[Haskell-beginners] record problem when updating fields with an IO a type

[Christian Maeder]

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?
>
>
>