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|
{
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
-- For ancient GHC 7.0.4
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances #-}
import Control.Monad (liftM, ap)
import Control.Applicative as A
}
%name parse exp
%tokentype { Token }
%error { parseError }
%monad { (MonadIO m) } { Parser m }
%lexer { lexer } { EOF }
%token ID { Id _ }
NUM { Num _ }
PLUS { Plus }
MINUS { Minus }
TIMES { Times }
LPAREN { LParen }
RPAREN { RParen }
%%
exp :: { AST }
: exp PLUS prod
{ Sum $1 $3 }
| prod
{ $1 }
prod :: { AST }
: prod TIMES neg
{ Prod $1 $3 }
| neg
{ $1 }
neg :: { AST }
: MINUS neg
{ Neg $2 }
| atom
{ $1 }
atom :: { AST }
: ID
{ let Id str = $1 in Var str }
| NUM
{ let Num n = $1 in Lit n }
| LPAREN exp RPAREN
{ $2 }
{
data Token =
Plus
| Minus
| Times
| LParen
| RParen
| Id String
| Num Int
| EOF
deriving (Eq, Ord, Show)
data AST =
Sum AST AST
| Prod AST AST
| Neg AST
| Var String
| Lit Int
deriving (Eq, Ord)
type Parser m = ExceptT () (Lexer m)
type Lexer m = StateT [Token] m
parseError :: MonadIO m => Token -> Parser m a
parseError tok =
do
liftIO (putStrLn ("Parse error at " ++ show tok))
throwError ()
lexer :: MonadIO m => (Token -> Parser m a) -> Parser m a
lexer cont =
do
toks <- get
case toks of
[] -> cont EOF
first : rest ->
do
put rest
cont first
parse :: (MonadIO m) => Parser m AST
parser :: (MonadIO m) =>
[Token]
-> m (Maybe AST)
parser input =
let
run :: (MonadIO m) =>
Lexer m (Maybe AST)
run =
do
res <- runExceptT parse
case res of
Left () -> return Nothing
Right ast -> return (Just ast)
in do
(out, _) <- runStateT run input
return out
main :: IO ()
main =
let
input = [Id "x", Plus,
Minus, Num 1, Times,
LParen, Num 2, Plus, Id "y", RParen]
expected = Sum (Var "x") (Prod (Neg (Lit 1)) (Sum (Lit 2) (Var "y")))
in do
res <- parser input
case res of
Nothing -> print "Test failed\n"
Just actual
| expected == actual -> print "Test works\n"
| otherwise -> print "Test failed\n"
-- vendored in parts of mtl
class Monad m => MonadIO m where liftIO :: IO a -> m a
instance MonadIO IO where liftIO = id
class Monad m => MonadState s m | m -> s where
put :: s -> m ()
get :: m s
newtype StateT s m a = StateT { runStateT :: s -> m (a, s) }
instance Monad m => Functor (StateT s m) where
fmap = liftM
instance Monad m => A.Applicative (StateT s m) where
pure = return
(<*>) = ap
instance Monad m => Monad (StateT s m) where
return x = StateT $ \s -> return (x, s)
m >>= k = StateT $ \s0 -> do
(x, s1) <- runStateT m s0
runStateT (k x) s1
instance Monad m => MonadState s (StateT s m) where
put s = StateT $ \_ -> return ((), s)
get = StateT $ \s -> return (s, s)
instance MonadIO m => MonadIO (StateT e m) where
liftIO m = StateT $ \s -> liftM (\x -> (x, s)) (liftIO m)
class Monad m => MonadError e m | m -> e where
throwError :: e -> m a
newtype ExceptT e m a = ExceptT { runExceptT :: m (Either e a) }
instance Monad m => Functor (ExceptT e m) where
fmap = liftM
instance Monad m => A.Applicative (ExceptT e m) where
pure = return
(<*>) = ap
instance Monad m => Monad (ExceptT e m) where
return = ExceptT . return . Right
m >>= k = ExceptT $ do
x <- runExceptT m
case x of
Left e -> return (Left e)
Right y -> runExceptT (k y)
instance MonadState s m => MonadState s (ExceptT e m) where
put s = ExceptT (liftM Right (put s))
get = ExceptT (liftM Right get)
instance MonadIO m => MonadIO (ExceptT e m) where
liftIO = ExceptT . liftM Right . liftIO
instance Monad m => MonadError e (ExceptT e m) where
throwError = ExceptT . return . Left
}
|
