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POCreation/Handling of lists of Monad.State's?
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copied!<p>I'm just in the process of learning haskell and I am sure that an elegant solution to the following problem exists. Given a function f that returns a stateful computation</p> <pre><code>f :: (Num a) => a -> a -> State [a] a f x y = modify ((2*x):) >> return (x+y) -- state and value are modified based on the passed values </code></pre> <p>I am looking for the cleanest way to generate multiple <code>State</code>'s from a given one recursively (as shown below). One solution I came up with is the following</p> <pre><code>g :: (Num a) => [a] -> (a, [a]) -> [(a,[a])] g [] _ = [] g (x:xs) aa@(a,b) = (new : next) where new = runState (f x a) b -- here the old value a is required! next = g xs aa </code></pre> <p>but I guess something like </p> <pre><code>g :: [a] -> [State [a] a] </code></pre> <p>should be possible and cleaner? I didn't succeed trying this and get errors from StateT that I can't figure out.</p> <p>Thank you!</p> <p><em>Background:</em> The code is a simplification of parts of a graph generator I am writing, where the state is the current adjacency vector. For each node, several edges may be created and therefore multiple state is required representing the different (partial) graphs.</p> <p><em>Edit</em>: (Try of) A description in words of the above function <code>g x (y, s)</code> in words:</p> <p>For a given list of values <code>x</code>, a single value <code>y</code> and a state <code>s</code>, compute recursively for each <code>x_i</code> in <code>x</code> a new value and state from <code>(y,s)</code> given by the stateful computation <code>f x_i y</code> and return the result as a list. </p> <p><em>Edit 2</em>: Example output:</p> <pre><code> g [1,2,3] (4,[2,3,4]) == [(5,[10,2,3,4]),(6,[12,2,3,4]),(7,[14,2,3,4])] </code></pre>

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