Safer env for recursives + variable identity.

[hishamhm]

The change with the "@" characters is a bit of paranoia, as different pairs of types may concatenate to the same string ("aa".."b" and "a".."ab") and this may cause evaluation errors in the future.

Note the change in subtype_variable for when the two entries have equal names. This is the important one. Please verify if this is semantically correct. Here it seems to solve #55, but I don't know if that's enough. Maybe this is just hiding the real problem.

I don't know if I'm understanding correctly the way the structural type system is supposed to work. With this patch, the code in #55 gives no errors, but the variant below still fails:

interface ELEM
   x: integer
end

interface OBJ
   parent: OBJ?
   atr: ELEM -- if I change ELEM to {"x":integer}, I no longer get an error
end

local function f(o:OBJ)
   local parent = o.parent
   if parent then
      f({atr={x=2}}) -- isn't this structurally equivalent to OBJ?
   end
end

[mascarenhas]

Yep, the code above should compile. What we need is a global environment that maps named types to their structure, with this environment being used throughout the type checker (including the subtype relation). Note that this is environment is not a type environment (type environments map variables names to types).

[hishamhm]

Yay, thank you!! So happy to see activity in this repo again! :-D

[mascarenhas]

I improved the name resolution code, and this example now works fine (fixed the loop in issue #50 too), but there is still more to be done. Ideally global type names like ELEM and OBJ should be left as is, so we can have mutually recursive global types. These types should also be exported along with the other names in the module, and modules that import this one should be able to access them:

local mod = require "pr56" -- pr56.tl is the module above
local e: mod.ELEM = { x = 10 }

[hishamhm]

Yes. I also missed a way to make one .tld require another (should be a separate issue?).

[hishamhm]

Ah, nevermind, that's #66.