Transform an attoparsec parser into a parser which fails if the number of bytes it consumes is not of a certain length

Question

Say I have an attoparsec parser, x.

I am looking to create a function f :: Int -> Parser a -> Parser a, such that if y = f n x, then:

• y fails if x fails
• y fails if x succeeds and x does not consume n bytes
• y succeeds otherwise

How would I go about doing this?

Answer 1

You can use match to implement it:

f n x = do
    (bs, res) <- match x
    guard (BS.length bs >= n)
    return res

You should check that this interacts with (<|>) in an acceptable way before putting it to heavy use.