[Haskell-cafe] GHC 7.10 GHC-API Changes and proposed changes.

Andrew Gibiansky andrew.gibiansky at gmail.com
Wed Oct 1 20:28:58 UTC 2014

I am very much in favor of this. IHaskell does not need this (...yet?), but
I have played around with writing other source-transforming tools, and
having to rely on HSE for parsing and reformatting and relying on GHC API
for interpreting and typechecking is a little bit of a pain.

Besides HaRe, I'd love a tool equivalent to `go fmt` which normalized stuff
like indentation, alignment, and usage of `where` vs `let`, and
automatically applied various changes (of course details are complicated);
that sort of thing requires good support for the sort of work you're doing
in HaRe.

Also, I *very strongly* support any work that goes into improving the GHC
API. Working with it was a nightmare at the beginning due to lack of
documentation, lack of exposed symbols, and other issues.

(That said, I'm also fine with using the dynamic typing approach that was
mentioned elsewhere to avoid an extra type parameter. Have the annotation
be a Dynamic or something. The motivation for this would be to make the
change less intrusive.)

-- Andrew

On Wed, Oct 1, 2014 at 11:34 AM, Alan & Kim Zimmerman <alan.zimm at gmail.com>

> There are a number of changes in GHC 7.10 that will make it easier for
> tool writers to use the GHC API.
> These include
> 1. More parser entrypoints, to explicitly parse fragments [Andrew
> Gibiansky]
>   The following parsers are now provided
>     parseModule, parseImport, parseStatement, ​ parseDeclaration,
>     parseExpression, parseTypeSignature, ​ parseFullStmt, parseStmt,
>     parseIdentifier, ​ parseType, parseHeader
> 2. No more landmines in the AST [Alan Zimmerman]
>   In the past it was difficult to work with the GHC AST as any generic
>   traversals had to carefully tiptoe around an assortment of panic and
>   undefined expressions. These have now been removed, allowing
>   standard traversal libraries to be used.
> There is a third change currently being discussed, and I would like
> feedback on it's desirability.
> 3. Introduce an annotation structure to the ParsedSource to record the
> location of uncaptured keywords.
>   At the moment the location of let / in / if / then / else / do
>   etc is not captured in the AST. This makes it difficult to
>   parse some source, transform the AST, and then output it again
>   preserving the original layout.
>   The current proposal, which can be seen at [1] and a proof of
>   concept implementation at [2] returns a structure keyed to each AST
>   element containing simply the specific SrcSpan's not already
>   captured in the AST.
>   This is the analogue of the Language.Haskell.Exts.Annotated.Syntax
>   from haskell-src-exts, except a custom SrcSpanInfo structure is
>   provided for each AST element constructor, and it is not embedded
>   within the AST.
>   [1] https://ghc.haskell.org/trac/ghc/wiki/GhcAstAnnotations
>   [2] https://phabricator.haskell.org/D297
> So, apart from HaRe, are there any other users who would be interested
> in using this?
> Regards
>   Alan
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