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

[Richard Eisenberg]

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> 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> 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 at joachim-breitner.de
>> http://www.joachim-breitner.de/
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