(Appologies to Malcolm for multiple copies. Say after me, Reply All!)
On 7/28/06, Malcolm Wallace <[EMAIL PROTECTED]> wrote:
"David House" <[EMAIL PROTECTED]> wrote:
> > 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
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