Declaration vs. expression style
(Difference between revisions)
(taken table from "History of Haskell")
(SPJ's filter example)
Revision as of 12:53, 3 July 2007
There are two main styles of writing functional programs, which are both supported by Haskell mainly because several language designers preferred these different styles.
As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude function
filter :: (a -> Bool) -> [a] -> [a]
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
There are characteristic elements of both styles.
|Function arguments on left hand side:||
f x = x*x
|Lambda abstraction:|| |
f = \x -> x*x
|Pattern matching in function definitions:||
f  = 0
f xs = case xs of  -> 0
|Guards on function definitions:||
f [x] | x>0 = 'a'
f [x] = if x>0 then 'a' else ...