Haskell函数，将单词列表转换为字符串

Question

For example:

wordsToString ["all","for","one","and","one","for","all"]
"all for one and one for all"

My code works without a type declaration:

wordsToString [] = ""
wordsToString [word] = word
wordsToString (word:words) = word ++ ' ':(wordsToString words)

But when I do the type check,it shows that it is a list of Chars which seems wrong to me as I'm supposed to declare the input as a list of strings and get a string as the output:

*Main> :type wordsToString
wordsToString :: [[Char]] -> [Char]

I want to change the declaration to wordsToString::[(String)]->[String] but it won't work

Answer 1

I want to change the declaration to wordsToString::[(String)]->[String] but it won't work

No, you want to change the declaration to wordsToString :: [String] -> String . You aren't getting a list of strings out, just a single one.

Answer 2

The function is called concat :

concat :: Foldable t => t [a] -> [a]
concat xs = foldr (++) [] xs

In your case, you want to insert a whitespace between the characters. This function is called intercalate :

intercalate :: [a] -> [[a]] -> [a]

It's defined in terms of intersperse .