Add ifThenElse and (?) to Data.Bool

[winter]

It seems this’s a very old request, see https://wiki.haskell.org/If-then-else <https://wiki.haskell.org/If-then-else>. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a
ifThenElse True  x _ = x
ifThenElse False _ y = y

infixr 1 ?
(?) :: Bool -> a -> a -> a
(?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax.  
+ Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes.
+ In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~
Winter
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