An EXPERT PROGRAMMER explains OO

Because I know you were all having so much trouble with this difficult concept.

http://theunixgeek.blogspot.com/2007/11/understanding-object-oriented.html

Prelude> mapM (putStrLn . show) [1..4]
1
2
3
4
[(),(),(),()]
Uhm, where did that list of empty tuples come from?

>>32
putStrLn :: String -> IO ()

>>33
So I map it along a list of one-element lists and one empty list. That's not a reason to show the last nil as empty tuples.

>>32
Nomads.  Haskell nomads.

>>34
?
The 1, 2, 3, 4 are printed as a result of calling putStrLn on each element. The [(),(),(),()] is the resulting list from your mapM call.

>>36
The [(),(),(),()] is the resulting list from your mapM call.
Huh?

The show part actually transforms the list to a list of strings and a nil at the end, right? Why would it return [(),(),(),()] ?

>>37
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>>37
Haskell. Haskell Nomads.

>>38
Konachan kawaii

>>37
No, the putStrLn . show part transforms the list of numbers to a list of ()'s, printing a bunch of strings as a side effect. The return value is [(),(),(),()]. It's printed because ghci assumes that any non-() value is potentially interesting.
If that's not what you want, make sure that the return value is (), e.g. by using mapM (putStrLn . show) [1..4] >> return (). However, an EXPERT HASKELL PROGRAMMER would use library functions to his advantage and simply write mapM_ print [1..4] or even putStr . unlines . map show $ [1..4].

>>41
I am a EXPERT HASKELL PROGRAMMER and agree whole-heartedly.