I m sorry for a question like this. But i m not too sure about the difference of : and ++ operator in haskell.
x:y:[] = [x,y]
also
[x] ++ [y] = [x,y]
as for the reverse function which arose this question for me,
reverse ::[a]->[a]
reverse [] = []
reverse (x:xs) = reverse(xs)++[x]
why doenst the following work?
reversex ::[Int]->[Int]
reversex [] = []
reversex (x:xs) = reversex(xs):x:[]
giving a type error.
: conses an element onto a list.
++ appends two lists.
The former has type
a -> [a] -> [a]
whereas the latter has type
[a] -> [a] -> [a]
The : operator is known as the "cons" operator and is used to append a head element to a list. So [] is a list and x:[] is appending x to the empty list making a the list [x]. If you then cons y:[x] you end up with th elist [y, x] which is the same as y:x:[].
The ++ operator is the list concatenation operator which takes two lists as operands and "combine" them into a single list. So if you have the list [x] and the list [y] then you can concatenate them like this: [x]++[y] to get [x, y].
Notice that : takes an element and a list while ++ takes two lists.
As for your code that does not work.
reversex ::[Int]->[Int]
reversex [] = []
reversex (x:xs) = reversex(xs):x:[]
The reverse function evaluates to a list. Since the : operator does not take a list as its first argument then reverse(xs):x is invalid. But reverse(xs)++[x] is valid.