deriving Typeable and Nat kinds
Iavor Diatchki
iavor.diatchki at gmail.com
Thu Apr 23 16:59:04 UTC 2015
Aha, I see what happened. What I said before was wrong---indeed the
`Typeable` instance for type-nats (and symbols) used to be implemented in
terms of `KnownNat` and `KnownSymbol`:
instance KnownNat n => Typeable (n :: Nat) where ...
When I updated the `Typeable` solver, I forgot about that special
case---I'll have a go at a fix.
-Iavor
On Thu, Apr 23, 2015 at 8:37 AM, Gabor Greif <ggreif at gmail.com> wrote:
> On 4/23/15, Iavor Diatchki <iavor.diatchki at gmail.com> wrote:
> > Hello,
> >
>
> Hi Iavor,
>
> > Apologies if my changes caused difficulties with your work---we made the
> > changes to `Typable` to preserve the soundness of the type system,
> > hopefully the new behavior is exactly equivalent to the old in the safe
> > cases.
>
> No need to apologize, I chose this way to be able to intervene fast
> when something breaks :-) The old behaviour might have been unsafe,
> though I cannot see why.
>
> >
> > Could you post an example of the code where the unwanted `Typeable p`
> > constraint appears? It seems entirely reasonable that if you want to
> solve
> > `Typeable (OUT p o)`, you'll have to provide `Typealbe p`, so I am not
> > seeing the whole picture.
>
> Here is a small example where an additional constraint surfaces:
>
>
> ------------------------------------------------------------------------------
> {-# LANGUAGE AutoDeriveTypeable, GADTs, DataKinds, KindSignatures,
> StandaloneDeriving #-}
>
> import GHC.TypeLits
> import Data.Typeable
>
> data Foo (n :: Nat) where
> Hey :: KnownNat n => Foo n
>
> deriving instance Show (Foo n)
>
> data T t where
> T :: (Show t, Typeable t) => t -> T t
>
> deriving instance Show (T n)
>
> {-
>
> ------------------- WITH ghci-7.11.20150407 (and newer)
> *Main> :t T Hey
> T Hey :: (Typeable n, KnownNat n) => T (Foo n)
>
> ------------------- WITH ghci-7.11.20150215 (old)
> *Main> :t T Hey
> T Hey :: KnownNat n => T (Foo n)
>
> -}
>
> ------------------------------------------------------------------------------
>
>
> >
> > To answer your question about `KnownNat p`---there is no relation between
> > it and `Typeable`. You may think if a `KnownNat x` constraint as just
> the
> > integer `x`. As it happens, the integer is closely related to the
> > `Typeable` instance for the number, so I think we *could* make it work so
>
> Yes, this relation I was alluding to.
>
> > that if you have `KnownNat p`, then you can get `Typeable p`, but this
> has
> > never worked previosly, so perhaps there is another issue.
>
> Maybe with my example from above it might be easier to debug it.
>
> >
> > -Iavor
>
> Thanks,
>
> Gabor
>
> >
> >
> >
> > On Thu, Apr 23, 2015 at 7:04 AM, Gabor Greif <ggreif at gmail.com> wrote:
> >
> >> Hi devs,
> >>
> >> between ghc-7.11.20150215 and ghc-7.11.20150407 I experienced a
> >> strange regression with deriving Typeable for data with Nat-kinded
> >> indices.
> >>
> >> Something like this used to work (I cannot post the whole thing,
> >> unfortunately)
> >>
> >> {{{
> >> data OTU (p :: Nat) (o :: Nat) = OTU { ...fields... } deriving (Show,
> >> Typeable)
> >> }}}
> >>
> >> With my ghc-7.11.20150407 (and later) strange obligations of the form
> >> `Typeable p` appear, rendering my code uncompilable. But there is no
> >> way I can see how I can convince the type checker that the Nat index
> >> is indeed Typeable. I *do* have a `KnownNat p` constraint around,
> >> though.
> >>
> >> The relevant changes to mainline appear to be
> >>
> https://github.com/ghc/ghc/commit/b359c886cd7578ed083bcedcea05d315ecaeeb54
> >>
> https://github.com/ghc/ghc/commit/3a0019e3672097761e7ce09c811018f774febfd2
> >>
> >> both by Iavor.
> >>
> >> So now my questions:
> >>
> >> 1) Is this a regression on mainline? (I surely hope so!) How did this
> >> work before?
> >> 2) Should `KnownNat p` imply `Typeable p` for the ability to have my
> >> `Typeable (OTU p o)`
> >> 3) if 2) is answered with "NO", how am I supposed to satisfy those
> >> constraints?
> >>
> >> Thanks and cheers,
> >>
> >> Gabor
> >>
> >>
> >> PS: Sometimes I feel like a canary doing my research-like programming
> >> with GHC-HEAD, and it is a mostly positive experience, but events like
> >> this make me shiver...
> >>
> >
>
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