Tart status update

I was out of town most of last week, and didn't get too much done on Tart. However, now that I'm back I'm continuing to work on type qualifiers and mutability.

Tart now allows type qualifiers (readonly, mutable, immutable) to be bound to a template parameter. The syntax looks like this:

def somefunc[%Mod(?)](arg:Mod(Object)) -> Mod(String);

Basically what we're saying here is that "Mod" is essentially a function that operates on types - that is, it takes a type as input, and returns a modified type as output. Moreover, the type inference engine can deduce what kind of function it is by examining the context in which the function is used. If 'arg' is an immutable object, then "Mod(?)" gets bound to "immutable(?)", which in turn causes the result of the function to be immutable as well.

More generally, each place where "Mod" appears in the function definition is a constraint. Function arguments are contravariant, so the type of 'arg' must be a subtype of Mod(Object). Function return values are covariant, so Mod(String) must be a subtype of whatever variable or parameter the return value is going to be assigned to. Given those two constraints, the compiler attempts to find a definition of Mod that satisfies both.

One downside of the current syntax is that it doesn't let you choose which type qualifiers the parameter is bound to - which is not a big issue right now, because a type currently can only have one qualifier, but may be an issue in the future.

Other than that there's not much to report. I still need to update the exception handling code to work with the new LLVM exception stuff, I just haven't gotten to it yet.