From Simple IO to Monad Transformers by J Adrian Zimmer

Author:J Adrian Zimmer
Language: eng
Format: epub
Publisher: J Adrian Zimmer

Here is a program you can compile and execute to test this. In it I have extracted the interior object from pairs getLine getLine, converted it to a string and printed it.

pairs :: Monad m => m t -> m u -> m (t,u)

pairs lst1 lst2 = do

x <- lst1

y <- lst2

return (x,y)

main = pairs getLine getLine >>= (\z -> putStrLn (show z))

This program, of course, silently asks for two lines of input. As a further exercise, you might want to change it so that it requests input with a "Input? " prefix. Then execution would look like

Input? line 1

Input? line 2

("line 1","line 2")

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