# Declaration vs. expression style

### From HaskellWiki

There are two main styles of writing functional programs, which are both supported by Haskell mainly because several language designers preferred these different styles.

filter

filter :: (a -> Bool) -> [a] -> [a]

## Contents |

## 1 Declaration style

filter p [] = [] filter p (x:xs) | p x = x : rest | otherwise = rest where rest = filter p xs

## 2 Expression style

filter = \p -> \ xs -> case xs of [] -> [] (x:xs) -> let rest = filter p xs in if p x then x : rest else rest

## 3 Comparison

There are characteristic elements of both styles.

Declaration style | Expression-style | ||

where |
let |
||

Function arguments on left hand side: | f x = x*x |
Lambda abstraction: | f = \x -> x*x |

Pattern matching in function definitions: | f [] = 0 |
case |
f xs = case xs of [] -> 0 |

Guards on function definitions: | f [x] | x>0 = 'a' |
if |
f [x] = if x>0 then 'a' else ... |