Re: deriving Typeable and Nat kinds

On 4/23/15, Iavor Diatchki 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 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...