Maybe this should be StringParser.Parser since it specifically deals with the parser type?

I named it Common because it contains more than just the Parser type, namely all the things that the CodeUnits parser and the CodePoints parser share but are not in Combinators.

But you have a lot more knowledge about naming conventions in the ecosystem, so should it be Parser instead? You tell me 😄

I may be misunderstanding you; all I see in this file are the parser type and instances, plus runParser and unParser for running the parser, fail for setting an error message, and printParserError for unpacking the error message.

namely all the things that the CodeUnits parser and the CodePoints parser share but are not in Combinators.

I guess my question is — what is being shared here beyond the Parser type and helpers for its error messages? If it's nothing, then I think StringParser.Parser is a more informative module name. If it's more than that, then Common makes more sense.

what is being shared here beyond the Parser type and helpers for its error messages?

That's pretty much it as you can see. So it's decided then :)