What is the difference between ++ and : in haskell?

问题

I don't get this--

Prelude> "hi"++"there"
"hithere"
Prelude> "hi":"there"

<interactive>:12:6:
    Couldn't match expected type `[Char]' with actual type `Char'
    Expected type: [[Char]]
      Actual type: [Char]
    In the second argument of `(:)', namely `"there"'
    In the expression: "hi" : "there"
Prelude> 

Why doesn't that also return "hithere"?

回答1:

The types. Try this in GCHi:

Prelude> :t (:)
(:) :: a -> [a] -> [a]
Prelude. :t (++)
(++) :: [a] -> [a] -> [a]

回答2:

I get it now. The first operator needs to be an element, not a list.

So if i did 'h':"ithere" it would return "hithere"

回答3:

The operator : is one of the constructors for lists. So "hello" is 'h':'e':'l':'l':'o':[]. You can imagine lists being defined like: (not real haskell syntax)

data List a = (:) a (List a) | []

: constructs a list by taking an element and a list. That's why the type is a->[a]->[a].

++ which concatenates lists is defined by using : as a primitive:

(++) :: [a] -> [a] -> [a]
(++) []     ys = ys
(++) (x:xs) ys = x : xs ++ ys

来源：https://stackoverflow.com/questions/11569650/what-is-the-difference-between-and-in-haskell