[Haskell-cafe] Functions with side-effects?

Daniel Carrera daniel.carrera at zmsl.com
Wed Dec 21 08:35:20 EST 2005

Thanks for all the help. I think things are much clearer now. And this bit:

> main = do putStrLn "Hello, what is your name?"
>           name <- getLine
>           putStrLn ("Hello, " ++ name ++ "!")

Looks quite straight forward.

I just wrote my very first IO program with Haskell:
main = do
         x <- getLine
         putStrLn( show(length x) )

That's not so scary. The next thing I need to figure out is how to act 
on the input data itself (not just its length). For example, if I wanted 
a program to output the nth element of the fibonacci sequence:
$ ./fib 12

My first inclination would be to write it as:
main = do
         x <- getLine
         putStrLn( show(fib x) )

Of course that won't work because x is an IO String and 'fib' wants an 
Int. To it looks like I need to do a conversion "IO a -> b" but from 
what Cale said, that's not possible because it would defeat the 
referential transparency of Haskell.

Hmm... there must be a way to solve this problem though...

In any event, thanks for the help. I'm learning :)

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