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