The sort of pragma you suggest would satisfy me. Pragmas like this don't bother me and make my job a fair bit easier. Too many, "don't worry about this; later" is exhausting. Too many, "don't worry about this; we're not even going to have time to cover it" is demoralizing.
On Thu, Feb 4, 2016 at 5:31 PM, Edward Z. Yang <[email protected]> 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). > > Edward > > Excerpts from Christopher Allen's message of 2016-02-04 15:20:34 -0800: > > This seems worse than FTP IMO. It's considerably noisier, considerably > > rarer a concern for Haskell programmers, and is wayyyy beyond the scope > of > > most learning resources. > > > > Is there a reason this isn't behind a pragma? > > > > On Thu, Feb 4, 2016 at 5:02 PM, Manuel M T Chakravarty < > [email protected] > > > wrote: > > > > > To be honest, I think, it is quite problematic if an obscure and > untested > > > language extension (sorry, but that’s what it is right now) bleeds > through > > > into supposedly simple standard functionality. The beauty of most of > GHC’s > > > language extensions is that you can ignore them until you need them. > > > > > > Has this ever been discussed more widely? I expect that every single > > > person teaching Haskell is going to be unhappy about it. > > > > > > Manuel > > > > > > > > > > Richard Eisenberg <[email protected]>: > > > > > > > > I agree with everything that's been said in this thread, including > the > > > unstated "that type for ($) is sure ugly". > > > > > > > > Currently, saturated (a -> b) is like a language construct, and it > has > > > its own typing rule, independent of the type of the type constructor > (->). > > > But reading the comment that Ben linked to, I think that comment is > out of > > > date. Now that we have levity polymorphism, we can probably to the > Right > > > Thing and make the kind of (->) more flexible. > > > > > > > > Richard > > > > > > > > On Feb 4, 2016, at 3:27 PM, Ryan Scott <[email protected]> > wrote: > > > > > > > >>> My understanding was that the implicitly polymorphic levity, did > (->) > > > not change because it's a type constructor? > > > >> > > > >> The kind of (->) as GHCi reports it is technically correct. As a > kind > > > >> constructor, (->) has precisely the kind * -> * -> *. What's special > > > >> about (->) is that when you have a saturated application of it, it > > > >> takes on a levity-polymorphic kind. For example, this: > > > >> > > > >> :k (->) Int# Int# > > > >> > > > >> would yield a kind error, but > > > >> > > > >> :k Int# -> Int# > > > >> > > > >> is okay. Now, if you want an explanation as to WHY that's the case, > I > > > >> don't think I could give one, as I simply got this information from > > > >> [1] (see the fourth bullet point, for OpenKind). Perhaps SPJ or > > > >> Richard Eisenberg could give a little insight here. > > > >> > > > >>> Also does this encapsulate the implicit impredicativity of ($) for > > > making runST $ work? I don't presently see how it would. > > > >> > > > >> You're right, the impredicativity hack is a completely different > > > >> thing. So while you won't be able to define your own ($) and be able > > > >> to (runST $ do ...), you can at least define your own ($) and have > it > > > >> work with unlifted return types. :) > > > >> > > > >> Ryan S. > > > >> ----- > > > >> [1] https://ghc.haskell.org/trac/ghc/wiki/NoSubKinds > > > >> > > > >> On Thu, Feb 4, 2016 at 2:53 PM, Christopher Allen < > [email protected]> > > > wrote: > > > >>> My understanding was that the implicitly polymorphic levity, did > (->) > > > not > > > >>> change because it's a type constructor? > > > >>> > > > >>> Prelude> :info (->) > > > >>> data (->) a b -- Defined in ‘GHC.Prim’ > > > >>> Prelude> :k (->) > > > >>> (->) :: * -> * -> * > > > >>> > > > >>> Basically I'm asking why ($) changed and (->) did not when (->) had > > > similar > > > >>> properties WRT * and #. > > > >>> > > > >>> Also does this encapsulate the implicit impredicativity of ($) for > > > making > > > >>> runST $ work? I don't presently see how it would. > > > >>> > > > >>> Worry not about the book, we already hand-wave FTP effectively. One > > > more > > > >>> type shouldn't change much. > > > >>> > > > >>> Thank you very much for answering, this has been very helpful > already > > > :) > > > >>> > > > >>> --- Chris > > > >>> > > > >>> > > > >>> On Thu, Feb 4, 2016 at 12:52 PM, Ryan Scott < > [email protected]> > > > wrote: > > > >>>> > > > >>>> Hi Chris, > > > >>>> > > > >>>> The change to ($)'s type is indeed intentional. The short answer > is > > > >>>> that ($)'s type prior to GHC 8.0 was lying a little bit. If you > > > >>>> defined something like this: > > > >>>> > > > >>>> unwrapInt :: Int -> Int# > > > >>>> unwrapInt (I# i) = i > > > >>>> > > > >>>> You could write an expression like (unwrapInt $ 42), and it would > > > >>>> typecheck. But that technically shouldn't be happening, since ($) > :: > > > >>>> (a -> b) -> a -> b, and we all know that polymorphic types have to > > > >>>> live in kind *. But if you look at unwrapInt :: Int -> Int#, the > type > > > >>>> Int# certainly doesn't live in *. So why is this happening? > > > >>>> > > > >>>> The long answer is that prior to GHC 8.0, in the type signature > ($) :: > > > >>>> (a -> b) -> a -> b, b actually wasn't in kind *, but rather > OpenKind. > > > >>>> OpenKind is an awful hack that allows both lifted (kind *) and > > > >>>> unlifted (kind #) types to inhabit it, which is why (unwrapInt $ > 42) > > > >>>> typechecks. To get rid of the hackiness of OpenKind, Richard > Eisenberg > > > >>>> extended the type system with levity polymorphism [1] to indicate > in > > > >>>> the type signature where these kind of scenarios are happening. > > > >>>> > > > >>>> So in the "new" type signature for ($): > > > >>>> > > > >>>> ($) :: forall (w :: Levity) a (b :: TYPE w). (a -> b) -> a -> b > > > >>>> > > > >>>> The type b can either live in kind * (which is now a synonym for > TYPE > > > >>>> 'Lifted) or kind # (which is a synonym for TYPE 'Unlifted), which > is > > > >>>> indicated by the fact that TYPE w is polymorphic in its levity > type w. > > > >>>> > > > >>>> Truth be told, there aren't that many Haskell functions that > actually > > > >>>> levity polymorphic, since normally having an argument type that > could > > > >>>> live in either * or # would wreak havoc with the RTS (otherwise, > how > > > >>>> would it know if it's dealing with a pointer or a value on the > > > >>>> stack?). But as it turns out, it's perfectly okay to have a levity > > > >>>> polymorphic type in a non-argument position [2]. Indeed, in the > few > > > >>>> levity polymorphic functions that I can think of: > > > >>>> > > > >>>> ($) :: forall (w :: Levity) a (b :: TYPE w). (a -> b) -> > a > > > -> b > > > >>>> error :: forall (v :: Levity) (a :: TYPE v). HasCallStack > => > > > >>>> [Char] -> a > > > >>>> undefined :: forall (v :: Levity) (a :: TYPE v). HasCallStack > => a > > > >>>> > > > >>>> The levity polymorphic type never appears directly to the left of > an > > > >>>> arrow. > > > >>>> > > > >>>> The downside of all this is, of course, that the type signature > of ($) > > > >>>> might look a lot scarier to beginners. I'm not sure how you'd > want to > > > >>>> deal with this, but for 99% of most use cases, it's okay to lie > and > > > >>>> state that ($) :: (a -> b) -> a -> b. You might have to include a > > > >>>> disclaimer that if they type :t ($) into GHCi, they should be > prepared > > > >>>> for some extra information! > > > >>>> > > > >>>> Ryan S. > > > >>>> ----- > > > >>>> [1] https://ghc.haskell.org/trac/ghc/wiki/NoSubKinds > > > >>>> [2] https://ghc.haskell.org/trac/ghc/ticket/11473 > > > >>>> _______________________________________________ > > > >>>> ghc-devs mailing list > > > >>>> [email protected] > > > >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > > >>> > > > >>> > > > >>> > > > >>> > > > >>> -- > > > >>> Chris Allen > > > >>> Currently working on http://haskellbook.com > > > >> _______________________________________________ > > > >> ghc-devs mailing list > > > >> [email protected] > > > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > > >> > > > > > > > > _______________________________________________ > > > > ghc-devs mailing list > > > > [email protected] > > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > > > > > _______________________________________________ > > > ghc-devs mailing list > > > [email protected] > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > > > > > -- Chris Allen Currently working on http://haskellbook.com
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