Pattern matching on built-in strings

[kyouko-taiga]

There's currently no way to match built-in strings with anything but identify. In other words, one can write the following:

match s
  with "one" :: 1
  with "two" :: 2

But one can't check for any pattern that would match only parts of the string (e.g. the character at specific position). Although less impactful, note that built-in integers expose a similar problem.

One solution could be to map the built-in strings to an algebraic type such as #cons(_: Char, _: String) or #empty, which would make them naturally expressible in the language. Hence, the syntax with double quotes would be merely a syntactic sugar for #cons("a", cons("b", ....

Another direction would be to provide a built-in function to extract a specific character (e.g. char(in:at:)) and use identity matching, e.g.:

match char(in: s, at: 0)
  with "a" :: "the string starts with 'a'"
  // ...

Note that both solutions would require a built-in Char type.