#14967: Optimizer Casting Caf with nominal type -------------------------------------+------------------------------------- Reporter: etn | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.4.1 Keywords: coerce, | Operating System: Unknown/Multiple nominal | Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When you make a function with a constant result, the optimizer will turn it into a cast of a CAF, even if doing so coerces a nominal parameter into a different one. Minimal Example program: {{{#!hs {-# LANGUAGE RoleAnnotations #-} import Debug.Trace (trace) type role Nom nominal data Nom a = Nom Int deriving Show class Gen g where gen :: g instance Gen (Nom a) where gen = trace "genD" $ Nom 0 main = print (gen :: Nom Int) >> print (gen :: Nom ()) >> print (gen :: Nom Char) }}} This program shows that only one value of type Nom is created and shared, even though doing so requires coercing a nominal role I discovered this while checking core for sharing after creating a constraint result caching mechanism. An IOref ended up being shared for multiple different constraint result holders -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14967 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler