#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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 you look at the test case for #13025 you'll see code like {{{ f :: (a~~b) => a -> b -> Bool f d x y = case HEq_sc d of (cobox :: a ~# b) -> True }}} From the Given dictionary `(a~~b)`, the contraint solver generates a given binding, just in case {{{ (cobox :: a~# b) = HEq_sc d }}} But because `cobox` is a ''coercion'' we evaluate this binding strictly, and so the desugarer produces the case expression above. Most redundant Given bindings are let-bound and hence discarded quickly by the occurrence analyser, but GHC doesn't discard cases (termination worries). So the task here is to remove the redundant Given in some way. (Of course, if the equality is actually used, we need to keep it.) NB: See `Note [The equality types story]` in `TysPrim` for the meanagerie of equality types. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler