Lua: trying to catch more edge cases

Lua doesn't have a conventional try/catch mechanism, but it does have
something that comes pretty close: pcall.

The pcall method works by passing it a function argument, which it
immediately executes.  The pcall method returns two values:  The first
value is the function execution status.. true means the function
completed successfully, false means an error was thrown.  The second
value returned by pcall is the successful return, or the thrown error
(respectively).

There's a few modifications necessary for interop with normal Haxe
syntax here.  First of all, Haxe "try" syntax can work on arbitrary
blocks of code, not necessarily just functions.  The lua generator needs
to create a small iife to execute the code as if it were a block.  It
then can catch errors normally.

However, that's not the end of the story... the "try" block in Haxe
itself may itself contain a return.  Normally, an iife doesn't have a
return value.  But, in this case, we'll have it return an "expected
return value".  If the function returns succesfully, and the return
value differs from what's expected, we'll return that new value instead.

Note that I'm using a lot of statically assigned temporary variables
here.  This isn't best practice, but I made a TODO to come back and
clean it up with tmp variable names guaranteed not to conflict.

genlua.ml

@@ -356,11 +356,11 @@ let rec gen_call ctx e el in_value =
 		concat ctx "," (gen_value ctx) el;
 		spr ctx ")";
 	| TField (e, ((FInstance _ | FAnon _) as ef)), el ->
-		gen_value ctx e; 
-		spr ctx ":"; 
-		print ctx "%s" (field_name ef); 
-		spr ctx "("; 
-		concat ctx "," (gen_value ctx) el; 
+		gen_value ctx e;
+		spr ctx ":";
+		print ctx "%s" (field_name ef);
+		spr ctx "(";
+		concat ctx "," (gen_value ctx) el;
 		spr ctx ")"
 	| _ ->
 		gen_value ctx e;
@@ -611,15 +611,22 @@ and gen_expr ctx e =
 		handle_break();
 		newline ctx;
 	| TTry (e,catchs) ->
-		spr ctx "try ";
+		(* TODO: add temp variables *)
+		spr ctx "local _expected_result = {}";
+		newline ctx;
+		spr ctx "local _status, _result = pcall(function() ";
 		gen_expr ctx e;
 		let vname =
 			let id = ctx.id_counter in
 			ctx.id_counter <- ctx.id_counter + 1;
-			"$e" ^ string_of_int id
+			(* TODO : More temp var cleanup *)
+			"_e" ^ string_of_int id
 		in
-		print ctx " catch( %s ) {" vname;
+		spr ctx " return _expected_result end)"; newline ctx;
+		spr ctx " if not _status then ";
 		let bend = open_block ctx in
+		newline ctx;
+		print ctx "local %s = _result" vname;
 		let last = ref false in
 		let else_block = ref false in
 		List.iter (fun (v,e) ->
@@ -666,10 +673,10 @@ and gen_expr ctx e =
 				spr ctx "} else ";
 				else_block := true
 		) catchs;
-		if not !last then print ctx "throw(%s)" vname;
+		if not !last then print ctx "error(%s)" vname;
 		bend();
 		newline ctx;
-		spr ctx "}";
+		spr ctx " elseif _result ~= _expected_result then return _result end";
 	| TSwitch (e,cases,def) ->
 		List.iteri (fun cnt (el,e2) ->
 		    if cnt == 0 then spr ctx "if " else spr ctx "elseif ";