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
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