Question

I have seen the solutions using iterate or replicate in order to apply a function n times. However, I don’t manage to use it inside a State monad.

This code works:

-- Stuff an empty game level with obstacles.
generateLevel :: Level -> State StdGen Level
generateLevel lvl =
    placeRandomWall lvl >>= placeRandomWall >>= placeRandomWall

This one works as well, unsurprisingly:

generateLevel :: Level -> State StdGen Level
generateLevel lvl =
    placeRandomWall =<< placeRandomWall =<< placeRandomWall lvl

However, this is not the same as:

generateLevel :: Level -> State StdGen Level
generateLevel lvl =
    (placeRandomWall =<< placeRandomWall =<< placeRandomWall) lvl

The latest complains about types. Therefore, I can’t foldl (=<<) id (relicate 42 placeRandomWall), nor can I iterate.

This makes sense, as iterate take a a -> a function, while what I have is a -> m a or something like that. So, I don’t really know how to go from there.

Answer 1

I think you’re looking for <=< and >=> from Control.Monad. They can be folded across a list you’ve made with replicate to create one big action.

Try foldr (<=<) return (replicate 42 placeRandomWall).