#3064: Very long compile times with type functions -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: closed Priority: low | Milestone: 7.12.1 Component: Compiler | Version: 6.10.1 (Type checker) | Keywords: Resolution: fixed | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Compile- | Related Tickets: time performance bug | Test Case: | perf/compiler/T3064 | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by goldfire): * milestone: 7.6.1 => 7.12.1 Comment: From the second-to-last bullet point in Simon's commit message, it looks like this is a breaking change and should not be merged into 7.10 since RC1 is already out. I have to say the breakage is a little disappointing here. Here is the real-life example from the Note: {{{ class Ord r => UserOfRegs r a where ... (i1) instance UserOfRegs r a => UserOfRegs r (Maybe a) where ... (i2) instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r CmmExpr where ... }}} Note the `Ord r` constraint in `(i2)`, which is newly-required by this change. What's disappointing here is that `Ord r` is "obviously" derivable from `UserOfRegs r CmmReg`! While I understand the reasoning in the Note (that `UserOfRegs r CmmReg` is not strictly smaller than the instance head, and therefore looking in its superclasses might, in perverse but realistic scenarios, might cause the superclass dictionary to loop), it's far from obvious from a user standpoint. Is it possible to alert the user to this problem in an error message? It seems, from the diff, that this is not yet done. And, should this perhaps be the first 7.12 release note? As it's a breaking change, I do think it should be called out in the user manual. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/3064#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler