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
--
Joachim Breitner
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