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

On Mar 15, 2019, at 1:04 PM, Iavor Diatchki 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 mailto: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 mailto:ghc-steering-committee-bounces@haskell.org> On Behalf Of Richard Eisenberg Sent: 14 March 2019 23:46 To: Iavor Diatchki mailto:iavor.diatchki@gmail.com> Cc: ghc-steering-committee@haskell.org mailto:ghc-steering-committee@haskell.org; Joachim Breitner mailto: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 mailto: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 mailto: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/... https://github.com/goldfirere/ghc-proposals/blob/printing-foralls/proposals/...

I like that it actually simplifies the UI (by removing :type +v), so I recommend acceptance.

Cheers, Joachim -- Joachim Breitner mail@joachim-breitner.de mailto:mail@joachim-breitner.de http://www.joachim-breitner.de/ http://www.joachim-breitner.de/

-----BEGIN PGP SIGNATURE-----

iQIzBAABCAAdFiEEGkYIf5Vdk8V8YFcbPZCKs/D79R8FAlyKaX8ACgkQPZCKs/D7 9R/7oxAAgU213SuV+1lr/JExnMSPNWD6s7CPTWdauLK+tHYUv1MD3HeZlBZb2pvN R/swC6jdm95vKXWeMVhfYGFGepOdMEKzqBWLDf3fCO29MlHlb4barwtEZOZ9AbaM DSyE+RjVQ35ATZPvfFTYdBacBrvCivoRWcUdi10ps6rizI86gKeCiG1PN5nVMNV+ cpvTpFWRm/xmbtU4xFWuZ9FjL0bfO6yJCueX07gTkdZYW4vnvWSdis6CInkQZ7SA hXbTEYNT9zMCGvJMG+NQN+5c0h3GJlUoTsNGd0PsQnkvoYiLjM52eM5hzkz5zFRr mMYT381T1nnIl505IFGuT/je5KAn7wTImIXvvclwDxfjCBn3J1IRVAbikgwlTU4M 9L+tlQiTKI8OZpbgpPjjRj4hWrnX1gzWiIGwtPYtQuPhkDhEvq0xex8qEKpd7waE r5klTYyZrC0LGaHyX5G5yunGLWqnlLKOHAPI20Z1bxVPtDJZ/4g6AoCYSNqBLsHt fMTFflHdKTlsCOETPGhaxgf6PP0WvZtNdgTrB2rL5jzKJSOKc1rVRFAdYIr5PP3H c4aZGce/ChyFazHeF7hH5UZ7yyB0gMc3T+do6I8vySp0YGeRweb36FU13CduMS05 rvzb6GIkNFz/Fx2MascE9kZtGvDT9BlxGXqJ9rL/huXquIwN6Gg= =1qbg -----END PGP SIGNATURE----- _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org mailto:ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org mailto:ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee