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

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 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@joachim-breitner.de http://www.joachim-breitner.de/ _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee