Heh. I already wrote the Phab differential weeks ago. But then I noticed
there's room for more equations, and wasn't sure where to stop.
If x x False = x
If x True False = x
If x True x = x
On Dec 29, 2017 10:27 AM, "Richard Eisenberg"
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