[ghc-steering-committee] #179: Tweak the printing of foralls, recommendation: accept

[Joachim Breitner]

Hi Iavor,

have these explanations helped your understanding here? Do they address
your concerns?

Cheers,
Joachim

Am Freitag, den 15.03.2019, 14:03 -0400 schrieb Richard Eisenberg:
> On Mar 15, 2019, at 1:04 PM, Iavor Diatchki <iavor.diatchki at gmail.com> wrote:
> > 
> > This is a bit orthogonal, but what would you expect to see in this case:
> > 
> > type T b = Bool
> > bar :: (a ~ Int) => a -> T b -> a
> > 
> > :t bar
> 
> Without -fprint-explicit-foralls: (a ~ Int) => a -> T b -> a
> With -fprint-explicit-foralls: forall a b. (a ~ Int) => a -> T b -> a
> 
> > :t bar @Bool
> 
> Without -fprint-explicit-foralls: (Bool ~ Int) => Bool -> T b -> Bool
> With -fprint-explicit-foralls: forall b. (Bool ~ Int) => Bool -> T b -> Bool
> 
> > I guess the answer to these depends on if `b` is quantified or not.
> 
> b *is* quantified -- the quantification happens on the type as the user writes it.
> 
> Richard
> 
> > 
> > 
> > 
> > 
> > On Fri, Mar 15, 2019 at 1:26 AM Simon Peyton Jones <simonpj at microsoft.com> wrote:
> > > This looks particularly confusing without the explicit `forall`, although it seems questionable either way.
> > >  
> > > Why?  GHC often suppresses foralls when printing types; and had -fprint-explicit-foralls to stop doing that.
> > >  
> > > Simon
> > >  
> > > From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Richard Eisenberg
> > > Sent: 14 March 2019 23:46
> > > 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] #179: Tweak the printing of foralls, recommendation: accept
> > >  
> > >  
> > > > On Mar 14, 2019, at 6:18 PM, Iavor Diatchki <iavor.diatchki at gmail.com> wrote:
> > > >  
> > > > bar :: forall a b. (a ~ Int) => a -> b -> a
> > > >  
> > > > Is it the case that `:t bar @Bool` will print
> > > >  
> > > > bar :: (Bool ~ Int) => Bool -> b -> Bool
> > > >  
> > > > This looks particularly confusing without the explicit `forall`, although it seems questionable either way.
> > > 
> > >  
> > > Yes, you understand correctly. `:t bar @Bool` will print that unfortunate type. I agree this is questionable. But I don't know how to do better. The current state of affairs is awkward in a different way, in that `:t bar @Bool` errors (that's OK) and `:t bar @Int` prints `forall {b}. Int -> b -> Int`, even though a further visible type application is possible. Perhaps worse, `:t bar` today prints `forall {b}. Int -> b -> Int` even though the next variable to be instantiated is `a`. This is why we have `:type +v`.
> > >  
> > > The direction in the proposal came about in wondering if it's possible to improve the mechanics of `:t` (without the `+v`). After we thought of doing maximal instantiation of inferred arguments (but no generalization), then we realized we could drop `:type +v` as redundant. I agree it's something of an uneasy compromise here, but I think any choice is going to have unfortunate behavior in some scenarios.
> > >  
> > > Richard
> > > 
> > > 
> > > >  
> > > > -Iavor
> > > >  
> > > >  
> > > >  
> > > > On Thu, Mar 14, 2019 at 7:47 AM Joachim Breitner <mail at joachim-breitner.de> wrote:
> > > > > Dear Committee,
> > > > > 
> > > > > Richard, in collarboration with Simon PJ, proposes some tweaks to how
> > > > > foralls are printed by GHC.
> > > > > 
> > > > > In particular, whenever a type variable is printed with an forall, it
> > > > > is printed with {a} unless it is a specified variable. And there are
> > > > > tweaks to what :type does with its argument that makes it more useful,
> > > > > and obviates the need for a separate :type +v.
> > > > > https://github.com/goldfirere/ghc-proposals/blob/printing-foralls/proposals/0000-printing-foralls.rst
> > > > > 
> > > > > I like that it actually simplifies the UI (by removing :type +v), so I
> > > > > recommend acceptance.
> > > > > 
> > > > > Cheers,
> > > > > Joachim
> > > > > -- 
> > > > > Joachim Breitner
> > > > >   mail at joachim-breitner.de
> > > > >   http://www.joachim-breitner.de/
> > > > > 
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> 
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-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/

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