Haskell: Printing out the contents of a list of tuples

Question

Basically what I need to do is write a function that takes in a list of type [(String, String)] and prints out the contents so that, line-by-line, the output looks like this:

FirstString : SecondString

FirstString : SecondString

..etc, for every item in the list. I've got the following code and it prints it out, but for some reason it prints out a line containing [(),()] at the end.

display :: Table -> IO ()
display zs = do { 
    xs <- sequence [putStrLn ( a ++ " = " ++ b) | (a, b) <- zs];
    print xs 
}

Is there anything I'm doing wrong?

Answer 1

The final print xs is unnecessary. sequence here is returning a bunch of ()s (the return value of putStrLn), and print is printing that out as well.

While you're at it, now that print xs is gone, you can get rid of the xs variable binding, and make sequence into sequence_ to throw away the return value, giving:

display :: Table -> IO()
display zs = sequence_ [putStrLn (a++" = "++b) | (a,b) <- zs]