Add facility to serialize binary values as url-safe base64 encoded strings

[line-o]

In XQuery 3.1 there are two XDM types to represent binary values xs:base64binary and xs:hexBinary.
The current draft adds binary literals as an additional option.

Thus, it is possible to base64 encode any value by casting a xs:base64binary to a xs:string.

xs:base64Binary("+w==") => xs:string()

There is no standard way to serialize those binary values to the URL safe variant of that encoding described in section 5 of RFC 4648.

The simplest workaround is replacing the unsafe characters of the alphabet (+ and /) and dropping the padding at the end with

xs:base64Binary("+w==") => translate("+/=", "-_")

This of course will only work for relatively small binary values. In order for processors to offer a performant and efficient way I see several options.

1. adding new type xs:base64BinaryUrlSafe whose string representation uses the adapted alphabet with - and _ and does not add padding at the end
2. a new function in fn namespace fn:encode-base64-url-safe($data as (xs:string | xs:base64Binary | xs:hexBinary)) as xs:string
3. a new function in bin namespace bin:encode-base64-url-safe($data as (xs:string | xs:base64Binary | xs:hexBinary)) as xs:string
4. add an output option that will serialize all binary values to base64 url-safe when cast to strings

Addendum

I am also wondering why binary values cannot be created from numeric literals. Especially now that we have the binary notation for integer literals and the xs:integer type is unbounded this would be a perfectly fine literal notation to create binary values from. At least as suitable as string literals that are currently allowed.

xs:hexBinary(0xfb) and xs:base64Binary(0b11111111)

[michaelhkay]

This seems a rather specialised requirement. I don't think we should be adding new functions for any job that anyone can come up with: we should be defining primitives that people can build on.

This of course will only work for relatively small binary values

Who says?

I am also wondering why binary values cannot be created from numeric literals

They can, using bin:hex, bin:binary etc plus a bit of string manipulation.