Get match length in parsec

Question

Parsec's parse pattern "(some_input)" input returns the parsed data (as I specified in pattern.

How to know how much of input have it consumed (the pattern is not anchored with eof)? I don't want to add length tracking though the all pattern's internals (if discards some parts of input).

Answer 1

1. It is not easy with Parsec;
2. If it is needed to skip header you can grab the rest of input using getInput;
3. May be other parser libraries can do this.

(the answer is based on comments to the question)