CODEWITHSUNDEEP
haskell
Sean
Sean

Reputation: 29772

How would I create an IO Bool that returns True for a while, then False thereafter?

Today I was trying to deepen my understanding of the IO monad, and I tried to write a function that take an Int n and returns an IO Bool than produces True n times, then False forever afterwards.

trueThenFalse :: Int -> IO Bool

Normally I'd address something like this with recursion, but here there seems to be nothing to recurse into.

How would I go about implementing this function? Is it possible and/or advisable to do so?

Upvotes: 2

Views: 555

Answers (1)

Louis Wasserman
Louis Wasserman

Reputation: 198103

It might make more sense as an IO (IO Bool), because you have to set up some state first:

trueThenFalse n = do
  holder <- newMVar n
  return (modifyMVar holder (\ k -> return (k - 1, k > 0)))

That creates a mutable holder to store the count of how many Trues are "left," and returns another IO operation which can be run multiple times, modifying the count and returning True if the count was > 0.

Upvotes: 6

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