#15378: Use TcLevels to decide about floating out of implications -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If we have an implication constraint `forall[3] sk[3]. [W] alpha[2] ~ Maybe b[2]`, GHC ''floats'' the constraint out of the implication. This is necessary to solve this particular wanted, because the implication is at level 3, while the unification variable `alpha` is at level 2; the variable is thus untouchable in the implication. Right now, computing which constraints can float out is done by looking at the free variables of the constraint and comparing against the set of skolem variables bound in the implication, like `sk[3]`, above (among a few other places). However, there is a much easier way: just use the levels. We can look at the Wanted and determine that it can float out because the maximum level in the wanted is a 2, and the implication is level 3. Thus, this ticket is to track that refactoring: use levels, not free variables, in determining what to float out of an implication. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15378 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler