port from which to read a Wasm
binary.
@end deffn

@deffn {Procedure} validated-wasm? obj
Return @code{#t} if @var{obj} is a validated Wasm object.
@end deffn

@deffn {Procedure} validated-wasm-ref validated-wasm
Unbox and return the Wasm module within @var{validated-wasm}.
@end deffn

@subsection Instantiation

@deffn {Procedure} instantiate-wasm wasm [#:imports '()]
Return a new Wasm instance for the validated Wasm module @var{wasm}.

@var{imports} is a nested association list of imported functions,
globals, memories, and tables.  Wasm imports are identified by a
module name and an object name.  Consider the following Wasm module
that computes 2D polar coordinates and prints them to a log:

@lisp
(use-modules (wasm resolve) (wasm vm) (wasm wat))

(define the-module
  (resolve-wasm
   (wat->wasm
    '(module
      (func $logf64 (import "debug" "logf64") (param f64))
      (func $cos (import "math" "cos") (param f64) (result f64))
      (func $sin (import "math" "sin") (param f64) (result f64))
      (func (export "polar") (param $r f64) (param $theta f64)
            (call $logf64 (f64.mul (local.get $r)
                                   (call $cos (local.get $theta))))
            (call $logf64 (f64.mul (local.get $r)
                                   (call $sin (local.get $theta)))))))))
@end lisp

This module requires three imported functions from two modules.  Thus
the module instantiation code would look like this:

@lisp
(define (logf64 x)
  (format #t "f64: ~a\n" x))

(define the-instance
  (instantiate-wasm (validate-wasm the-module)
                    #:imports `(("debug" . (("logf64" . ,logf64)))
                                ("math" . (("cos" . ,cos)
                                           ("sin" . ,sin))))))
@end lisp

@end deffn

@subsection Globals

@deffn {Procedure} make-wasm-global value mutable?
Return a new Wasm global containing @var{value}.  When @var{mutable?}
is @code{#f}, the value cannot be modified later.
@end deffn

@deffn {Procedure} wasm-global? obj
Return @code{#t} if @var{obj} is a Wasm global.
@end deffn

@deffn {Procedure} wasm-global-ref global
Return the current value within @var{global}.
@end deffn

@deffn {Procedure} wasm-global-set! global val
Set the value within @var{global} to @var{val}.  An exception is
raised if @var{global} is immutable.
@end deffn

@deffn {Procedure} wasm-global-mutable? global
Return @code{#t} if @var{global} is mutable.
@end deffn

@subsection Memories

@deffn {Procedure} make-wasm-memory size [#:limits (make-limits 1 #f)]
Return a new Wasm linear memory containing @var{size} 64KiB pages.
@var{limits} determines the lower and upper bounds of how many pages
this memory can store.  The default limits are a minimum of 1 page and
no maximum page limit.  @xref{Data types} for more information on
limit objects.
@end deffn

@deffn {Procedure} wasm-memory? obj
Return @code{#t} if @var{obj} is a Wasm memory.
@end deffn

@deffn {Procedure} wasm-memory-bytes memory
Return the current bytevector representing the pages of @var{memory}.
@end deffn

@deffn {Procedure} wasm-memory-size memory
Return the size of @var{memory} in 64KiB pages.
@end deffn

@deffn {Procedure} wasm-memory-limits memory
Return the limits of @var{memory}
@end deffn

@deffn {Procedure} wasm-memory-grow! memory n
Increase the size of @var{memory} by @var{n} pages.  An exception is
raised if growing by @var{n} exceeds the limits of @var{memory}.
@end deffn

@subsection Tables

@deffn {Procedure} make-wasm-table size [#:limits (make-limits 1 #f)]
Return a new Wasm reference table containing @var{size} element slots.
@var{limits} determines the lower and upper bounds of how many
elements this table can store.  The default limits are a minimum of 1
element and no maximum element limit.  @xref{Data types} for more
information on limit objects.
@end deffn

@deffn {Procedure} wasm-table?
Return @code{#t} if @var{obj} is a Wasm table.
@end deffn

@deffn {Procedure} wasm-table-size table
Return the size of @var{table}.
@end deffn

@deffn {Procedure} wasm-table-ref table i
Return the reference at the @var{i}th index in @var{table}.
@end deffn

@deffn {Procedure} wasm-table-set! table i x
Set the @var{i}th element of @var{table} to @var{x}, a Wasm reference
type.
@end deffn

@deffn {Procedure} wasm-table-fill! table start fill length
Fill the elements of @var{table} from @var{start} to @var{start} +
@var{length}, exclusive, with the value @var{fill}.
@end deffn

@deffn {Procedure} wasm-table-copy! table at elems start length
Copy the block of elements from vector @var{elems}, from @var{start}
to @var{start} + @var{length}, exclusive, to @var{table}, starting at
@var{at}.
@end deffn

@deffn {Procedure} wasm-table-grow! table n init
Increase the size of @var{table} by @var{n} elements.  An exception is
raised if growing by @var{n} exceeds the limits of @var{table}.
@end deffn

@subsection Observation

Every Wasm instruction evaluated by interpreter can be observed via
the @code{current-instruction-listener} parameter.  Use this hook to
instrument Wasm modules.

The following instruction listener would print every instruction's
name on a separate line:

@lisp
(define (log-instr instr path instance stack blocks locals)
  (display (car instr))
  (newline))

(parameterize ((current-instruction-listener log-instr))
  ...)
@end lisp

@defvar {Variable} current-instruction-listener
The current instruction observation hook which is invoked
@emph{before} each instruction evaluation.  Must be a procedure that
accepts the following arguments:

@enumerate
@item @strong{Instruction:}
The symbolic Wasm instruction to be evaluated.
@item @strong{Path:}
The symbolic location of the instruction within the Wasm module.
@item @strong{Instance:}
The instance that is evaluating the instruction.
@item @strong{Stack:}
The Wasm value stack.
@item @strong{Blocks:}
The Wasm block stack, which is just a list of prompt tags.
@item @strong{Locals:}
The Wasm function locals.
@end enumerate

@end defvar

The Wasm value stack is a special data type with the following API:

@deffn {Procedure} wasm-stack? obj
Return @code{#t} if @var{obj} is a Wasm value stack.
@end deffn

@deffn {Procedure} wasm-stack-items stack
Return the values on @var{stack} as a list.
@end deffn

@node Contributing
@chapter Contributing

Found a bug?  Let us know!  Made an improvement?  Show us!  Issues can
be filed and pull requests can be submitted on
@url{https://gitlab.com/spritely/guile-hoot,GitLab}.

@node License
@chapter License

@emph{(C) 2023 David Thompson}

@emph{Both Guile Hoot and this manual are released under the terms of
the following license:}

@include apache-2.0.texi

@node Index
@unnumbered Index

@printindex fn

@bye