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Jason McCarty reported on Haskell-cafe that this code used to work with GHC 7.6 but in GHC 7.8 it requires AllowAmbiguousTypes:

-- The code below is simplified from code that computes a tensor product of
-- a tensor with an identity matrix whose size is determined from the
-- shapes of the input and output tensors.
{-# LANGUAGE DataKinds, KindSignatures, TypeFamilies, TypeOperators #-}
--{-# LANGUAGE AllowAmbiguousTypes #-}
module Tensors where
import GHC.TypeLits

type family (as :: [Nat]) ++ (bs :: [Nat]) :: [Nat]
type instance '[] ++ bs = bs
type instance (a ': as) ++ bs = a ': (as ++ bs)

data Tensor (s :: [Nat]) = Tensor -- content elided

-- apparently GHC reduces (++) enough to see that n is determined
leftUnit :: Tensor s -> Tensor ('[n, n] ++ s)
leftUnit Tensor = Tensor

-- accepted in 7.6, not accepted in 7.8 without AllowAmbiguousTypes
rightUnit :: Tensor s -> Tensor (s ++ '[n, n])
rightUnit Tensor = Tensor

-- also accepted without AllowAmbiguousTypes
outsideUnit :: Tensor s -> Tensor ('[n] ++ s ++ '[n])
outsideUnit Tensor = Tensor

useleftUnit :: Tensor '[1,1,2]
useleftUnit = leftUnit Tensor -- type of Tensor is inferred

userightUnit :: Tensor '[1,2,2]
userightUnit = rightUnit (Tensor :: Tensor '[1]) -- type must be provided

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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