[ghc-steering-committee] TypeApplications for Overloaded Literals (#129), Recommendation: accept

[Simon Peyton Jones]

I’m on the fence:

  *   Benefit: slight
  *   Cost: slight

But the spec is distressingly ad-hoc.  Magical behaviour if

  *   RebindableSyntax is no
  *   The ‘fromInteger’ in scope is not ‘Prelude.fromInteger’

So I lean to rejection.  Unless some stronger motivations emerge.

Simon

From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Richard Eisenberg
Sent: 06 May 2018 20:27
To: Iavor Diatchki <iavor.diatchki at gmail.com>
Cc: ghc-steering-committee at haskell.org; Joachim Breitner <mail at joachim-breitner.de>
Subject: Re: [ghc-steering-committee] TypeApplications for Overloaded Literals (#129), Recommendation: accept

I'm also unconvinced about this proposal. It's hard to specify, requiring that Prelude.fromInteger be magical. Note that if someone defined a separate Num class (under -XNoImplicitPrelude -XRebindableSyntax) that's fully equivalent to Prelude.Num, it wouldn't work. Also, Joachim's point about `:t 5` is a poor motivation: `:t` does not give you helpful information w.r.t. type applications. You need `:t +v`. With -fprint-explicit-foralls,

λ> :t +v 5
5 :: forall {p}. Num p => p

The appearance of the braces in the output means that 5 is not available for visible type application (the variable p is *inferred*). So if there's any unexpected behavior in the current implementation, that's due to ill-informed expectations, not misbehavior.

In the end, I would prefer to reject this proposal in favor of Joachim's casual proposal here: https://mail.haskell.org/pipermail/ghc-steering-committee/2018-May/000541.html (and expanded upon by my subsequent post).

Richard


On May 6, 2018, at 12:27 PM, Iavor Diatchki <iavor.diatchki at gmail.com<mailto:iavor.diatchki at gmail.com>> wrote:

I am not convinced, if that's the sole motivation.  By that reasoning you might expect that `(-5) @Int` or `(5+1) @Int` would also work, but they wouldn't.



On Sun, May 6, 2018 at 8:47 AM Joachim Breitner <mail at joachim-breitner.de<mailto:mail at joachim-breitner.de>> wrote:
Hi,

Am Sonntag, den 06.05.2018, 15:42 +0000 schrieb Iavor Diatchki:
> Is there a reason why we should write `5 @Int` as opposed to `5 :: Int`?

Because we can. Or rather, we should can, because

   Prelude> :t 5
   5 :: Num t => t
   Prelude> :t 5 @Double
   <interactive>:1:1: error:
       • Cannot apply expression of type ‘t0’
         to a visible type argument ‘Double’
       • In the expression: 5 @Double

is confusing and inconsistent with the user’s expectation after
learning about when they can use ExplicitTypeApplications (unless one
knows about the syntactic sugar involved.)

> Also, there appears to be a typo in the spec, the part which specifies translations for integers (1 turned into 0?)

thanks, fixed.


(BTW, all of you are owners of the repository and should have the
necessary permissions to edit pull requests directly.)

Cheers,
Joachim

--
Joachim Breitner
  mail at joachim-breitner.de<mailto:mail at joachim-breitner.de>
  http://www.joachim-breitner.de/
_______________________________________________
ghc-steering-committee mailing list
ghc-steering-committee at haskell.org<mailto:ghc-steering-committee at haskell.org>
https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
_______________________________________________
ghc-steering-committee mailing list
ghc-steering-committee at haskell.org<mailto: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/20180508/4e602aeb/attachment.html>