Re: New type of ($) operator in GHC 8.0 is problematic

As the instigator of these most recent changes: - Yes, absolutely, ($)'s type is quite ugly. In other areas, I've tried to hide the newfound complexity in the type system behind flags, but I missed this one. I consider the current output to be a bug. - It's conceivable to have a flag -fdefault-levity, on by default, which looks for levity polymorphism while printing and instantiates all levity variables to Lifted before printing. That would fix the type of ($). Of course, users could specify -fno-default-levity. Would this make you happy? - There's a real drawback to flags like -fdefault-levity (and, relatedly, -fprint-explicit-kinds, -fprint-explicit-foralls, -fprint-explicit-coercions, -fprint-equality-relations; the last two are new in 8.0): they hide things from unsuspecting users. We already get a steady trickle of bug reports stemming from confusion around hidden kinds. Users diligently try to make a minimal test case and then someone has to point out that the user is wrong. It's a waste of time and, I'm sure, is frustrating for users. I'm worried about this problem getting worse. - It's interesting that the solution to the two problems Takenobu pulls out below (but others have hinted at in this thread) is by having an alternate Prelude for beginners. I believe that having an alternate beginners' Prelude is becoming essential. I know I'm not the first one to suggest this, but a great many issues that teachers of Haskell have raised with me and posts on this and other lists would be solved by an alternate Prelude for beginners. - Separate from a full alternate Prelude, and as Iavor suggested, we could just have two ($) operators: a simple one with no baked-in magic or levity polymorphism, and then a levity-polymorphic, sneakily impredicative one. This would be dead easy. - Edward is right in that (->) isn't really levity-polymorphic. Well, it is, but it's ad hoc polymorphism not parametric polymorphism. Perhaps in the future we'll make this more robust by actually using type-classes to control it, as we probably should. - The case with (->) is different than that with (). (() :: Constraint) and (() :: *) are wholly unrelated types. () is not constraintyness-polymorphic. It's just that we have two wholly unrelated types that happen to share a spelling. So there are hacks in the compiler to disambiguate. Sometimes these hacks do the wrong thing. If we had type-directed name resolution (which I'm not proposing to have!), this would get resolved nicely. - The reason that the foralls get printed in ($)'s type is that kind variables appear in the type variables' kinds. GHC thinks that printing the foralls are useful in this case and does so without a flag. This is not directly related to the levity piece. If you say `:t Proxy`, you'll get similar behavior. Bottom line: We *need* an alternate Prelude. But that won't happen for 8.0. So in the meantime, I propose -fdefault-levity, awaiting your approval. Richard On Feb 5, 2016, at 8:16 AM, Takenobu Tani wrote:

Hi,

I'll worry about the learning curve of beginners. Maybe, beginners will try following session in their 1st week.

ghci> :t foldr ghci> :t ($)

They'll get following result.

Before ghc7.8:

Prelude> :t foldr foldr :: (a -> b -> b) -> b -> [a] -> b

Prelude> :t ($) ($) :: (a -> b) -> a -> b

Beginners should only understand about following:

* type variable (polymorphism)

After ghc8.0:

Prelude> :t foldr foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b

Prelude> :t ($) ($) :: forall (w :: GHC.Types.Levity) a (b :: TYPE w). (a -> b) -> a -> b

Beginners should understand about following things, more:

* higher order polymorphism (t m) * type class (class t =>) * universal quantification (forall) * kind (type::kind) * levity (lifted/unlifted)

I think it's harder in their 1st week. I tried to draw informal illustrations about Foldable, but beginners may need ghci-beginner’s mode or something?

Sorry I don't still have good idea.

Of course I like Haskell's abstraction :)

Regards, Takenobu

2016-02-05 18:19 GMT+09:00 Joachim Breitner : Hi,

Am Freitag, den 05.02.2016, 09:22 +0200 schrieb Roman Cheplyaka:

...

On 02/05/2016 01:31 AM, Edward Z. Yang wrote:

...

I'm not really sure how you would change the type of 'id' based on a language pragma.

How do people feel about a cosmetic fix, where we introduce a new pragma, {-# LANGUAGE ShowLevity #-} which controls the display of levity arguments/TYPE. It's off by default but gets turned on by some extensions like MagicHash (i.e. we only show levity if you have enabled extensions where the distinction matters).

Yes, I am surprised this isn't the way it's been done. The levity arguments should totally be hidden unless requested explicitly.

I'd only expect this to be a ghc flag (-fshow-levity), not a language pragma, since it should only affect the way types are /shown/.

shouldn’t this already happen, based on -fprint-explicit-kinds? At least I would have expected this.

So we probably either want to make sure that -fno-print-explicit-kinds also prevents forall’ed kind variables, or add a new flag of that (heh) kind.

Greetings, Joachim

-- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org

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