[Haskell-cafe] RFC: revisiting the simplified haskell-src-exts AST

Bertram Felgenhauer bertram.felgenhauer at googlemail.com
Sat Jul 30 22:22:19 UTC 2016

Dear all,

the latest major release of haskell-src-exts (version 1.18) removed
the annotation free variant of the Haskell AST. It occurred to me that
using ghc's pattern synonyms it is possible to offer a light-weight
simplified view on the annotated AST. For example,
Language.Haskell.Exts.Syntax defines

  data Name l = Ident l String | Symbol l String

and we can provide a simplified view with all annotations equal to ()
by doing

  {-# LANGUAGE PatternSynonyms #-}
  import qualified Language.Haskell.Exts.Syntax as H

  type Name = H.Name ()
  pattern Ident a = H.Ident () a
  pattern Symbol a = H.Symbol () a

Because the type isn't changed, no code has to be reimplemented to
work with this AST.

A first rough cut at an implementation can be found on github [1].
Before releasing it on hackage (which requires some more polishing)
I'd appreciate feedback on the approach. Would you consider using
such a package? Is there a better approach? Also any thoughts on
usability (e.g., how to approach documentation) would be welcome.

Cheers, Bertram

[1] https://github.com/int-e/haskell-src-exts-simple

P.S. This grew out of an effort to update lambdabot to HSE-1.18,
which made me feel awful about all the superfluous and ugly () in
the resulting code. I also have some numbers showing that the
annotation-free view makes updating the code smoother:

Using the annotation-free view, the update touches 75 lines:
 src/Lambdabot/Plugin/Haskell/Eval.hs     |   18 +-
 src/Lambdabot/Plugin/Haskell/Pointful.hs |  201 +++++++++++++++----------------
 src/Lambdabot/Plugin/Haskell/Pretty.hs   |   14 +-
 src/Lambdabot/Plugin/Haskell/UnMtl.hs    |   62 ++++-----
 src/Lambdabot/Plugin/Haskell/Undo.hs     |  115 ++++++++---------
 5 files changed, 72 insertions(+), 75 deletions(-)

Using the haskell-src-exts 1.18 AST, the update touches 208 lines:
 src/Lambdabot/Plugin/Haskell/Pointful.hs |   87 ++++++++++++++-----------------
 src/Lambdabot/Plugin/Haskell/Pretty.hs   |   12 ++--
 src/Lambdabot/Plugin/Haskell/UnMtl.hs    |   10 +--
 src/Lambdabot/Plugin/Haskell/Undo.hs     |   34 ++++++------
 5 files changed, 204 insertions(+), 206 deletions(-)

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