### Re: Closed Type Family Simplification

Richard Eisenberg <eir <at> cis.upenn.edu>

2014-08-14 11:42:42 GMT

Erik's analysis agrees with mine: type families can never be partially applied, and the fact that Ian's
latest example is accepted as a syntactically correct program is a straightforward bug in GHC. If you try
that code on GHC 7.8.2, it will fail.
Type families can ***never*** be partially applied. (Data families are another story, though...)
Richard
On Aug 14, 2014, at 5:17 AM, Erik Hesselink <hesselink <at> gmail.com> wrote:
> I think partially applied type synonyms (and type families) are
> supposed to be disallowed, but I recently found out that since GHC
> 7.8, this is not the case anymore, but you get very weird other type
> errors instead. I filed a bug [0], perhaps yours is a similar issue.
>
> Erik
>
> [0] https://ghc.haskell.org/trac/ghc/ticket/9433#ticket
>
> On Thu, Aug 14, 2014 at 5:48 AM, Ian Milligan <ianmllgn <at> gmail.com> wrote:
>> This is enough to demonstrate this error:
>>
>> {-# LANGUAGE TypeFamilies #-}
>> {-# LANGUAGE RankNTypes #-}
>>
>> module TypeFamilyTest where
>>
>>
>> type family B f g a where B f g a = f (g a)
>> newtype C f a = C (f a)
>> t :: (forall a. a -> f a) -> (forall a. a -> g a) -> a -> C (B f g) a
>> t f g a = C (f (g a))
>>
>> Is it not possible to use type families in this way?
>>
>>
>> On Wednesday, August 13, 2014 8:39:09 PM UTC-7, Ian Milligan wrote:
>>>
>>> I suspect the problem happens because I am trying to use a partially
>>> applied type family with a type constructor. However, in trying to replicate
>>> the problem I ran into another strange error message with the following
>>> program (using tagged and constraints as I was trying to keep it as close to
>>> the original program as possible)
>>>
>>> {-# LANGUAGE TypeFamilies #-}
>>> {-# LANGUAGE ConstraintKinds #-}
>>> {-# LANGUAGE RankNTypes #-}
>>> {-# LANGUAGE ScopedTypeVariables #-}
>>> {-# LANGUAGE TypeOperators #-}
>>>
>>> module TypeFamilyTest where
>>>
>>> import GHC.Prim
>>> import Data.Constraint
>>> import Data.Tagged
>>> import Data.Proxy
>>>
>>> type family A :: * -> Constraint
>>> type family B f g a where B f g a = f (g a)
>>> newtype C f = C (forall a. Tagged a (A a :- A (f a)))
>>>
>>> t :: forall f g a. C f -> C g -> C (B f g)
>>> t (C x) (C y) = C z where
>>> z :: forall a. Tagged a (A a :- A (f (g a)))
>>> z = Tagged (trans (proxy x (Proxy :: Proxy (g a))) (proxy y (Proxy ::
>>> Proxy a)))
>>>
>>> This fails with the error message
>>>
>>> TypeFamilyTest.hs:21:19:
>>> Couldn't match type ‘a1’ with ‘g a1’
>>> ‘a1’ is a rigid type variable bound by
>>> a type expected by the context: Tagged a1 (A a1 :- A (f a1))
>>> at TypeFamilyTest.hs:21:17
>>> Expected type: Tagged a1 (A a1 :- A (f a1))
>>> Actual type: Tagged a1 (A a1 :- A (f (g a1)))
>>> Relevant bindings include
>>> z :: forall a. Tagged a (A a :- A (f (g a)))
>>> (bound at TypeFamilyTest.hs:23:5)
>>> y :: forall a. Tagged a (A a :- A (g a))
>>> (bound at TypeFamilyTest.hs:21:12)
>>> t :: C f -> C g -> C (B f g) (bound at TypeFamilyTest.hs:21:1)
>>> In the first argument of ‘C’, namely ‘z’
>>> In the expression: C z
>>>
>>> for some reason C (B f g) expects Tagged a1 (A a1 :- A (f a1)) instead of
>>> Tagged a1 (A a1 :- A (f (g a1)))?
>>>
>>> On Wednesday, August 13, 2014 6:54:45 PM UTC-7, Richard Eisenberg wrote:
>>>>
>>>> Your operating assumption sounds right. Do you have a complete, minimal
>>>> example showing the error? If not, I recommend using -fprint-explicit-kinds
>>>> to see if kinds are getting in your way at all.
>>>>
>>>> Richard
>>>>
>>>> On Aug 13, 2014, at 8:02 PM, Ian Milligan <ianm... <at> gmail.com> wrote:
>>>>
>>>>> When a closed type family has only one instance it seems like it should
>>>>> never fail to simplify. Yet this doesn't appear to be the case. When I
>>>>> defined (in GHC 7.8.3) the closed type family
>>>>> type family (:.:) f g a where (:.:) f g a = f (g a)
>>>>> I get errors such as
>>>>> 'Could not deduce (Object c3 ((:.:) f g a) ~ Object c3 (f (g a)))'
>>>>> (where Object is a Constraint family), indicating that f (g a) is not
>>>>> being substituted for (:.:) f g a as desired. Any idea why this happens?
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>>>>
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>>
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