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copied!<p>What you want to do is to keep track of the value of variables during <em>evaluation</em> of expressions, not during parsing. Let's assume you parse your expressions into the following types:</p> <pre><code>data Expr = Literal Int | Variable Var | Assign Var Expr | Add Expr Expr | ... newtype Var = Var String deriving (Ord, Eq, Show) </code></pre> <p>Then you could simply pass a Map around your evaluation function with the current value of all variables:</p> <pre><code>import qualified Data.Map as M import Control.Monad.State data Expr = Literal Int | Variable Var | Assign Var Expr | Add Expr Expr newtype Var = Var String deriving (Ord, Eq, Show) -- Each Expr corresponds to a single line in your language, so -- a = 2+1 -- a + 2 -- corresponds to -- [Assign (Var "a") (Add (Literal 2) (Literal 1)), -- Add (Variable (Var "a")) (Literal 2)] eval :: [Expr] -> Int eval es = last $ evalState (mapM eval' es) M.empty -- M.empty :: M.Map Var Int where eval' (Literal n) = return n eval' (Variable v) = do vs <- get case M.lookup v vs of Just x -> return x _ -> error $ "variable " ++ show v ++ " is undefined!" eval' (Assign v ex) = do x <- eval' ex modify (M.insert v x) return x eval' (Add a b) = do x <- eval' a y <- eval' b return (x+y) </code></pre> <p>Of course, there's nothing to prevent you from evaluating expressions as you parse them, eliminating the need for an abstract syntax tree such as this. The general idea is the same there; you'll need to keep some state with you during the entire parsing, that keeps track of the current value of all your variables.</p>

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