101 lines
2.6 KiB
Haskell
101 lines
2.6 KiB
Haskell
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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| Lambda [(Pattern, [AST])]
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| Call String [AST]
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| StrConst String
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| IntConst Int
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deriving (Show, Eq)
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data Pattern = VarP String
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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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| FnV [(Pattern, [AST])] -- pattern->body bindings
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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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initialState = M.fromList [("id", FnV [(VarP "x", [Var "x"])])]
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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 (Lambda pats) =
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return $ FnV pats
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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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eval (Call name args) = get >>= \m ->
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case M.lookup name m of
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Just fn@(FnV _) ->
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do
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xargs <- mapM eval args
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applyMany fn xargs
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Nothing -> error $ "call: name " ++ name ++ " doesn't exist or is not a function"
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patternBindings :: Pattern -> Value -> Maybe Env
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patternBindings (VarP n) v = Just $ M.fromList [(n, v)]
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-- applies many arguments to a function
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applyMany :: Value -> [Value] -> InterpState Value
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applyMany fn@(FnV _) (arg:xs) =
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apply fn arg >>= \value ->
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applyMany value xs
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applyMany value [] = return value
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applyMany _ xs = error "couldn't apply all arguments"
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-- applies a function
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apply :: Value -> Value -> InterpState Value
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apply (FnV pats) arg =
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apply' pats
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where
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apply' ((pat, body):xs) =
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case patternBindings pat arg of
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Just env' -> -- satisfies
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do
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env <- get
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put $ M.union env env'
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foldr1 (>>) $ map eval body
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Nothing -> error "doesn't satisfy"
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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) initialState
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main = do
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print $ evalProgram prg
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print $ evalProgram prg2
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print $ evalProgram prg3
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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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lam arg body = Lambda [(VarP arg, [body])]
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prg3 = [ Def "add" (lam "x" $ lam "y" $ Add (Var "x") (Var "y")),
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Call "add" [IntConst 5, IntConst 1]] |