Currently, we have (in Data.Type.Bool):

> -- | Type-level "If". @If True a b@ ==> @a@; @If False a b@ ==> @b@
> type family If cond tru fls where
>   If 'True  tru  fls = tru
>   If 'False tru  fls = fls

I propose adding a new equation, thus:

> -- | Type-level "If". @If True a b@ ==> @a@; @If False a b@ ==> @b@
> type family If cond tru fls where
>   If b same same = same
>   If 'True  tru  fls = tru
>   If 'False tru  fls = fls

This new equation would allow If to reduce when we don't know the condition but we do know that both branches are the same. All three equations are *compatible* (a technical term defined in the closed type families paper), meaning that GHC ignores the ordering between them and will just choose whichever equation matches.

Any objections?

Thanks,
Richard
_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries