Hello everybody, I have a piece of code that gives me headaches for some time now. Simply put, I would like to know which is the best way to overpass a "Couldn't match expected type * against inferred type *"-error and an "Occurs check: cannot construct the infinite type:"-error in the following situation: {-# OPTIONS -fglasgow-exts #-}
module Simple where import Text.ParserCombinators.Parsec
data HData a = O | C a deriving (Eq,Ord,Show) data IN l = IN Int (HData l) deriving (Eq,Ord,Show) data CH l = CH Char (HData l) deriving (Eq,Ord,Show) -- data type is well-defined: sample = C(IN 0 (C(CH 'a' (C(IN 1 (C(CH 'b' (C(IN 2 O)))))))))
embeddedParser types = do string "end"; spaces; return O {- <|> do let h = head types let t = tail types case h of 1 -> do aux <- pInt rest <- embeddedParser $t++[h] return $ C (IN aux rest) 2 -> do aux <- pCh rest <- embeddedParser $t++[h] return $ C (CH aux rest) _ -> error "unallowed type" -} pInt = do n <- fmap read $ many1 digit; return $ fromInteger n pCh = do c <- letter; return $ c simple = embeddedParser [1,2]
-- the above result from sample I would like to get by running -- parseTest simple "0a1b2end"
The way I see it, the defined datatype works but I am a bit clueless about how to modify the parser to accept things of the type (e.g.): HData (IN (CH (IN (CH (IN a))))) (and in general of any finite type embedded like this). Thanks in advance for your help, George