#9098: Don't use unsafe coerce when desugaring lazy pattersn ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Christiaan Baaij asks (on ghc-devs): when I ask for the desugaring of: {{{ module PatError where paterror :: Maybe Int -> Int paterror (Just i) = i }}} I get the following: {{{ PatError.paterror = \ (ds_dIS :: Data.Maybe.Maybe GHC.Types.Int) -> break<1>() case ds_dIS of _ [Occ=Dead] { __DEFAULT -> (\ _ [Occ=Dead, OS=OneShot] -> Control.Exception.Base.patError @ GHC.Types.Int "PatError.hs:4:1-21|function paterror"#) GHC.Prim.void#; Data.Maybe.Just i_aqG -> break<0>(i_aqG) i_aqG } }}} Where `Control.Exception.Base.patError` is applied to `Int`. However, when I compile the almost identical code with an irrefutable pattern: {{{ module PatError2 where patError :: Maybe Int -> Int patError ~(Just i) = i }}} I get the following core: {{{ PatError2.paterror = \ (ds_dIT :: Data.Maybe.Maybe GHC.Types.Int) -> break<1>() let { i_aqG :: GHC.Types.Int [LclId, Str=DmdType] i_aqG = case ds_dIT of _ [Occ=Dead] { __DEFAULT -> (\ _ [Occ=Dead, OS=OneShot] -> (Control.Exception.Base.irrefutPatError @ () "PatError2.hs:4:1-22|(Data.Maybe.Just i)"#) `cast` (UnivCo representational () GHC.Types.Int :: () ~# GHC.Types.Int)) GHC.Prim.void#; Data.Maybe.Just i_aqG -> i_aqG } } in break<0>(i_aqG) i_aqG }}} Where `Control.Exception.Base.patError` is first applied to `()` after which it is casted to `GHC.Types.Int`. Out of the `[Note]` for the `UnivCo` coercion: {{{ The UnivCo ("universal coercion") serves two rather separate functions: - the implementation for unsafeCoerce# - placeholder for phantom parameters in a TyConAppCo }}} It seems to be used in the `unsafeCoerce#` role here. My question is: why can't the irrefutable pattern be translated to `Control.Exception.Base.irrefutPatError' applied to 'Int'? Instead of using this 'cast'? I find the cast-less version much more readable. Greetings, Christiaan -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9098 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler