#12901: Levity polymorphic expressions mustn't be floated out -------------------------------------+------------------------------------- Reporter: hsyl20 | Owner: hsyl20 Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple LevityPolymorphism | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I have been trying to add the following code into GHC: {{{#!hs {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ViewPatterns #-} pattern AppOp1 :: PrimOp -> Arg CoreBndr -> CoreExpr pattern OpVal :: PrimOp -> Arg CoreBndr pattern AppOp1 op x = App (OpVal op) x pattern OpVal op <- Var (isPrimOpId_maybe -> Just op) where OpVal op = Var (mkPrimOpId op) }}} It triggers the following CoreLint error: {{{ <no location info>: warning: [RHS of lvl_spfb :: r] RuntimeRep-polymorphic binder: lvl_spfb :: (r :: TYPE rep) }}} Indeed: {{{#!hs $mAppOp1 :: forall (r :: TYPE rep). CoreExpr -> (PrimOp -> Arg CoreBndr -> r) -> (Void# -> r) -> r [LclIdX, Arity=3] $mAppOp1 = \ (@ (rep_agRe :: RuntimeRep)) (@ (r_agRf :: TYPE rep)) (scrut_agRh :: CoreExpr) (cont_agRi :: PrimOp -> Arg CoreBndr -> r) (fail_agRj :: Void# -> r) -> let { fail_spbW :: Void# -> r [LclId, Arity=1] fail_spbW = \ _ [Occ=Dead, OS=OneShot] -> fail_agRj void# } in let { lvl_spfb :: r [LclId] lvl_spfb = fail_spbW void# } in case scrut_agRh of { __DEFAULT -> fail_spbW void#; App ds_dj2u x_afoV -> $mOpVal @ rep @ r ds_dj2u (\ (op_afoU :: PrimOp) -> cont_agRi op_afoU x_afoV) (\ _ [Occ=Dead] -> lvl_spfb) }}} According to the levity polymorphism paper `lvl_spfb` shouldn't have been floated out in a let-binding. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12901 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler