[Haskell-cafe] Printing a random list

[Bryan Catanzaro]

Thanks for the response, it does compile after I juggled some  
parentheses around.  And also I appreciate the pointer to the better  
way of making a random list.  So that problem is solved.

However, when I ran my random list generator, the interpreter had a  
stack overflow.  Here's my code again:
---
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))
---

It seems that this code somehow tries to evaluate every element of the  
infinite list defined by randomList.  Can you tell me why it is not  
lazily evaluating this list?  I can get around this by changing main  
to do this instead:

---
       main = do
         myRandomList <- sequence(take 10 (randomList (0::Int) 255))
         putStrLn(show(myRandomList))
---

But I don't understand why sequence(randomList (0::Int) 255) actually  
tries to evaluate the entire infinite list, instead of just lazily  
defining a list with the proper types, that I evaluate later when I  
take elements from it.

Thanks for your help!

- bryan

On Jun 8, 2008, at 4:33 PM, Don Stewart wrote:

> catanzar:
>> 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
>    [11,90,187,119,240,57,241,52,143,86]
>
> Cheers,
>  Don