Ternary operator

With a bit of work, we can define a ternary conditional operator in Haskell. Courtesy of Andrew Baumann. This appears only to be valid in Hugs?

import qualified Prelude

data Cond a = a : a

infixl 0 ?
infixl 1 :

(?) :: Prelude.Bool -> Cond a -> a
Prelude.True  ? (x : _) = x
Prelude.False ? (_ : y) = y

test = 1 Prelude.< 2 ? "yeah" : "no!"

Another version that works in GHC.

data Cond a = a :? a

infixl 0 ?
infixl 1 :?

(?) :: Bool -> Cond a -> a
True  ? (x :? _) = x
False ? (_ :? y) = y

test = 1 < 2 ? "Yes" :? "No"

Further reading

• The GHC Users Guide has a section on Rebindable Syntax.
• Techniques for Embedding Postfix Languages in Haskell