CODEWITHSUNDEEP
haskelltype-familiesassociated-types
Clinton
Clinton

Reputation: 23135

Associated type family fails, works when made standalone

When I have code like the following:

class C where
  type T t1 t2
  ...

instance C (X t) where
  type T (X t) (t a b) = a
  ...

I get the error (with GHC head):

• Polymorphic type indexes of associated type ‘T’
    (i.e. ones independent of the class type variables)
    must be distinct type variables

Presumably this is because (t a b) is not a simple variable like the error implies it should be.

But I can simply reorganise the code like the following:

type family T t1 t2

class C where
  ...

type instance T (X t) (t a b) = a

instance C (X t) where
  ....

And then everything seems to work fine now.

Because a slightly messier syntax, do I lose anything by taking type family definitions out of classes, or are in-class definitions just syntactic sugar so taking them out is a relatively cost free workaround?

Upvotes: 2

Views: 51

Answers (1)

chi
chi

Reputation: 116139

This

type family T t1 t2
type instance T (X t) (t a b) = a

roughly works like this

class K t1 t1 where
   type T t1 t2
instance K (X t) (t a b) where
   type T (X t) (t a b) = a

Note how all the variables t1,t2 are parameters of class K. Indeed, this is required when variables occur more than once (i.e. non-linearly), as the posted error states:

• Polymorphic type indexes of associated type ‘T’
    (i.e. ones independent of the class type variables)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    must be distinct type variables

So, either you add all these type variables to your class, or - to the same effect - you move the type family outside the class.

This compiles in GHC 8.0.

{-# LANGUAGE TypeFamilies, PolyKinds #-}

data X (t :: k) = X

class C t1 where
  type T t1 t2

instance C (X (t :: * -> * -> *)) where
  type T (X t) (t a b) = a

Upvotes: 4

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