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

[Richard Eisenberg]

But is this summary correct:

> - @ denotes a type application if it is preceded by a non-identifier character and succeeded by a non-whitespace character
> - @ denotes an as-pattern if is preceded by an identifier character or succeeded by a whitespace character
> 
> This means that `f@ Int` is an as-pattern.

I still say this is an awkward twist, but I see what you're getting at.

Richard

> On Aug 20, 2018, at 9:05 AM, Simon Peyton Jones <simonpj at microsoft.com> wrote:
> 
> I think the goal is to preserve the syntactic space for
> 	f ((f -> Just x) @ (g -> Just y)) = ...
> as an "and-pattern" which matches both patterns and binds both x and y.  And also perhaps
> 
> 	f ((f -> Just x)@(g -> Just y)) = ...
> 
> by narrowing the situations in which "@" introduces a type argument to just
>     <space>@<type>
> 
> with white space before, but not after the "@".
> 
> And do to his in both terms and patterns.
> 
> Simon
> 
> |  -----Original Message-----
> |  From: ghc-steering-committee <ghc-steering-committee-
> |  bounces at haskell.org> On Behalf Of Richard Eisenberg
> |  Sent: 19 August 2018 02:32
> |  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)
> |  
> |  So what's the new rule? Is it:
> |  
> |  - @ denotes a type application if it is preceded by a non-identifier
> |  character and succeeded by a non-whitespace character
> |  - @ denotes an as-pattern if is preceded by an identifier character or
> |  succeeded by a whitespace character
> |  
> |  This means that `f@ Int` is an as-pattern.
> |  
> |  I think the new rule just adds another twist to an already too-
> |  complicated plot.
> |  
> |  I'm not worried about backward-compat issues here (echoing Joachim's
> |  sentiments) but I don't see the advantage to this new spec.
> |  
> |  Richard
> |  
> |  > On Aug 17, 2018, at 1:27 PM, Joachim Breitner <mail at joachim-
> |  breitner.de> wrote:
> |  >
> |  > Hi,
> |  >
> |  > Am Freitag, den 17.08.2018, 13:00 -0400 schrieb Eric Seidel:
> |  >> I've always thought of "@Int" as a single syntactic unit, so I'd be
> |  happy to disallow spaces between the @ and the type.
> |  >
> |  > not opposed in principle, but if we go that route, it should also
> |  > apply to type applications in expressions, for consistency. Are we
> |  > willing to potentially break code out there?  (Well, it’s an
> |  > extension, the fix is simple and fully backward-compatible, and most
> |  > people probably got it right in the first place, so maybe breaking is
> |  > not too bad.)
> |  >
> |  > Cheers,
> |  > Joachim
> |  >
> |  >
> |  > --
> |  > Joachim Breitner
> |  >  mail at joachim-breitner.de
> |  >
> |  >
> |  https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.jo
> |  > achim-
> |  breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7Ca3c9
> |  >
> |  e99884a34ec6670d08d605739f1f%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C
> |  >
> |  0%7C636702391484409758&sdata=mFolm%2BaXnNO%2FMdniUugnuqcotyGViJbtf
> |  > qK%2FKD1VM0I%3D&reserved=0
> |  > _______________________________________________
> |  > ghc-steering-committee mailing list
> |  > ghc-steering-committee at haskell.org
> |  > https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-
> |  committ
> |  > ee
> |  
> |  _______________________________________________
> |  ghc-steering-committee mailing list
> |  ghc-steering-committee at haskell.org
> |  https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-
> |  committee

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-steering-committee/attachments/20180820/f1648b40/attachment-0001.html>