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.

Comparison

As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude function filter:

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

Declaration style

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

Expression style

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

Syntactic elements

There are characteristic elements of both styles.

Declaration style		Expression-style
where clause		let expression
Function arguments on left hand side:	f x = x*x	Lambda abstraction:	f = \x -> x*x
Pattern matching in function definitions:	f [] = 0	case expression:	f xs = case xs of [] -> 0
Guards on function definitions:	f [x] | x>0 = 'a'	if expression:	f [x] = if x>0 then 'a' else ...

See also

• Let vs. Where
• History of Haskell (in section 4.4)