from objfuncs to objclosures

at compile time, at the end of a functions block compilation we now emit a new instruction OP_CLOSURE that the VM will use at runtime to wrap our function objects within a new closure object. the idea is that we’re going to use this closure object to store references to closed over variables (upvalues).

as a refresher, each time we create a compiler instance per function declaration, we also create a new function object via newFunction.

 static void function(FunctionType type) {
    Compiler compiler;
    initCompiler(&compiler, type);
    beginScope();
    ...
    ObjFunction* function = endCompiler();
    // emit a closure instruction!
    emitBytes(OP_CLOSURE, makeConstant(OBJ_VAL(function)));
  }
  
  ...
  
 static void initCompiler(Compiler* compiler, FunctionType type) {
  compiler->enclosing = current;
  compiler->function = NULL;
  compiler->type = type;
  compiler->localCount = 0;
  compiler->scopeDepth = 0;
  compiler->function = newFunction();
  current = compiler;
  if (type != TYPE_SCRIPT) {
    current->function->name = copyString(parser.previous.start,
                                         parser.previous.length);
  }

now at the end of the function compilation we make sure to emit an OP_CLOSURE so that at runtime, we use that opcode to wrap the raw ObjFunction in a closure and push it onto the stack.

below is the disassembly of fun foo() { fun bar(){} }

> fun foo() { fun bar(){} }
== bar ==
0000    1 OP_NIL
0001    | OP_RETURN
== foo ==
0000    1 OP_CLOSURE          0 <fn bar>
0002    | OP_NIL
0003    | OP_RETURN
== <script> ==
0000    1 OP_CLOSURE          1 <fn foo>
0002    | OP_DEFINE_GLOBAL    0 'foo'
0004    2 OP_NIL
0005    | OP_RETURN
          [ <script> ]
0000    1 OP_CLOSURE          1 <fn foo>
          [ <script> ][ <fn foo> ]
0002    | OP_DEFINE_GLOBAL    0 'foo'
          [ <script> ]
0004    2 OP_NIL
          [ <script> ][ nil ]
0005    | OP_RETURN

there’s a couple of interesting things about this design choice

every function, regardless of whether they close over variables, will be treated like a closure at runtime. this adds both overhead through the creation of each closure function and indirection
closed over values are stored on the clojure instead of the function, which nicely reflects the reality that we made have multiple different closures of the same function!

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