Is there a better way to do this?
This problem is similar to adding numbers to each tree item
Marc
Yes. The feature you wanted is "replicateM".
The point of a State monad is you probably never have to touch the State data
constructor:
module Main where
import Control.Monad.State
import Data.Tree
type Supply = State Int
unique :: Supply Int
unique = do
value <- get
put (succ value)
return value
createTree :: Int -> Supply (Tree Int)
createTree 4 = do
me <- unique
return (Node me [])
createTree level = do
me <- unique
children <- replicateM level (createTree (succ level))
return (Node me children)
main = do
putStrLn $ drawTree $ fmap show $ evalState (createTree 2) 0
Or even more tersely:
> import Control.Monad
>
createTree :: Int -> Supply (Tree Int)
createTree 4 = liftM (`Node` []) unique
createTree level = liftM2 Node unique (replicateM level (createTree (succ
level)))
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