Splicing types.. should this work?

25 Jan 2010

      Now that type-splicing works in TH, and TH has type-family support, I
was wondering if the following example should compile (with 6.12.1):
...
{-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, TypeFamilies,
             FlexibleInstances, OverlappingInstances #-}
...
module Sample where
...
import Control.Monad
import Language.Haskell.TH
...
class Foo a where
    type FooType a
...
createInstance :: Q Type -> Q Dec
...
createInstance t = instanceD (return [])
  (conT ''Foo `appT` t) [
          tySynInstD ''FooType [t] (conT ''String)
      ]
...
createInstance' :: Q Type -> Q Dec
createInstance' t = liftM head [d|
    instance Foo $t where
        type FooType $t = String|]
the function 'createInstance' compiles without a problem, but it's
(near) equivalent
written using TH quotations + splices fails with the error:

Sample.lhs:22:10:
    Type indexes must match class instance head
    Found `t_aMn' but expected `t_aMl'
    In the associated type instance for `FooType'
    In the instance declaration for `Foo $t'
    In the Template Haskell quotation
      [d|
          instance Foo $t where
              type instance FooType $t = String |]

The compiler seems to not be able to determine that the type spliced
in the class instance head will match the type spliced in the type
instance.

The first version works fine for my purposes, but was curious whether
the failure of the 2nd was a bug or a feature.

Thanks,
Rob

Splicing types.. should this work?

Robert Greayer