A comparison of LogicT and Stream (from Olegs FBackTrack.hs) Stream has better termination criteria!

logict_vs_fbacktrack.hs

{-
To install dependencies:

cabal install logict
cabal install stream-monad

-}

import Control.Monad.Logic
import Control.Monad.Stream


choose :: (MonadPlus m) => [a] -> m a 
choose [] = mzero
choose (x:xs) = return x `mplus` choose xs

odds :: (MonadPlus m) => m Int
odds = choose [1,3..]

two :: (Monad m) => m Int
two = return 2

filterEvens :: (MonadPlus m) => m Int -> m Int
filterEvens m = do
  x <- m
  if even x then return x else mzero


test1 :: MonadLogic m => m Int
test1 = filterEvens (odds `interleave` two)

test2 :: MonadLogic m => m Int
test2 = filterEvens odds `interleave` filterEvens two

-- Spits out a 2, and then loops
test1_LogicT :: [Int]
test1_LogicT = observeAll $ test1

-- loops
test2_LogicT :: [Int]
test2_LogicT = observeAll $ test2

-- spits out a 2 and then loops
test2_Stream :: [Int]
test2_Stream  = runStream $ test2