CODEWITHSUNDEEP
haskellparsec
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Reputation: 13037

Parsec: parse string that satisfies some predicate

Parsec already have a satisfy parser which succeeds when a given predicate returns True when applied to given char:

punctuationOrSymbol = satisfy isPunctuation <|> satisfy isSymbol

How can I do something similar for a whole string? I need it because I want to parse an URI but only if it is valid. For that, I want to leverage isURI function in network-uri package. Something like:

uri :: ParsecT s u m URI
uri = parseURI <$> satisfyForString isURI

I suspect that it has something to do with token function, but I'm new to Haskell and I haven't been able to understand it well.

Upvotes: 0

Views: 461

Answers (1)

Cubic
Cubic

Reputation: 15693

ParsecT is an instance of Alternative, so you can use guard to do something like this:

predicateP :: (a -> Bool) -> Parsec s u a -> Parsec s u a
predicateP pr p = do
   x <- p
   guard $ pr x
   return x

I wouldn't be surprised if that already existed as a combinator somewhere, but I'm not super familiar with parsec.

Now that only gives you a way to apply a predicate; To apply that to a string you'll have to get a string which you think is or might be a URL first. How you get that depends on your domain (when parsing XML you'll most likely find those in attributes or text contents for instance).

Upvotes: 4

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