Ternary operator: Difference between revisions

Latest revision as of 23:08, 24 July 2021

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"

@@ Line 18: / Line 18: @@
 </haskell>
+Another version that works in GHC.
+<haskell>
+data Cond a = a :? a
+infixl 0 ?
+infixl 1 :?
+(?) :: Bool -> Cond a -> a
+True  ? (x :? _) = x
+False ? (_ :? y) = y
+test = 1 < 2 ? "Yes" :? "No"
+</haskell>
 == Further reading ==
-* [http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#rebindable-syntax Rebindable syntax]
+* {{GHCUsersGuide|exts/rebindable_syntax||a section on Rebindable Syntax}}
-* [http://www.eecs.usma.edu/webs/people/okasaki/pubs.html#hw02 Techniques for Embedding Postfix Languages in Haskell]
+* [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.121.1890&rep=rep1&type=pdf Techniques for Embedding Postfix Languages in Haskell]
 [[Category:Code]]
 [[Category:Idioms]]