Excellent point. I'll fix this. https://ghc.haskell.org/trac/ghc/ticket/9098 Thanks Simon | -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of | Christiaan Baaij | Sent: 09 May 2014 15:29 | To: ghc-devs@haskell.org | Subject: Why is an irrefutable pattern desugared to a 'cast', while an | incomplete pattern is not? | | Dear list, | | 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 | | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://www.haskell.org/mailman/listinfo/ghc-devs