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

[Simon Peyton Jones]

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<mailto: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<mailto: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<mailto:mail at joachim-breitner.de>
  http://www.joachim-breitner.de/

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