Am 01.04.2011 16:28, schrieb Manfred Lotz:

Hi there, I have a problem where I like to update a record with an IO a. Not quite sure how to describe it.

Here is a minimal example: I get a list of numbers from the command line and I like to add those numbers n and randome numbers from a range from [0..n] in two fields of a record.

<---------------------------snip----------------------------> module Main where

import System.Environment.UTF8 import System.Random

data NumRec = NumRec { mxV :: Int, mxR :: Int } deriving (Show,Read)

initNumRec = NumRec { mxV = 0, mxR = 0 }

toInt s = read s :: Int

think about the type of addRandom!

addRandom m n = do let mxv = mxV m let mxr = mxR m

change:

let r = rand n m { mxV = mxv + n, mxR = mxr + r }

to: r <- rand n return m {....}

rand :: Int -> IO Int rand max = getStdRandom (randomR (0, max))

main = do args<- getArgs print args let ilist = map toInt args let mixed = foldl addRandom initNumRec ilist

use Control.Monad.foldM. I'm not sure if the following will work: mixed <- foldM addRandom initNumRec ilist Cheers Christian

print mixed <---------------------------snap---------------------------->

I get the following error when compiling:

[1 of 1] Compiling Main ( minimal.hs, minimal.o )

minimal.hs:22:23: Couldn't match expected type `Int' with actual type `IO Int' In the second argument of `(+)', namely `r' In the `mxR' field of a record In the expression: m {mxV = mxv + n, mxR = mxr + r}

How can I correct the compile error?