How do I get Parsec to let me call `read` :: Int?

Question

I\'ve got the following, which type-checks:

p_int = liftA read (many (char \' \') *> many1 digit <* many (char \' \'))

Now, as the fu

Answer 1

How do I get Parsec to let me call read :: Int?

A second answer is "Don't use read".

Using read is equivalent to re-parsing data you have already parsed - so using it within a Parsec parser is a code smell. Parsing natural numbers is harmless enough, but read has different failure semantics to Parsec and it is tailored to Haskell's lexical syntax so using it for more complicated number formats is problematic.

If you don't want to go to the trouble of defining a LanguageDef and using Parsec's Token module here is a natural number parser that doesn't use read:

-- | Needs @foldl'@ from Data.List and 
-- @digitToInt@ from Data.Char.
--
positiveNatural :: Stream s m Char => ParsecT s u m Int
positiveNatural = 
    foldl' (\a i -> a * 10 + digitToInt i) 0 <$> many1 digit