weird behaviour of context resolution with FlexibleContexts and TypeFamilies

[Ganesh Sittampalam]

Hi Dimitrios,

Done: http://hackage.haskell.org/trac/ghc/ticket/4981

BTW although I said in my original email that this happened on GHC 6.12 
as well as GHC 7, it looks like that's not the case for the cutdown 
example. I'm fairly sure that it happened with my original code; let me 
know if it's important to know and I'll go back and check on that.

Cheers,

Ganesh

On 26/02/2011 02:01, Dimitrios Vytiniotis wrote:
>
> Hi Ganesh, you are right Simon's answer is not correct.
>
> The cause of your problem is I believe quite involved -- I think I 
> know what's going on (Simon: it seems to be
> an overlap problem indeed but between a different instance and given 
> arising from a superclass when trying to
> solve a 'silent parameter' wanted) but I can't build the compiler on 
> my OS X tonight to verify my thoughts.
>
> I will hopefully be able to say more over the weekend. Can you submit 
> a bug report in the meanwhile?
>
> Or I will do it tomorrow.
>
> Thanks!
> d-
>
>
>
> ------------------------------------------------------------------------
> *From:* glasgow-haskell-users-bounces at haskell.org 
> [glasgow-haskell-users-bounces at haskell.org] on behalf of Ganesh 
> Sittampalam [ganesh at earth.li]
> *Sent:* Friday, February 25, 2011 4:39 PM
> *To:* Simon Peyton-Jones
> *Cc:* glasgow-haskell-users at haskell.org; Sittampalam, Ganesh
> *Subject:* Re: weird behaviour of context resolution with 
> FlexibleContexts and TypeFamilies
>
> Hi Simon,
>
> You talk about the timing of application of the instance declaration
>   instancePatchInspect (PrimOf p)) => Conflict p
>
> but the constraint is actually defined in the class declaration, and I 
> don't have any instance declarations for Conflict p itself.
> classPatchInspect (PrimOf p)) => Conflict p
>
> Does that make a difference to your answer, or do you mean that the 
> constraint in the class declaration automatically gives rise to the 
> same behaviour?
>
> Ganesh
>
> On 25/02/2011 09:06, Simon Peyton-Jones wrote:
>>
>> You are doing something very delicate here, akin to overlapping 
>> instances.
>>
>> You have an instance
>>
>> instancePatchInspect (PrimOf p)) => Conflict p
>>
>> and a function
>>
>> clever :: (Conflict (OnPrim p), ..) => ...
>>
>> So if a constraint (Conflict blah) arises in the RHS of clever, the 
>> instance declaration will immediately apply; and then the type check 
>> fails.  But if it just so happens to precisely match the provided 
>> constraint (Conflict (OnPrim p)), you want to use the provided 
>> constraint.  In effect the type signature and the instance overlap.
>>
>> Arguably, GHC should refrain from applying the instance if there is 
>> any possibility of a “given” constraint matching.  Currently it’s a 
>> bit random; but it’s a very weird situation.
>>
>> But first, is this really what you intend?
>>
>> Simon
>>
>> *From:*glasgow-haskell-users-bounces at haskell.org 
>> [mailto:glasgow-haskell-users-bounces at haskell.org] *On Behalf Of 
>> *Sittampalam, Ganesh
>> *Sent:* 24 February 2011 07:41
>> *To:* glasgow-haskell-users at haskell.org
>> *Subject:* weird behaviour of context resolution with 
>> FlexibleContexts and TypeFamilies
>>
>> Hi,
>>
>> If I build the code below with -DVER=2, I get a complaint about 
>> PatchInspect (PrimOf p) being missing from the context of 
>> cleverNamedResolve.
>>
>> This doesn't happen with -DVER=1 or -DVER=3
>>
>> I presume that type class resolution is operating slightly 
>> differently in the different cases, but it's quite confusing - in the 
>> original code joinPatches did something useful and I was trying to 
>> inline the known instance definition. I would have expected it to be 
>> consistent between all three cases, either requiring the context or not.
>>
>> Is it a bug, or just one of the risks one takes by using 
>> FlexibleContexts?
>>
>> I've tried this with GHC 6.12.3 and with 7.0.2RC2.
>>
>> Cheers,
>>
>> Ganesh
>>
>> {-# LANGUAGE CPP, TypeFamilies, FlexibleContexts #-}
>> module Class ( cleverNamedResolve ) where
>>
>> data FL p = FL p
>>
>> class PatchInspect p where
>> instance PatchInspect p => PatchInspect (FL p) where
>>
>> type family PrimOf p
>> type instance PrimOf (FL p) = PrimOf p
>>
>> data WithName prim = WithName prim
>>
>> instance PatchInspect prim => PatchInspect (WithName prim) where
>>
>> class (PatchInspect (PrimOf p)) => Conflict p where
>>     resolveConflicts :: p -> PrimOf p
>>
>> instance Conflict p => Conflict (FL p) where
>>     resolveConflicts = undefined
>>
>> type family OnPrim p
>>
>> #if VER==1
>> class FromPrims p where
>>
>> instance FromPrims (FL p) where
>>
>> joinPatches :: FromPrims p => p -> p
>> #else
>> #if VER==2
>> joinPatches :: FL p -> FL p
>> #else
>> joinPatches :: p -> p
>> #endif
>> #endif
>>
>> joinPatches = id
>>
>> cleverNamedResolve :: (Conflict (OnPrim p)
>>                       ,PrimOf (OnPrim p) ~ WithName (PrimOf p))
>>                    => FL (OnPrim p) -> WithName (PrimOf p)
>> cleverNamedResolve = resolveConflicts . joinPatches
>>
>>
>>
>>
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