
Bryan Catanzaro wrote:
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.
You are correct.
Can you tell me why it is not lazily evaluating this list?
Whenever you use IO, there is a baton being passed along behind the scenes. The baton is called "RealWorld#" and it represents the fact that interactions with global state and the outside world have to be serialized. In particular, whenever you use the global random number generator, a global state variable has to be updated. This has to be serialized, thus the baton has to be passed along from one action to the next. When you "sequence" a list of IO actions, you are effectively sending the baton along that list, and you don't get it back until the end of the list is reached. Your code is sending the baton into an infinite list of actions, never to be returned.
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)) ---
Now you are sending the baton into a list of only 10 actions. The baton comes back, and the program goes on. If you don't know in advance how many random numbers you need, and if you are satisfied with the global random number generator, then Don Stewart's solution is a better approach. Don Stewart wrote:
main = do g <- newStdGen print (take 10 (randomRs (0,255) g :: [Int]))
If you want to be able to reproduce the same sequence of random numbers, for example for testing and debugging purposes, then you can use mkStdGen to create your own random number generator, independent of the global one. The catch is that you will have to thread the state of the random number generator through your code. Once you learn about monads (if you haven't already), you'll recognize that you can use the State monad for your random number generator.