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

[Simon Peyton Jones]

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 at p2) and (p1 @ p2) free for patterns.

Simon

From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Richard Eisenberg
Sent: 17 August 2018 17:18
To: Joachim Breitner <mail at joachim-breitner.de>
Cc: ghc-steering-committee at 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 at Int 5 would not parse.)


On Aug 17, 2018, at 12:16 PM, Richard Eisenberg <rae at cs.brynmawr.edu<mailto:rae at 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 at joachim-breitner.de<mailto: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
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