Release notes for Agda version 2.6.3

Installation and infrastructure

Agda supports GHC versions 8.0.2 to 9.2.1.

Language

The new option --erased-cubical turns on a variant of Cubical Agda (see #4701).

When this variant of Cubical Agda is used glue (and some related builtins) may only be used in erased settings. One can import regular Cubical Agda code from this variant of Cubical Agda, but names defined using Cubical Agda are (mostly) treated as if they had been marked as erased. See the reference manual for more details.

The GHC and JS backends can compile code that uses --erased-cubical if the top-level module uses this flag.

This feature is experimental.

The parameter arguments of constructors and record fields are now marked as erased (#4786).

For instance, the type of the constructor c below is now {@0 A : Set} → D A, and the type of the record field R.f is {@0 A : Set} → R A → A`:

  data D (A : Set) : Set where
    c : D A

  record R (A : Set) : Set where
    field
      f : A

The options --subtyping and --no-subtyping have been removed (see #5427).
The cubical interval I now belongs to its own sort, IUniv, rather than SSet. For J : ISet and A : J → Set l, we have (j : J) → A : Set l, that is, the type of functions from a type in ISet to a fibrant type is fibrant.
A new reflection primitive getInstances : Meta → TC (List Term) was added to Agda.Builtin.Reflection. This operation returns the list of all possibly valid instance candidates for a given metavariable. For example, the following macro instantiates the goal with the first instance candidate, even if there are several:
```
macro
pickWhatever : Term → TC ⊤
pickWhatever hole@(meta m _) = do
  (cand ∷ _) ← getInstances m
    where [] -> typeError (strErr "No candidates!" ∷ [])
  unify hole cand
pickWhatever _ = typeError (strErr "Already solved!" ∷ [])
```
The reflection primitives getContext and inContext use a nominal context List (Σ String λ _ → Arg Type) instead of List (Arg Type) for printing type information better. Similarly, extendContext takes an extra argument of type String.
macro definitions can now be used even when they are declared as erased. For example, this is now accepted: ```agda macro @0 trivial : Term → TC ⊤ trivial = unify (con (quote refl) [])

test : 42 ≡ 42 test = trivial


* A new reflection primitive `formatErrorParts : List ErrorPart → TC String`
  is added. It takes a list of `ErrorPart` and return its formatted string.

* A new constructor `pattErr : Pattern → ErrorPart` of `ErrorPart` for reflection
  is added.

Syntax
------

* It is now OK to put lambda-bound variables anywhere in the
  right-hand side of a syntax declaration. However, there must always
  be at least one "identifier" between any two regular "holes". For
  instance, the following syntax declaration is accepted because `-`
  is between the holes `B` and `D`.

  ```agda
  postulate
    F : (Set → Set) → (Set → Set) → Set

  syntax F (λ A → B) (λ C → D) = B A C - D

Syntax can now use lambdas with multiple arguments (#394).

Example:

  postulate
    Σ₂ : (A : Set) → (A → A → Set) → Set

  syntax Σ₂ A (λ x₁ x₂ → P) = [ x₁ x₂ ⦂ A ] × P

Library management

Library files below the "project root" are now ignored (see #5644).

For instance, if you have a module called A.B.C in the directory Root/A/B, then .agda-lib files in Root/A or Root/A/B do not affect what options are used to type-check A.B.C: .agda-lib files for A.B.C have to reside in Root, or further up the directory hierarchy.

Pragmas and options

Profiling options are now turned on with a new --profile flag instead of abusing the debug verbosity option. (See #5781.)

Performance

Meta-variables can now be saved in .agdai files, instead of being expanded. This can affect performance. (See #5731.)

Meta-variables are saved if the pragma option --save-metas is used. This option can be overridden by --no-save-metas.

The new option --syntactic-equality[=FUEL] can be used to limit how many times the syntactic equality shortcut is allowed to fail (see #5801).

If FUEL is omitted, then the syntactic equality shortcut is enabled without any restrictions.

If FUEL is given, then the syntactic equality shortcut is given FUEL units of fuel. The exact meaning of this is implementation-dependent, but successful uses of the shortcut do not affect the amount of fuel. Currently the fuel is decreased in the failure continuations of the implementation of the syntactic equality shortcut.

The idea for this option comes from András Kovács' smalltt.

Compiler backends

Both the GHC and JS backends now refuse to compile code that uses --cubical.

Note that support for compiling code that uses --erased-cubical has been added to both backends (see above).

LaTeX backend

DOT backend

The new option --dependency-graph-include=LIBRARY can be used to restrict the dependency graph to modules from one or more libraries (see #5634).

Note that the module given on the command line might not be included.

The generated graphs no longer contain "redundant" edges: if a module is imported both directly and indirectly, then the edge corresponding to the direct import is omitted.

JSON API

The JSON API now represents meta-variables differently, using objects containing two keys, id and module, both with values that are (natural) numbers. See #5731.

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