Mapping over Either's Left

Question

Somewhere in my app I receive an Either ParserError MyParseResult from Parsec. Downstream this result gets some other parsing done over using other libs. During tha

Answer 1

This can be done easily with lens:

import Control.Lens

over _Left (+1) $ Left 10   => Left 11
over _Left (+1) $ Right 10  => Right 10
over _Right (+1) $ Right 10 => Right 11