将字符串列表连接到一个字符串haskell

Question

I have a list of strings (see below) how do I concatenate these strings into one list containing one string.

["hello","stack","overflow"] 

to

["hellostackoverflow"]

I am just allowed to import Data.Char and Data.List

Answer 1

Consider each string in a list as a list of characters

["hello","stack","overflow"] :: [[Char]]

Concatenation is a process of connecting several lists into one. It must have a following type:

concat :: [[a]] -> [a]

If you will have such a function, you'll get a half of job done. You are looking for a way to get

["hellostackoverflow"]

as a result of concatenation. Once again, look at it's type:

["hellostackoverflow"] :: [[Char]]

It is the same type as you had at the beginning except that there is only one element in a list. So now you need a function which puts something into a list. It must have a type

putToList :: a -> [a]

Once you'll have both concat and putToList functions, your solution will be almost ready. Last thing you need to do is to compose it like that:

myConcatenation = putToList . concat

---

I suggest you to use Hoogle to search existing function by it's type.

Answer 2

You can also use the list monad to reduce the list to a single string, then re-wrap the result in a list.

> [["hello", "stack", "overflow"] >>= id]
["hellostackoverflow"]

The preceding avoids explicitly use of Control.Monad.join :

> import Control.Monad
> [join ["hello", "stack", "overflow"]
["hellostackoverflow"]

Answer 3

concat ["hello","stack","overflow"]

"hellostackoverflow"