# Case

### From HaskellWiki

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select :: a -> [(Bool, a)] -> a | select :: a -> [(Bool, a)] -> a | ||

select def = maybe def snd . List.find fst | select def = maybe def snd . List.find fst | ||

− | + | -- = fromMaybe def . lookup True | |

+ | -- = maybe def id . lookup True | ||

select exDefault | select exDefault | ||

Line 29: | Line 30: | ||

Alternative implementations are | Alternative implementations are | ||

<haskell> | <haskell> | ||

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− | |||

{- a purely functional implementation of if-then-else -} | {- a purely functional implementation of if-then-else -} | ||

if' :: Bool -> a -> a -> a | if' :: Bool -> a -> a -> a | ||

Line 65: | Line 64: | ||

exDefault | exDefault | ||

</haskell> | </haskell> | ||

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=== Using syntactic sugar === | === Using syntactic sugar === |

## Latest revision as of 18:31, 15 March 2012

## Contents |

## [edit] 1 Question

Can I have acase

## [edit] 2 Answer

There are several approaches to this problem.

### [edit] 2.1 Using functions

#### [edit] 2.1.1 select

We can do this nicely with a function implemented in Haskell:

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

Unfortunately this function is not in the Prelude. It is however in the utility-ht package.

#### [edit] 2.1.2 nested 'if'

Alternative implementations are

{- 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'

See if-then-else.

Alternatively you can unrollfoldr

if' cond1 ex1 $ if' cond2 ex2 $ if' cond3 ex3 $ exDefault

#### [edit] 2.1.3 infix operator

If you useif'

?

then because of partial application it will work nicely together with '$' for the else clause.

infixl 1 ? (?) :: Bool -> a -> a -> a (?) = if' cond1 ? ex1 $ cond2 ? ex2 $ cond3 ? ex3 $ exDefault

### [edit] 2.2 Using syntactic sugar

#### [edit] 2.2.1 Guards

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.

#### [edit] 2.2.2 List comprehensions

An alternative sugarful approach is to use list comprehensions.

head $ [ ex1 | cond1 ] ++ [ ex2 | cond2 ] ++ [ ex3 | cond3 ] ++ [ exDefault ]