Create concatenate function in Haskell: [String] - String

Question

I'm having a lot of trouble getting this function to work: ``` concatenate :: [String] -> String ``` It is designed to simply take a list of strings and return a single string that is the result of the concatenations of each element of the list from head to tail. I'm trying to stay within the `map`, `foldl`, and `foldr` functions. I feel like I know what the concept of these functions do well enough, but the most common problem I'm running into is that I'm having a conflict of types. GHC will expect a [Char] for example, and I'll put in code that is apparently trying to use a [[Char]] without me knowing it. For example: `concatenate (x:xs) = foldr (++) x (concatenate xs)` And I get the following compile error: ``` Couldn't match type `Char' with `[Char]' Expected type: [[Char]] Actual type: String In the return type of a call of `concatenate' In the third argument of `foldr', namely `(concatenate xs)' In the expression: foldr (++) x (concatenate xs) ``` I'm very new to Haskell, so ple…

Accepted Answer

You actually don't need the recursive call there. The function `foldr` already simulates a recursive call. All you need to do is use:

```
concatenate :: [String] -> String
concatenate ls = foldr (++) "" ls
```

And remember that there's a `concat` function already, which is more generic, as it works on any list of list (as opposed to simply list of strings).

Create concatenate function in Haskell: [String] -> String

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