> For an implementation that supports URI/URLs (and opening ports from URLs),
> I think it is better if the URL API is similar to filename URI.
> For example, I think it makes sense to use the same API to extract
> parts from a file: URL and from a filename object - in an implementation
> supports working with URLs.

It's probably true that the APIs should at least be symmetrical.

> I see good reasons why the filename API should be designed to be a subset
> of a more general API that supports URLs, even for implementations
> that only support the former.
> (1) It provides a path (so to speak) for an implementation without URLs
> to be extended to one that supports URLs.

This is good.

> (2) It allows code to be more portable: If a program or library works
> with namesnames portably, it will probably also worth with URLs in
> an implementation that supports them, with little or no changes needed.
> (3) It seems inelegant to have to use different APIs if working with
> a filename vs working with a file: URL.
>
> Note these benefits do not require filepath or URLs as distinct types:
> You can generally just work with strings, and distinguish them by syntax.

There are important security and performance implications to opening a
URI in place of a file. I've found generic URI opening useful mainly in
simple scripts. If any `open` call in a complex application can turn
into a `open-uri`, a security audit is difficult.

> (Though there are some corner cases, especially if you support both
> relative filenames and relative URIs: You can't syntactically distinguish
> them, which could be an issue in the case of special characters, such as
> '%':
> They should be literal in filenames, and escapes in URIs.)

In the case of URIs, it may even be best to have different APIs for
different URI schemes: http, ftp, mailto... There are subtle differences
in how seemingly similar cases are treated, and implementations don't
always follow the same standards (when they follow any at all).