#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Let's leave aside the deferred-type-error case for now. Other than that, I claim that '''every evidence variable should be non- bottom''', and so * Can be evaluated strictly, and/or * Its evaluation can be discarded if its payload is not required This is true even of recursively-defined dictionaries; see `Note [Recursive superclasses]` in `TcInstDcls`. (A recursively-defined dictionary may be infinite, so you can't evaluate it hyper-strictly, but it should not be bottom.) But dictionaries still ''can'' be defined recursively (unlike say unboxed values). Now, I believe all this is true of the programs generated by source Haskell, but nothing in Core checks this invariant; and in Core you could certainly declare a bottom dictionary `(let d = d in ...)`. I don't know how to make it a statically-checkable invariant of Core, but I expect that it'd be possible to make a decent conservative approximation that was enough to capture the programs generated from source Haskell. There's even an old flag `-fdicts-strict` that I would like to get back to, which flirted with using call-by-value for evidence variables; c.f. #2436 That leaves deferred type errors. But let's not allow the tail to wag the dog. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler