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darkf/interp.hs

Last active Dec 25, 2015
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Barebones interpreter in Haskell showing `State s a`
import Control.Monad.State (State, runState, evalState, get, put)
import qualified Data.Map as M
data AST = Add AST AST
| Def String AST
| Var String
| StrConst String
| IntConst Int
deriving (Show, Eq)
data Value = IntV Int
| StrV String
deriving (Show, Eq)
type Env = M.Map String Value -- an environment
type InterpState = State Env -- interpreter state (pass along the global environment)
(IntV l) +$ (IntV r) = IntV (l + r)
(StrV l) +$ (StrV r) = StrV (l ++ r)
l +$ r = error $ "cannot + " ++ show l ++ " and " ++ show r
eval :: AST -> InterpState Value
eval (IntConst i) = return $ IntV i
eval (StrConst s) = return $ StrV s
eval (Var var) = get >>= \m ->
case M.lookup var m of
Just v -> return v
Nothing -> error $ "unbound variable " ++ var
eval (Def name v') = do
v <- eval v'
m <- get
put $ M.insert name v m
return v
eval (Add l r) = do
l <- eval l
r <- eval r
return $ l +$ r
evalProgram :: [AST] -> Value -- fold the state from each node and return the result
evalProgram nodes = evalState (foldr1 (>>) $ map eval nodes) M.empty
main = do
print $ evalProgram prg
print $ evalProgram prg2
where
prg = [Def "x" (IntConst 5),
Def "y" (IntConst 3),
Add (Var "x") (Var "y")]
prg2 = [Add (StrConst "hi ") (StrConst "there")]
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