Question

Prelude> mapM putStrLn ["a", "b"]
a
b
[(),()]
Prelude> mapM_ putStrLn ["a", "b"]
a
b

Why first version shows third line and second does not and where does third line comes from. I would not expect it.

Answer 1

If you put the mapM version in a standalone program, compile it with ghc, and run it, you don’t get a third line from it, either:

$ cat demo.hs
main = mapM putStrLn [ "a", "b" ]
$ ghc demo.hs
$ ./demo
a
b
$

That [(),()] you see in ghci is just the return value of the mapM call; ghci automatically displays the value of every expression you enter. (This is why ghci is called a Read-Evaluate-Print Loop, or REPL; the “Print” part is what you’re seeing here.)

While mapM creates a list containing the return value of every putStrLn call (so you get one () for each element in the list), mapM_ discards those return values and returns IO (), which ghci doesn’t bother to display. So you don’t see an extra line from ghci in that case.