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module Hint.Eval where
import Control.Applicative ((<|>))
import qualified Data.Map as M
import Data.Maybe (fromJust)
import Hint.Data
type Env = M.Map Identifier Expr
data Scope = Scope
{ env :: [Env] }
deriving (Show, Eq)
basic :: Scope
basic = Scope mempty
enter :: Env -> Scope -> Scope
enter e (Scope es) = Scope (e : es)
leave :: Scope -> Scope
leave (Scope [] ) = error "bad"
leave (Scope (_ : es)) = Scope es
scopeLookup :: Identifier -> Scope -> Maybe Expr
scopeLookup _ (Scope []) = Nothing
scopeLookup i (Scope (e : es)) = M.lookup i e <|> scopeLookup i (Scope es)
eval :: Scope -> Expr -> Maybe Expr
eval _ (Literal l) = Literal <$> pure l
eval s (Ident i) = scopeLookup i s
eval scope (Bind name val rest) =
eval (enter (M.fromList [(name, fromJust $ eval scope val)]) scope) rest
eval s (If cond a b) = do
Literal (Boolean c) <- eval s cond
if c then eval s a else eval s b
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