This sounds similar to ChoiceT from the monadLib package. Did you know ChoiceT? greetings, Sjoerd On Jul 15, 2009, at 3:33 PM, Yair Chuchem wrote:
A new "generator" package has been uploaded to Hackage.
It implements an alternative list monad transformer, a list class, and related functions.
The difference from mtl/transformers's ListT is that mtl is a monadic action that returns a list: newtype ListT m a = ListT { runListT :: m [a] } generator's is a monadic list: data ListItem l a = Nil | Cons { headL :: a, tailL :: l a } newtype ListT m a = ListT { runListT :: m (ListItem (ListT m) a) } A short example program which reads numbers from the user and interactively sums them up: import Control.Monad.ListT (ListT) import Data.List.Class (execute, joinM, repeat, scanl, takeWhile) import Prelude hiding (repeat, scanl, takeWhile)
main = execute . joinM . fmap print . scanl (+) 0 . fmap (fst . head) . takeWhile (not . null) . fmap reads . joinM $ (repeat getLine :: ListT IO (IO String)) I also wrote an example/blog-post about using ListT to add an undo option to the classic game of "hamurabi": http://mashebali.blogspot.com/2009/07/charlemagne-disraeli-and-jefferson.htm...
Another interesting observation is that "ListT [] a" is a tree of "a"s. The module Data.List.Tree includes functions to prune and search such trees (dfs, bfs, bestFirstSearchOn, etc). This can be useful for modularizing code that uses the list monad for combinatoric search by decoupling tree creation from processing and pruning.
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-- Sjoerd Visscher sjoerd@w3future.com