Question

I have a Haskell function that returns a monad, declared as follows:

data Options = Options {
    optGames :: Int,
    optSuits :: Int,
    optVerbose :: Bool
  } deriving Show

playGame :: Options -> StateT StdGen (WriterT [String] IO)) Bool

This function plays a single game of solitaire, then returns a boolean indicating a win or loss, along with a log in the WriterT monad.

I would like to call this function a set number of times, each time using the “next” value of the random generator (StdGen), and concatenating the Bool return values into a list.

I tried creating a recursive function to do the calls, but can’t figure out how to pass the monad into each next iteration.

I would like to emulate

initial state >>= playGame >>= playGame ... -- repeat N times

and collect all of the resulting Bool values, as well as the log entries from the WriterT monad.

What is the best way to do this?

Answer 1

I think you’re looking for replicateM. This repeats the given action a specified number of times, returning the result as a list. So replicateM n playGame corresponds to playing the game n times and getting a list of the results back.