Fixity resolution enhancement

[niklasbroberg]

Originally requested by @ndmitchell, two related improvements:

1. If a parse results in a fixity error it should be reported through ParseError, not by raising error.
2. The applyFixities function is very limited. A better signature would be:

applyFixities :: a -> ([FixityError],a) 

Now when there is a fixity error you get back a list of the errors, and a revised document with as many fixities resolved as possible. This would be particularly useful for HLint: http://code.google.com/p/ndmitchell/issues/detail?id=302

And more generally, it should suit any user of HSE. With a simple FixityError -> ParseError function the apply fixities could easily be reused for the main parsing.

[robinp]

It would be nice if the applyFixities could also avoid the "ambigous fixities" - so not just retroactively modify a successful AST, but prevent generating the AST from failing at the first place. For example an imported module defines the (otherwise ambigous) fixities - HSE has no way knowing about this, but the client (haskell-names for example) can supply the information.

[robinp]

Ignore previous comment, just found that can supply fixities in ParseMode.

[mgsloan]

We've run into this issue a number of times, when directly using haskell-src-exts for the FP Haskell Center, and also when calling hlint. applyFixities actually has a monad in its type just to be able to call "fail". However, ParseResult's monad instance does not define fail, so it's just error.

I'm going to make a pull request to both remove the SrcSpanInfo and add a definition for fail.

[ndmitchell]

@mgsloan How do you run into it with HLint? HLint is meant to try again if it fails due to fixities with no fixity resolution, and then goes back and does something custom. Having a standard supported solution would be very useful, but I don't think you should see any big problems in HLint at the moment.

[mgsloan]

@ndmitchell Ahh, you're right - I misremembered that. I think it was an issue in some other usage of HSE (parsing module names, import lists).

This bug currently also affects fay, and really anyone that uses the default parse mode - https://github.com/faylang/fay/blob/master/src/Fay/Compiler/Misc.hs#L325. It was just due to a missing fail = ParseFailed noLoc in its monad instance.