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 ... |
