Doing IO in foldr repeats lines?

Scott Turner
Sat, 20 Jan 2001 17:08:48 -0500

At 21:17 2001-01-20 +0000, Ian Lynagh wrote:
> main = do _ <- foldl foo (return 14) ["qq\n", "ww\n", "ee\n"]
>           putStr ""                           
> foo :: IO Int -> String -> IO Int             
> foo io_l s = do l <- io_l                     
>                 () <- putStr s                
>                 io_l                          

It repeats lines
>and I really don't understand why. Is the code re-evaluated every time
>foldl is expanded or something?

Since the result type of foo is IO Int, it is building an IO action, in
other words it is calculating a sequence of effects.  The combined effects
are performed by main.

Repetition occurs due to the structure of foo.  Notice that its action
consists of first performing io_l, then putting a string, and then
performing io_l _again_.  If io_l prints the lines qq, ww, and qq, then the
result will involve two such triplets.

Try this.  Remove either of the occurrences of io_l from the do block, and
you will see quite different results.

Scott Turner