Why is an irrefutable pattern desugared to a 'cast', while an incomplete pattern is not?

[Simon Peyton Jones]

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 at haskell.org] On Behalf Of
| Christiaan Baaij
| Sent: 09 May 2014 15:29
| To: ghc-devs at 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
| 
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