Type Synonyms cause Ambiguous Types
The following code fails to compile:
{-# LANGUAGE DataKinds, TypeOperators, TypeFamilies,
KindSignatures, ConstraintKinds #-}
import GHC.TypeLits
type a / b = FDiv a b
type a ** b = FMul a b
type family FDiv a b where
FDiv 11648 128 = 91
type family FMul a b where
FMul 64 91 = 5824
type family FGCD a b where
FGCD 128 448 = 64
FGCD 128 5824 = 64
type family FLCM a b where
FLCM 128 5824 = 11648
data CT (m :: Nat) (m' :: Nat)
type H0 = 128
type H1 = 448
type H0' = 11648
type H1' = 5824
main' =
let x = undefined :: CT H0 H0'
in foo x :: CT H1 H1'
foo x = bug x
type Ctx2 e r s e' r' =
(e ~ FGCD r e', r' ~ FLCM r e', e ~ FGCD r s)
bug :: (Ctx2 e r s e' r', e' ~ (e ** (FDiv r' r)))
=> CT r r' -> CT s s'
bug = undefined
with the error
Could not deduce ((~) Nat (FGCD r e'0) (FGCD r s))
...
The type variable ‘e'0’ is ambiguous
When checking that ‘foo’ has the inferred type
foo :: forall (r :: Nat) (s :: Nat) (s' :: Nat) (e' :: Nat).
((~) Nat (FGCD r s) (FGCD r e'),
(~) Nat (FMul (FGCD r s) (FDiv (FLCM r e') r)) e') =>
CT r (FLCM r e') -> CT s s'
Probable cause: the inferred type is ambiguous
However, if I change the definition of bug
to:
bug :: (Ctx2 e r s e' r', e' ~ (e ** (r' / r)))
=> CT r r' -> CT s s'
bug = undefined
that is, I use the type synonym /
for FDiv
, then the code suddenly compiles. This seems like a different bug than #11248 (closed) because that ticket is about transitivity of constraint synonyms, while this example is broken simply by using a type synonym.
It's possible that this is also related to #10338, but I'm not sure.