    \documentclass{article}
    \usepackage{agda}
    \setlength{\parindent}{0pt}
    \setlength{\parskip}{1ex}
    \begin{document}
    The let should be properly aligned in the following code:

    \begin{code}
    id : (A : Set) → A → A
    id A =
      let
        id' : B → B
        id' = λ a → a
      in id'
      where
        B : Set
        B = A
    \end{code}

    This should also work if the new block is following after the
    keyword, as opposed to on a new line.

    \begin{code}
    di : (A : Set) → A → A
    di A =
      let  id' : B → B
           id' = λ a → a
      in id'
      where  B : Set
             B = A
    \end{code}
    \end{document}