Parsing funny arrows

[Vladislav Zavialov]

Hi Csongor,

I believe the reason for this failure is that  a -> @m b  gets parsed as  a -> @(m b).
Why is that? Because a ‘btype’ includes type-level application.

If you replace the ‘btype’ after PREFIX_AT with an ‘atype’, this particular issue should go away. At least that’s my hypothesis, I haven’t tested it.

- Vlad

> On 29 Aug 2020, at 01:32, Csongor Kiss <kiss.csongor.kiss at gmail.com> wrote:
> 
> Hello devs,
> 
> I am trying to modify GHC's parser to allow the following syntax in types:
> 
>   a -> @m b
> 
> but my naive attempt was unsuccessful:
> 
> type :: { LHsType GhcPs }
>         : btype                        { $1 }
>         | btype '->' PREFIX_AT btype ctype  ...
> 
> For example when I try to parse the following code (and turn on the lexer debug log):
>   
>   test :: a -> @m b
>   test = undefined
> 
> I get the following 
> 
> token: ITvarid "test"
> token: ITdcolon NormalSyntax
> token: ITvarid "a"
> token: ITrarrow NormalSyntax
> token: ITtypeApp
> token: ITvarid "m"
> token: ITvarid "b"
> token: ITsemi
> 
> Parse.hs:2:1: error:
>     parse error (possibly incorrect indentation or mismatched brackets)
>   |
> 2 | test = undefined
> 
> 
> I don't have much experience with hacking on the parser so I'm likely missing something obvious.
> Could someone please point at what I might be doing wrong?
> 
> Thanks in advance.
> 
> Cheers,
> Csongor
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