在Haskell中获取列表中单词的长度

Question

I started to write a couple functions where I could count specific words in a sentence and so on, but now I need to count the lengths of the words in a list. So I have a list where each element is a word and want to count the characters of this word. I couldn't find any solution to do that, since Haskell doesn't use loops. Is there anyway to do this?

Answer 1

If you think you need a loop when working in Haskell, you're probably looking for recursion. We'll want to recurse through the list of strings, applying the length function to each one. The general pattern of recursion through a list where a function is applied to each value (which you'll become very familiar with as you work through Haskell) looks roughly like this:

func f []     = []
func f (x:xs) = f x : func f xs

So we could solve the problem that way (specializing the function, rather than accepting it as an argument):

wordLengths :: [String] -> [Int]
wordLengths []     = []
wordLengths (x:xs) = length x : wordLengths xs

However, since this behavior is so common, there exists a function map that takes care of all of this for us. Its type signature is (a -> b) -> [a] -> [b] , so we know to pass it a function ( length ) and a list of things (our String s). If we map the length function to each value in your list of strings, we'll get your desired result:

wordLengths :: [String] -> [Int]
wordLengths = map length

Example usage:

ghci>> wordLengths ["abc","defg"]
[3,4]