On 6/20/06, Taro Ikai
Here's a code fragment I have from working with Hal Daume's "Yet Another Haskell Tutorial". I cannot figure out how to iterate over the result from askForNumbers that returns "IO [Integer]", and print them.
The "<-" operator seems to take IO off of the results from actions like getContents, which returns type "IO String", but I cannot seem to use it to take IO off of lists with it. What am I supposed to do?
Also, as a side question, what should I be returning from the do function?
code>>>>
module Main where
import IO
main = do hSetBuffering stdin LineBuffering s <- askForNumbers {-- This compiles --} map show s {-- But this doesn't. Why? --}
s is a list of numbers, "map show s" is a list of Strings. It is not an IO [Strings] so you can't use it like an action. Think about what you're trying to do here, you map "show" on a list of ints, but what do you do with the result? Nothing. Why would you *want* the above to compile? It would just convert a bunch of numbers into strings and then forget about them! So what you want is probably something like: let xs = map show s The rule of thumb is, "use 'let' for regular values, and (<-) for actions". If you want to print all of them you could do something like: printList [] = return () printList (x:xs) = do putStrLn x printList xs On the other hand, this operation seems generally useful, so why not generalise it a bit? mapAction [] = return () mapAction a (x:xs) = do a x mapAction a xs And in fact, this function already exists! It's called mapM_ (the M is for Monad). You could use this by passing in "putStrLn" as the action to this function. /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862