#11248: Ambiguous Types with Constraint Synonyms -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- 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) type Ctx1 e r s e' r' = (Ctx2 e r s e' r', e' ~ (e ** (r' / r))) bug :: (Ctx1 e r s e' 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 move the `e' ~ ...` constraint from `Ctx1` to `Ctx2` or to the context of `bug`, it compiles as expected. Somehow, GHC misses the constraint when it is in `Ctx1`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11248 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler