Haskell函数与“拆分”案例

Question

How can I make a "split"ting case inside an function like this:

getBase64 :: String -> [Char]
getBase64 code
  | ("[&":c:r) = c
  | _          = ' '

This is so not working cause:

main.hs:7:11: Not in scope: ‘c’

main.hs:7:13: Not in scope: ‘r’

main.hs:7:18: Not in scope: ‘c’

The error is clear to me, but I'm a newcommer and don't know how I can do my "wish" ...

Thanks for help!

Answer 1

You are trying to match a pattern, but since you are using the | cond = expr | cond = expr notation, you are actually using a guard. Visit this page for more details about guards: https://en.wikibooks.org/wiki/Haskell/Control_structures#if_and_guards_revisited . | ("[&":c:r) | ("[&":c:r) actually means "a list constructed from the string "[&" and value c and value r ", and Haskell compiler complains the variables c and r do not exist.

Here is how to define functions with multiple cases:

getBase64 ("[&":c:r) = c
getBase64 _ = ' '

However, this won't work because "[&" is a string, and Haskell thinks the first parameter is a list of strings. A string is a list of characters and you need to fix your pattern like this:

getBase64 ('[':'&':c:r) = c
getBase64 _ = ' '

Look at these examples for more details about pattern matching against characters in a string: https://stackoverflow.com/a/1602296/303939 and https://stackoverflow.com/a/3851126/303939

Then there is another problem: getBase64 returns [Char] . If that's what you intended, your code should be fixed like this:

getBase64 ('[':'&':c:r) = [c]
getBase64 _ = " "     -- or [' ']