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

[Mateusz Kowalczyk]

On 10/01/2014 07:34 PM, Alan & Kim Zimmerman wrote:
> 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

Awesome.

> 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.

Also awesome.

> 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

I imagine that basically *any* tool that ever cares about source
locations of things would benefit. Naming existing specifics is
difficult because they currently no doubt use some other way of dealing
with the problem.

I want to say ‘yes, we could use this to implement a bunch of stuff in
Yi!’ but at the same time if HaRe can do it then we should simply hook
up into that and never care for it ourselves. So while I appreciate SPJ
making multiple attempts at reaching out for wider feedback, I think
that in actual fact HaRe would be *the* big user here. It is hard to
come up and say ‘if GHC gets this then I'll write XYZ’ because it might
be the case that ‘if GHC gets this then HaRe will be able to do XYZ
which we can use’.

I'd also like to say that it seems like it would make lexer-stuff easier
but I am unsure whether that is even remotely related: this is for what
parser spits out and the lexer would give us that information anyway,
right? I think the lexer is currently not exposed either way…

In conclusion, I think that if it makes it possible for HaRe to
implement better transformations then it's a good change. I can't
comment on the implementation impact here, this is merely from ‘do we
want this or not’ point of view.

-- 
Mateusz K.