Syntax for list construction / concatenation

Question

I've only been at Haskell for two days now, and was wondering what the difference between the two function definitions below are:

Prelude> let swap (x1:x2:xs) = x2:x1:xs
Prelude> swap [1..5]
[2,1,3,4,5]
Prelude> let swap' (x1:x2:xs) = [x2] ++ [x1] ++ xs
Prelude> swap' [1..5]
[2,1,3,4,5]

That is, what makes x2:x1:xs different from [x2] ++ [x1] ++ xs ? Please and thanks.

Answer 1

The type signatures are a good place to start:

(:) :: a -> [a] -> [a]
(++) :: [a] -> [a] -> [a]

You can find these out with :type (:) and :type (++) in ghci.

As you can see from the type signatures, both are used to produce lists.

The : operator is used to construct lists (and to take them apart again for pattern matching). To make a list [1,2,3] you just build it up with 1 : 2 : 3 : []. The first element of : is the item to add on the front of the list, and the second element is either a list (also built up with : or the empty list signified by []).

The ++ operator is list concatenation. It takes two lists and appends them together. [1,2,3] ++ [4,5,6] is legal, whereas 1 ++ [1,2,3] is not.