make DSL locations more deterministic

[JonathanAldrich]

The goal of this would be to simplify the spec of Wyvern, with benefits for implementation, for the understandability of error messages, and for learning the language. The cost is a slightly higher annotation burden.

Right now, anytime there is a random indent, we assume it is a DSL unless a keyword like "type" or "def" or "match" leads us to expect an indent for something else.

Sometimes this leads to weird error messages, where the user indented and hears something about a DSL but they never intended that. There are also strange interactions with comments, particularly /* / style. If you indent and add a comment, the lexer infers a DSL, which then must be heuristically stripped out (see DSLLines() in WyvernParser.jj). Since / / may wrap lines, and DSLs can wrap lines but require indentation (which / */ does not), there are weird error cases, like indenting, starting a comment, and continuing the comment without indentation.

It would be better if you can always determine if line X+1 is a DSL by looking at line X. That would require some signal. We could always require a ~, so that if would look like

if (x > 0) ~
    something
  else
    something else

Or we could be pythonic:

if (x > 0):
    something
  else:
    something else

[potanin]

In my head this is related to the lambdas issue, it is all about what language makes explicit and what it makes implicit as you add more features. The original goal with TSL's was to make them everywhere and thus it is "normal" to expect DSL errors everywhere because everything is TSL of some sort - almost every kind of expression. Surely making DSL's explicit will detract from this goal of Wyvern (the goal of composable language)?