i have these functions:
item :: Parser Char
item = Parser i
where i [] = []
i (x:xs) = [(x,xs)]
many :: Eq a=> Parser a -> Parser [a]
many p = many1 p +++ return []
many1 :: Eq a=> Parser a -> Parser [a]
many1 p = do v <- p
vs <- many p
returns (v:vs)
Im getting strange results applying differents strings. If i execute:
parse (many item) "x:=0"
i get
[('x',0)]
while if i use another string longer like "if x=0 then x:=0 else x:=1" it looks like go in loop. Doing some attempts it seems that if the string is longer more than 19 chars the function doesnt end. It's strange because on other string long less 19 chars it works good. What it could be?
Other definitions:
newtype Parser a = Parser { parse :: String -> [(a, String)] }
instance Monad Parser where
return t = Parser $ \s -> [(t, s)]
m >>= k = Parser $ \s -> [(x, y) | (u, v) <- parse m s, (x, y) <- parse (k u) v]
(+++) :: Eq a => Parser a -> Parser a -> Parser a
p +++ q = Parser $ \s -> if((parse p s)==[]) then parse q s else parse p s
Your implementation of (+++) doesn’t do what you think it does. In particular, it will only return successful parsed from one of its arguments, rather than from both. Here is how to do what you want:
(+++) :: Parser a -> Parser a -> Parser a
p +++ q = Parser $ \s -> parse p s ++ parse q s
Although this doesn’t remove duplicates so be warned that you can end up with an explosion of parses by doing eg many (many item).