[Haskell-cafe] Printing a random list

Don Stewart dons at galois.com
Sun Jun 8 19:33:11 EDT 2008

> I'm just starting out with Haskell, and I could use some help.  I'm  
> trying to create a random list and print it out, which seems simple  
> enough, but has been giving me problems.  Here's what I have:
> module Main
>     where
>       import IO
>       import Random
>       randomList :: Random a => a -> a-> [IO a]
>       randomList lbound ubound = randomRIO(lbound, ubound) :  
> randomList lbound ubound
>       main = do
>         myRandomList <- sequence(randomList(0::Int 255))
>         putStrLn(show(take(10 myRandomList)))
> -----
> So, I have tried to make a randomList action which defines an infinite  
> random list, bounded by lbound and ubound.  It seems that to print  
> this, I need to convert between randomList, which is of type [IO a] to  
> something like IO [a], which is what sequence should do for me.  Then  
> I just want to print out the first 10 elements.
> I'm currently getting the error "Only unit numeric type pattern is  
> valid", pointing to 0::Int 255 in the code.  I'm not sure what this  
> means.

Missing parenthesis around the (0 :: Int) type annotation.

> I'm sure I'm looking at this the wrong way, since I'm new to Haskell  
> and haven't quite wrapped my head around it yet.  Maybe you can fix  
> the problem by showing me a more Haskell approach to creating a random  
> list and printing it...  =)

For lists, best to use the randomRs function,

    import System.Random

    main = do
        g <- newStdGen
        print (take 10 (randomRs (0,255) g :: [Int]))

Running it:

    $ runhaskell A.hs


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