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POPropogation of State Monad
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5385363
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2011-03-22T00:31:02.150
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2011-03-22T01:46:04.777
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I have the following function for walking around "edges" of the "graph" of my game world. It alters the state of the world--specifically, the player's location. I need to report a message alerting the player of their change in location as well. So I could either return a tuple of (message, newWorld), or I could use a State monad. (Right? I'm new to this stuff.) Here's my attempt at the monad approach: <pre><code>walk dir = do world <- get let attempt = filter (\e -> edgeDirection e == dir) $ edges edges = (worldEdges world) M.! playerLoc playerLoc = playerLocation $ worldPlayer world case attempt of [] -> return "You can't go that way." (edge : _) -> do put world' return message where world' = world { worldPlayer = player' } player' = (worldPlayer world) { playerLocation = loc } loc = edgeLocation edge message = "You go " ++ (downcase $ show dir) </code></pre> The thing is, now the function that calls <code>walk</code> has a State monad to deal with. Do I <code>runState</code> it right there, or do I have that function use State monads too, like this: <pre><code>parseWalk dir = let direction = case dir of ('e' : _) -> Just East ('n' : _) -> Just North ('s' : _) -> Just South ('w' : _) -> Just West ('u' : _) -> Just Up ('d' : _) -> Just Down _ -> Nothing in case direction of Just d -> walk d Nothing -> return "You can't go that way" </code></pre> Then what about <code>parseWalk</code>'s caller, and so forth? Should I try to keep this State monad passed around as long as possible? Should I be seeing a lot of <code>MonadState ...</code> in my type signatures? Sincerely, Confused in California
Tags
<design><haskell><state><monads>
Title
Propogation of State Monad
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1. COSince you're walking and modifiying a large data structure, you may want to look at using a Zipper.
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