Haskell：如何打印用逗号分隔的列表的每个元素

Question

I am trying to print list with comma. I have list like ["1","2","3"] and I want to print 1,2,3 How can I do that?

I tried:

printList xs = mapM_ (\(a) -> do
                      putStr a
                      putStr (",")) xs

But I dont know how to remove the last comma.

Answer 1

You can use intercalate . It'll insert the comma between each element of the list and concatenate the resulting list of strings to turn it into a single string.

import Data.List

toCommaSeparatedString :: [String] -> String
toCommaSeparatedString = intercalate ","

ghci> toCommaSeparatedString ["1","2","3"]
"1,2,3"

Answer 2

This is a bit of an XY problem : as Benjamin Hodgson shows, you're better off turning your list into a string, and then printing that – you want as much of your logic outside of the IO monad as possible.

But of course, even if your question is somewhat in the wrong direction from the start, it has an answer! Which is that, for example, you could write this:

printList :: [String] -> IO ()
printList [] = return ()
printList [x] = putStr x
printList (x:xs) = do
  putStr x
  putStr ","
  printList xs

Benjamin's answer is better . But this one might elucidate IO monad code and do -notation a bit more.