On Mar 15, 2019, at 1:04 PM, Iavor Diatchki <iavor.diatchki@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@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@haskell.org> On Behalf Of Richard Eisenberg
Sent: 14 March 2019 23:46
To: Iavor Diatchki <iavor.diatchki@gmail.com>
Cc: ghc-steering-committee@haskell.org; Joachim Breitner <mail@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@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@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@joachim-breitner.de
  http://www.joachim-breitner.de/

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