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

I am fine with the change. Something that I've realized from the last few discussions we've had is that the underlying feature I find questionable is the bit where GHC infers a variable ordering from the written signatures (left to write, but kinds before types, and synonyms matter). On Wed, Apr 17, 2019 at 12:02 AM Joachim Breitner wrote:

Hi Iavor,

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

Cheers, Joachim

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On Mar 15, 2019, at 1:04 PM, Iavor Diatchki wrote:

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This is a bit orthogonal, but what would you expect to see in this

case:

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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

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: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

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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

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On Fri, Mar 15, 2019 at 1:26 AM Simon Peyton Jones <

simonpj@microsoft.com> wrote:

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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 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

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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

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The direction in the proposal came about in wondering if it's

Am Freitag, den 15.03.2019, 14:03 -0400 schrieb Richard Eisenberg: though the next variable to be instantiated is `a`. This is why we have `:type +v`. 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.

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Richard

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-Iavor

On Thu, Mar 14, 2019 at 7:47 AM Joachim Breitner <

mail@joachim-breitner.de> wrote:

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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/...

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I like that it actually simplifies the UI (by removing :type

+v), so I

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recommend acceptance.

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

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_______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/

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