# Case

### From HaskellWiki

(Difference between revisions)

(links) |

## Revision as of 15:24, 13 March 2007

Can I have acase

You can make use of some syntactic sugar of Haskell, namely of guards.

case () of _ | cond1 -> ex1 | cond2 -> ex2 | cond3 -> ex3 | otherwise -> exDefault

Alternatively, one could simply factor out a function(/value) and use guards in the argument patterns.

Why sticking to syntactic sugar? We can do it nicely with a function implemented in Haskell:

select :: a -> [(Bool, a)] -> a select def = maybe def snd . List.find fst select exDefault [(cond1, ex1), (cond2, ex2), (cond3, ex3)]

Alternative implementations are

select' def = fromMaybe def . lookup True {- a purely functional implementation of if-then-else -} if' :: Bool -> a -> a -> a if' True x _ = x if' False _ y = y select'' = foldr (uncurry if')

select''

select

if

if'

zipWith3

zipWith3 if'

If you don't like the parentheses for the pairs, you can also define

data SelectBranch a = (:->) { condition :: Bool, expression :: a } select :: a -> [SelectBranch a] -> a select def = maybe def expression . List.find condition select exDefault [cond1 :-> ex1, cond2 :-> ex2, cond3 :-> ex3]

It is also possible to define a ternary operator '?' like in C. Because of partial application it will work nicely together with '$' for the else cause.

infixl 1 ? (?) :: Bool -> a -> a -> a True ? v = const v False ? _ = id cond1 ? ex1 $ cond2 ? ex2 $ cond3 ? ex3 $ exDefault