Re: [Haskell-cafe] Combining State and List Monads
Ah, egg in my face, I appear to have misremembered how ListT is implemented ^_^ http://www.haskell.org/haskellwiki/ListT_done_right may be relevant. Edward Excerpts from Edward Z. Yang's message of Sat Aug 25 01:51:40 -0400 2012: Hello Henry, In such cases, it is often worth thinking about how you would implement such a scheme manually, without using pre-existing monads. You will quickly see that the two candidate types: s - ([a], s) [s - (a, s)] both will not work (exercise: what semantics do they give?) In fact, you must use continuation passing style, and you must resume the computation with the latest state value you would extracted from the last run. See the LogicT monad for how to implement list-like monads in continuation passing style. Cheers, Edward Excerpts from Henry Laxen's message of Sat Aug 25 00:35:37 -0400 2012: Dear Cafe, It seems to me there should be some simple way of doing this, but thus far it eludes me. I am trying to combine the State and List monads to do the following: countCalls = do a - [1..2] b - [1..2] modify (+1) return (a,b) where with some combination of ListT, StateT, List, State, or who knows what would result in: ([(1,1),(1,2),(2,1),(2,2)],4) assuming we initialize the state to 0 Is there any way to make this happen? Thanks in advance. Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Combining State and List Monads
Dear Cafe, It seems to me there should be some simple way of doing this, but thus far it eludes me. I am trying to combine the State and List monads to do the following: countCalls = do a - [1..2] b - [1..2] modify (+1) return (a,b) where with some combination of ListT, StateT, List, State, or who knows what would result in: ([(1,1),(1,2),(2,1),(2,2)],4) assuming we initialize the state to 0 Is there any way to make this happen? Thanks in advance. Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combining State and List Monads
Hello, the state should be inside, to count, so type is ListT (State Int) (Int,Int) the runner is then runState (runListT countCalls) 0 but [] is not of type ListT m so you need to wrap it in ListT . return import Control.Monad.List import Control.Monad.State import Control.Monad.Instances countCalls :: ListT (State Int) (Int,Int) countCalls = do a - ListT . return $ [1..2] b - ListT . return $ [1..2] modify (+1) return (a,b) regards paolino 2012/8/25 Henry Laxen nadine.and.he...@pobox.com Dear Cafe, It seems to me there should be some simple way of doing this, but thus far it eludes me. I am trying to combine the State and List monads to do the following: countCalls = do a - [1..2] b - [1..2] modify (+1) return (a,b) where with some combination of ListT, StateT, List, State, or who knows what would result in: ([(1,1),(1,2),(2,1),(2,2)],4) assuming we initialize the state to 0 Is there any way to make this happen? Thanks in advance. Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combining State and List Monads
Hello Henry, In such cases, it is often worth thinking about how you would implement such a scheme manually, without using pre-existing monads. You will quickly see that the two candidate types: s - ([a], s) [s - (a, s)] both will not work (exercise: what semantics do they give?) In fact, you must use continuation passing style, and you must resume the computation with the latest state value you would extracted from the last run. See the LogicT monad for how to implement list-like monads in continuation passing style. Cheers, Edward Excerpts from Henry Laxen's message of Sat Aug 25 00:35:37 -0400 2012: Dear Cafe, It seems to me there should be some simple way of doing this, but thus far it eludes me. I am trying to combine the State and List monads to do the following: countCalls = do a - [1..2] b - [1..2] modify (+1) return (a,b) where with some combination of ListT, StateT, List, State, or who knows what would result in: ([(1,1),(1,2),(2,1),(2,2)],4) assuming we initialize the state to 0 Is there any way to make this happen? Thanks in advance. Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
