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The following code fails to compile:

{-# LANGUAGE PolyKinds, DataKinds, TypeFamilies, KindSignatures, GADTs, TypeOperators
 #-}

data Nat = Zero | Succ Nat

type family Sing (a :: k) :: k -> *

data SNat n where
  SZero :: SNat Zero
  SSucc :: SNat n -> SNat (Succ n)

data SList (a :: [k]) where
  SNil :: SList '[]
  SCons :: Sing h h -> SList t -> SList (h ': t)

type instance Sing (a :: Nat) = SNat
type instance Sing (a :: [k]) = SList

term :: SList '[ '[Zero], '[]]
term = SCons (SCons SZero SNil) (SCons SNil SNil)

The error generated is

/Users/rae/temp/Scratch.hs:27:15:
    Couldn't match type `Sing [Nat] ((':) Nat 'Zero ('[] Nat))'
                  with `SList Nat'
    Expected type: Sing
                     [Nat] ((':) Nat 'Zero ('[] Nat)) ((':) Nat 'Zero ('[] Nat))
      Actual type: SList Nat ((':) Nat 'Zero ('[] Nat))
    In the return type of a call of `SCons'
    In the first argument of `SCons', namely `(SCons SZero SNil)'
    In the expression: SCons (SCons SZero SNil) (SCons SNil SNil)

/Users/rae/temp/Scratch.hs:27:40:
    Couldn't match type `Sing [Nat] ('[] Nat)' with `SList Nat'
    Expected type: Sing [Nat] ('[] Nat) ('[] Nat)
      Actual type: SList Nat ('[] Nat)
    In the first argument of `SCons', namely `SNil'
    In the second argument of `SCons', namely `(SCons SNil SNil)'
    In the expression: SCons (SCons SZero SNil) (SCons SNil SNil)

It seems that the Sing kind-indexed type family isn't quite working. My guess is that the problem is that we really want, say, Sing '[Zero] to be SList Nat, where Nat is the implicit kind parameter to SList. But, we can't say that. I would hope that the explicit kind annotation on the result of Sing would fix the implicit kind parameter to SList, but it doesn't seem to.

This was tested on 7.5.20120420.

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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