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POReturning a value with a state monad
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2012-11-22T15:58:15.853
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I'm new to Monads and Haskell in general and trying to understand how to return a value when using them. My code looks something like the following: <pre><code>foo :: A -> B foo a = do b <- fooC a (C 0) -- want to return just (B "b") fooC :: A -> C -> State MyState B fooC a c = return (B "b") </code></pre> I tried using <code>snd (snd b)</code>, but apparently <code>State MyState B</code> is not a tuple? How can I return the desired value <code>(B "b")</code>? Edit: Taking Daniel's advice into account, the rewrite looks like this: <pre><code>data MyState = MyState String data C = C Int foo :: String -> String -- want to return just "b" foo a = evalState (fooC a) (C 0) fooC :: String -> Int -> State MyState String fooC a c = return "b" </code></pre> That still results in a compilation error: <pre><code>Couldn't match expected type `State s0 String' with actual type `Int -> State MyState String' In the return type of a call of `fooC' Probable cause: `fooC' is applied to too few arguments In the first argument of `evalState', namely `(fooC a)' In the expression: evalState (fooC a) (C 0) </code></pre> Edit 2: Fixed! Final version looks as follows: <pre><code>import Control.Monad.State data MyState = MyState String data C = C Int foo :: String -> String -- want to return just (B "b") foo a = evalState (fooC a (C 0)) (MyState "whatever") fooC :: String -> C -> State MyState String fooC a c = return "b" main = print(foo("test")) -- prints "b" </code></pre>
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<haskell><monads><state-monad>
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Returning a value with a state monad
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1. USWesley Tansey
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