[oever]

Value space is the set of values a type can have. A boolean has only two values in its value space. An unsigned byte has 256 possible values, so does a signed byte.

A string enumeration has a limited number of values. E.g. type A ("Yes" | "No" | "Maybe") has three values and is a superset of type B ("Yes" | "No"). A function that accepts type A can also accept type B as valid input.

If the value space is defined by a regular expression, as is often the case, the mentioned library could be used to check, at compile-time, which type are subsets of others.

[_a_a_a_]

Thank you. I guess I misread.

"For example, e-mails and urls are a special syntax. Their value space..." seemed to talk about the 'value space' of strings (these being e-mails and urls), not types (of e-mails and urls), which confused me.

[acchow]

It is bout the 'value space' of strings. Think of all possible strings. That is the entire value space of strings. Not every possible string is an email. Only a subset of this value space is a valid email. This subset is the 'value space' of strings which are valid emails.