53 lines
1.3 KiB
Haskell
53 lines
1.3 KiB
Haskell
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import Control.Monad.State (State, runState, evalState, get, put)
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import qualified Data.Map as M
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data AST = Add AST AST
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| Def String AST
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| Var String
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| StrConst String
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| IntConst Int
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deriving (Show, Eq)
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data Value = IntV Int
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| StrV String
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deriving (Show, Eq)
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type Env = M.Map String Value -- an environment
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type InterpState = State Env -- interpreter state (pass along the global environment)
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(IntV l) +$ (IntV r) = IntV (l + r)
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(StrV l) +$ (StrV r) = StrV (l ++ r)
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l +$ r = error $ "cannot + " ++ show l ++ " and " ++ show r
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eval :: AST -> InterpState Value
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eval (IntConst i) = return $ IntV i
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eval (StrConst s) = return $ StrV s
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eval (Var var) = get >>= \m ->
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case M.lookup var m of
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Just v -> return v
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Nothing -> error $ "unbound variable " ++ var
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eval (Def name v') = do
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v <- eval v'
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m <- get
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put $ M.insert name v m
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return v
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eval (Add l r) = do
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l <- eval l
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r <- eval r
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return $ l +$ r
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evalProgram :: [AST] -> Value -- fold the state from each node and return the result
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evalProgram nodes = evalState (foldr1 (>>) $ map eval nodes) M.empty
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main = do
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print $ evalProgram prg
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print $ evalProgram prg2
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where
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prg = [Def "x" (IntConst 5),
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Def "y" (IntConst 3),
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Add (Var "x") (Var "y")]
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prg2 = [Add (StrConst "hi ") (StrConst "there")]
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