[ghc-steering-committee] Discussion on #155 Type Variable in Labmdas

[Joachim Breitner]

Hi,

there is lots of fence-sitting here (and I am also on that particular
fence). But to make shake the fence: Let’s do it! I think people will
find good and surprising uses for this feature.

Iavor, your original message did not carry a concrete recommendation.
Did the discussion help you to form an opinion?

Cheers,
Joachim


Am Donnerstag, den 04.04.2019, 10:10 +0000 schrieb Simon Peyton Jones
via ghc-steering-committee:
> I really am on the fence.   Good things:
>  
> Richard’s first motivating example, where we still need Proxy, is quite convincing.
>  
> It fills out an obvious gap, with the right sort of intro/elim duality with visible type application.
>  
> And I like that it gives us a language in which to talk about System F elaboration of the program, if and when we want to.  E.g. we can say: if you write
>  
> f x = Just x
>  
> it is as if you had written
>  
> f :: forall a. a -> Maybe a
> f = \@a \(x::a). Just x
>  
> Less good:
> It’s still incomplete concerning (B) because we can’t talk about dictionary bindings.
> It adds more complexity.
> We are not under real pressure to do this now.
>  
> I’d like to hear from a broader range of opinion.
>  
> Simon
>  
> From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Iavor Diatchki
> Sent: 03 April 2019 17:46
> To: Eric Seidel <eric at seidel.io>
> Cc: ghc-steering-committee <ghc-steering-committee at haskell.org>
> Subject: Re: [ghc-steering-committee] Discussion on #155 Type Variable in Labmdas
>  
> Hello,
>  
> perhaps it is time to come up with some sort of decision here.  Based on the replies to this thread we seem to have the following opinions:
>   1. Eric and Richard seem to be quite keen on the feature
>   2. Simon is on the fence, but likes it because it introduces System F vocabulary to Haskell
>   3. I am skeptical of the proposal as is, as I think it adds additional complexity to the language (more non-orthogonal features) without significant payoff.
>  
> Does anyone else have anything else to add?
>  
> -Iavor
>  
>  
>  
> On Tue, Mar 26, 2019 at 6:48 PM Eric Seidel <eric at seidel.io> wrote:
> > On Tue, Mar 26, 2019, at 13:17, Iavor Diatchki wrote:
> > > My concern is that the notation certainly suggests that you are binding 
> > > types with the @ syntax, but in really it is still the type signature 
> > > that guides the binding of the variables and the @ parameters just 
> > > duplicate the information from the type signature.
> > 
> > But you are binding types with the @ syntax. The proposal gives a number of examples where the @-bound type variable is bound by a different name (or not at all) in the type signature. Many are contrived, to demonstrate where the binders are allowed, but the higher-rank and proxy-eliding examples look compelling to me.
> > 
> > We also already allow repeated value binders in Haskell. When I write a function in equational style, I have to rebind each argument in each alternate equation. Sometimes this is noisy and I'll prefer a single equation with an explicit `case`. But for functions where the body is sizable, I find the repeated binders to be quite helpful because the scopes are smaller. I can easily see the same benefit applying to type binders. Ultimately, I think this comes down to a matter of style, and I favor letting Haskell programmers pick the style that works best for them.
> > 
> > Eric
> > _______________________________________________
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> > ghc-steering-committee at haskell.org
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> 
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-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/

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