Declaration vs. expression style
There are two main styles of writing functional programs, which are both supported by Haskell mainly because several language designers preferred these different styles.
In the declaration style you formulate an algorithm in terms of several equations that shall be satisfied.
In the expression style you compose big expressions from small expressions.
As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude function
filter :: (a -> Bool) -> [a] -> [a]
filter p  =  filter p (x:xs) | p x = x : rest | otherwise = rest where rest = filter p xs
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:||
|Pattern matching in function definitions:||
|Guards on function definitions:||