Note that this rule is documented, in brief, in the manual:

> A visible type application is preceded with an @ sign. (To disambiguate the syntax, the @ must be preceded with a non-identifier letter, usually a space. For example, read@Int 5 would not parse.)

On Aug 17, 2018, at 12:16 PM, Richard Eisenberg <rae@cs.brynmawr.edu> wrote:

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@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@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

_______________________________________________
ghc-steering-committee mailing list
ghc-steering-committee@haskell.org
https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee