[ghc-steering-committee] Discussion about "Type Application in Patterns" (#126)

[Richard Eisenberg]

The rule is this, implemented in the lexer:

With -XTypeApplications on, the character immediately before an @ determines how it is lexed.
1. If the character is a legal end-of-identifier character (respecting the state of the -XMagicHash flag), then the @ is an as-pattern.
2. Otherwise, then the @ is the beginning of a type application.

I abbreviate the rule as talking about whitespace sensitivity, because if the character preceding the @ is, say, a +, then we lex +@ as a user-defined operator.

This rule has been implemented since GHC 8.0 and applies in patterns as well as expressions. I don't think anyone noticed. :) So it works reasonably well. It's a silly rule and I'd be happy to do better, but I don't think it's strictly necessary to aim for better here.

It would be reasonable to warn when a user writes an as-pattern that doesn't conform to this rule (with -XNoTypeApplications).

Richard

> On Aug 17, 2018, at 1:37 AM, Joachim Breitner <mail at joachim-breitner.de> wrote:
> 
> Hi Richard,
> 
> Am Donnerstag, den 16.08.2018, 22:28 -0400 schrieb Richard Eisenberg:
>> Considering this future of relaxed ordering requirements on data
>> constructors isn't compulsory, but I do think it's better if we don't
>> paint ourselves into a corner around this.
> 
> I agree.
> 
> But so what do we do? Already now require that there is no space
> between @ and the following token when the user wants to use type
> applications in the pattern?
> 
> And would we also require the user to add a space after an at-pattern?
> In which case we should probably start a deprecation cycle for the
> currently legal
> 
>   foo (a @b) = …
> 
> 
> Cheers,
> Joachim
> 
> -- 
> Joachim Breitner
>  mail at joachim-breitner.de
>  http://www.joachim-breitner.de/
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