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?