如何实现计算单词字母数量的函数？基于给定的stringLength函数签名

Got it, let's walk through how to build this function that counts the number of characters in a string. There are a couple of ways to do this, depending on whether you want a concise, idiomatic solution or want to dig into the recursive underpinning.

The Idiomatic Haskell Approach #

Since in Haskell, a String is just a type synonym for [Char] (a list of characters), we can leverage the built-in length function directly. It's designed to work on any list, so it fits perfectly here:

stringLength :: String -> Int
stringLength = length

That's it! This is the most Haskell-y way to do it—no need to reinvent the wheel when the standard library has exactly what we need.

Manual Recursive Implementation (For Learning) #

If you want to understand how the length calculation works under the hood, you can write a recursive version using pattern matching:

stringLength :: String -> Int
-- Base case: empty string has 0 characters
stringLength [] = 0
-- Recursive case: take the first character (we don't care about its value, so use _),
-- add 1 to the length of the remaining string
stringLength (_:xs) = 1 + stringLength xs

Let's break this down:

• When we pass an empty string ([]), we return 0—this stops the recursion.
• For any non-empty string, we split it into the first character (_) and the rest of the string (xs). We add 1 (for that first character) to the result of calling stringLength on the remaining characters. This repeats until we hit the empty string base case.

Testing the Function #

You can test it out with some examples to make sure it works:

main :: IO ()
main = do
  print $ stringLength "hello"    -- Output: 5
  print $ stringLength ""         -- Output: 0
  print $ stringLength "Haskell!" -- Output: 8

内容的提问来源于stack exchange，提问作者Eric Zhang