Use sharing in the Alternative instance of Maybe

[Michael Snoyman]

On Wed, Dec 14, 2011 at 2:17 PM, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> On 14 December 2011 12:51, Michael Snoyman <michael at snoyman.com> wrote:
>> Out of curiosity, does this actually bypass an extra allocation, or is
>> GHC smart enough to notice the possibility to share?
>
> With optimizations on, GHC seems to be smart enough. Since the
> optimized core of:
>
> plus1 Nothing  r = r
> plus1 (Just x) _ = Just x
>
> Looks like:
>
> Maybe.plus1 =
>  \ (@ a_acI)
>    (ds_dcO :: Data.Maybe.Maybe a_acI)
>    (r_abo :: Data.Maybe.Maybe a_acI) ->
>    case ds_dcO of wild_X6 {
>      Data.Maybe.Nothing -> r_abo;
>      Data.Maybe.Just x_abp -> wild_X6
>    }
>
> Which is equivalent to the optimized core of:
>
> plus2 Nothing r = r
> plus2 l       _ = l
>
> Which looks like:
>
> Maybe.plus2 =
>  \ (@ t_acG)
>    (ds_dcK :: Data.Maybe.Maybe t_acG)
>    (r_abq :: Data.Maybe.Maybe t_acG) ->
>    case ds_dcK of wild_X7 {
>      Data.Maybe.Nothing -> r_abq;
>      Data.Maybe.Just ipv_scU -> wild_X7
>    }
>
> With -O0 the core of plus1 looks like:
>
> Maybe.plus1 =
>  \ (@ a_acG)
>    (ds_dcM :: Data.Maybe.Maybe a_acG)
>    (r_abm :: Data.Maybe.Maybe a_acG) ->
>    case ds_dcM of _ {
>      Data.Maybe.Nothing -> r_abm;
>      Data.Maybe.Just x_abn -> Data.Maybe.Just @ a_acG x_abn
>    }
>
> Cheers,
>
> Bas

Thank you, that's good to know.

Michael