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

I've always thought of "@Int" as a single syntactic unit, so I'd be happy to disallow spaces between the @ and the type. On Fri, Aug 17, 2018, at 12:21, Simon Peyton Jones via ghc-steering-committee wrote:

Ha. We could tighten this up further to require a non-blank immediately after the @ for a type application. Thus f @Int or g @(Tree Int) but not f @ Int g @ (Tree Int)

That would leave (p1@p2) and (p1 @ p2) free for patterns.

Simon

From: ghc-steering-committee On Behalf Of Richard Eisenberg Sent: 17 August 2018 17:18 To: Joachim Breitner Cc: ghc-steering-committee@haskell.org Subject: Re: [ghc-steering-committee] Discussion about "Type Application in Patterns" (#126)

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 mailto: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 mailto: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

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