Re: [Haskell-cafe] if-then-else as rebindable syntax (was Re: Why does Haskell have the if-then-else syntax?)

(Appologies to Malcolm for multiple copies. Say after me, Reply All!) On 7/28/06, Malcolm Wallace wrote:

"David House" wrote:

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Or perhaps (?:) or something like that,

This has come up a few times on #haskell, and the consensus is that a tertiary (?:) operator isn't possible because of the deep specialness of (:). However, you can simulate it pretty well:

infixr 1 ? (?) :: Bool -> (a, a) -> a True ? (t, _) = t False ? (_, t) = t

length "hello" > 4 ? ("yes it is!", "afraid not")

HaXml has a lifted version of C's tertiary operator, which matches C's syntax even more closely:

data ThenElse a = a :> a infixr 3 ?>, :>

(?>) :: (a->Bool) -> ThenElse (a->b) -> (a->b) p ?> (f :> g) = \c-> if p c then f c else g c

You can drop it back down to the term level easily enough:

(?>) :: Bool -> ThenElse a -> a p ?> (t :> e) = if p then t else e

Because the operators are right associative, you don't need parens when you use it:

length "hello" == 4 ?> "yes it is!" :> "afraid not"

I've used this trick as well with a ThenElse data constructor, but that's just for aesthetical reasons, wanting the : first. You could just as easily say e.g. (?) :: Bool -> (a,a) -> a (?) p = if p then fst else snd (<:>) :: a -> a -> (a,a) a <:> b = (a,b) and use it like length "hello" == 4 ? "yup" <:> "nope" The benefit over Malcolm's version is that the (?) operator becomes useful in its own right, like David's version. /Niklas