#8634: Relax functional dependency coherence check ("liberal coverage condition") -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: feature | Status: new request | Milestone: 7.10.1 Priority: high | Version: 7.7 Component: Compiler | Keywords: Resolution: | Operating System: Unknown/Multiple Differential Revisions: Phab:D69 | Type of failure: None/Unknown Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: Unknown | Blocked By: | Related Tickets: #1241, | #2247, #8356, #9103, #9227 | -------------------------------------+------------------------------------- Comment (by rwbarton): You can write your program with `AllowAmbiguousTypes`: add `{-# LANGUAGE AllowAmbiguousTypes, ScopedTypeVariables #-}` and change `tst` to {{{ tst :: forall a s b. (Property a s, Property s b) => a -> b tst a = property (property a :: s) }}} Your program could be equally ambiguous if it used GHC 7.6-style/dys- functional dependencies. The difference is that GHC 7.6 assumes that in `Property a s`, `a` determines `s` if and only if there is a fundep `a -> s`, though neither implication is necessarily true. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:37 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler