Created
January 4, 2019 09:22
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Parsec tutorial
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module Parser where | |
-- won't work because type class instances must not be type synonyms | |
-- type Parsera res = String -> (Either String res, String) | |
-- data Parser res = Parser (String -> (Either String res, String)) | |
-- parse (Parser p) = p | |
-- or with `newtype` | |
newtype Parser res = Parser { parse :: String -> (Either String res, String) } | |
anyChar :: Parser Char | |
anyChar = Parser nextCh | |
where nextCh [] = (Left "End of string", []) | |
nextCh (x:xs) = (Right x, xs) | |
satisfy :: (Char -> Bool) -> Parser Char | |
satisfy f = anyChar >>= nextP | |
where nextP a = | |
if f a | |
then pure a | |
else Parser (\s -> (Left "", s)) | |
char :: Char -> Parser Char | |
char c = satisfy (==c) | |
-- Monadic API | |
instance Functor Parser where | |
fmap = undefined | |
instance Applicative Parser where | |
(<*>) = undefined | |
pure a = Parser (\s -> (Right a, s)) | |
instance Monad Parser where | |
(>>=) = undefined | |
return = pure | |
-- Maybe samples | |
ma :: Maybe Int | |
ma = Just 42 | |
plus :: Num a => a -> a -> a | |
plus a b = a + b | |
res :: Int | |
res', res'' :: Maybe Int | |
res = plus 42 0 | |
res' = plus <$> ma <*> Nothing | |
res'' = (<*>) (fmap plus ma) Nothing |
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