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

Another option is to use Bifunctor instance of Either. Then you have

first :: (a -> b) -> Either a c -> Either b c

(Also Bifunctor can be used to traverse over the first part of (a,b).)