#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 sulzmann): Here's a brief summary of my current understanding: At all times we demand (the) Consistency (Condition). For instance, the following example violates the Consistency Condition. class C a b | a -> b instance C Int Bool instance C Int Char The Consistency Condition is the minimal requirement to guarantee type safety. (1) The Coverage Condition guarantees termination and confluence for all programs. Recall confluence means that we find unique answers. (2) The Weak Coverage Condition guarantees (local) confluence assuming that we can establish (local) termination. That is, if the type inferencer terminates for some constraint arising from some program text (aka goal constraint) then we know the answer is unique. (3) So what happens if we drop coverage (but of course still demand consistency)? Then we can neither guarantee termination nor confluence (which is obviously bad for predictable type inference). The example I gave still enjoys termination but in general we encounter non-confluence. danilo2 mentions some specific application scenario where for the specific constraints arising out of the program text we seem to find unique answers. I agree that the naming of GHC flags (UndecidableInstances, DysfunctionalDependencies) connected to (2) and (3) is not very helpful and rather confusing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:33 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler