Implement deferred word, conditional resolution

Due to the non-trivial way tokens were previously processed the compiler could not safely perform tail-call elimination. Thus the _slang_ evaluation depth was restricted by the maximum call stack size.

This issue is mitigated by introducing deferred word resolution - i.e. pushing expanded tokens onto a buffer stack and processing them in an explicit loop.

This change ties into the implementation of the language's conditional primitive. The previous implementation did implicitly not support direct nesting of conditional expressions such as:

	truthA if
		mainBranch
		truthB if
			secondaryMainBranch
		then else
	then
		alternativeBranch
	else

This issue is now made explicit by disallowing direct nesting of conditionals as depicted above. Appropriate exceptions are generated when required and the conditional primitive is reimplemented in a more robust fashion under the assumption that this rule holds. Note that nesting is still fully supported iff. the nested conditional is contained in a deferredly evaluated word. As a positive side effect this will make it slightly harder to generate unreadable code by forcing developers to write simpler words.

The main change of the conditional primitive lies in deferring branch token processing until the conditional expression is concluded by `else`. This is achieved by capturing non-dropped tokens in an internal buffer akin to how the word definition operator `§` is implemented.
The branch buffer is discharged after `else` is evaluated. Discharging is triggered via the newly introduced `result` method of the primitive evaluation module. This avenue for injecting tokens into the processing stream may be used by further primitives in the future.

1 files changed, 0 insertions, 137 deletions

diff --git a/src/primitives/impl.d b/src/primitives/impl.d
deleted file mode 100644
index 9fddee5..0000000
--- a/src/primitives/impl.d
+++ /dev/null
@@ -1,137 +0,0 @@
-module src.primitives.impl;
-
-import std.stdio;
-
-import src.stack;
-import src.definition;
-
-Token[string] variables;
-bool          drop_mode;
-
-bool definition_start() {
-	src.definition.start;
-	return true;
-}
-
-bool binary_op_variable_bind() {
-	string name  = stack.pop.get!string;
-	Token  value = stack.pop;
-
-	variables[name] = value;
-	return true;
-}
-
-bool unary_op_variable_resolve() {
-	string name = stack.pop.get!string;
-
-	if ( name in variables ) {
-		stack.push(variables[name]);
-	}
-
-	return true;
-}
-
-bool unary_conditional_if() {
-	drop_mode = !stack.pop.get!bool;
-	return true;
-}
-
-bool n_ary_conditional_then() {
-	drop_mode = !drop_mode;
-	return true;
-}
-
-bool n_ary_conditional_else() {
-	drop_mode = false;
-	return true;
-}
-
-bool binary_op_add() {
-	int b = stack.pop.get!int;
-	int a = stack.pop.get!int;
-
-	stack.push(a + b);
-
-	return true;
-}
-
-bool binary_op_multiply() {
-	int b = stack.pop.get!int;
-	int a = stack.pop.get!int;
-
-	stack.push(a * b);
-
-	return true;
-}
-
-bool binary_op_divide() {
-	int b = stack.pop.get!int;
-	int a = stack.pop.get!int;
-
-	if ( b == 0 ) {
-		throw new Exception("division by 0 undefined");
-	} else {
-		stack.push(a / b);
-	}
-
-	return true;
-}
-
-bool binary_op_modulo() {
-	int b = stack.pop.get!int;
-	int a = stack.pop.get!int;
-
-	if ( b == 0 ) {
-		throw new Exception("modulo 0 undefined");
-	} else {
-		stack.push(a % b);
-	}
-
-	return true;
-}
-
-bool unary_op_io_print() {
-	writeln(stack.top);
-	return true;
-}
-
-bool unary_op_stack_pop() {
-	stack.pop;
-	return true;
-}
-
-bool unary_op_stack_dup() {
-	stack.push(stack.top);
-	return true;
-}
-
-bool binary_op_stack_swp() {
-	auto b = stack.pop;
-	auto a = stack.pop;
-
-	stack.push(b);
-	stack.push(a);
-
-	return true;
-}
-
-bool binary_cond_lt() {
-	int b = stack.pop.get!int;
-	int a = stack.pop.get!int;
-
-	stack.push(a < b);
-	return true;
-}
-
-bool binary_cond_eq() {
-	auto b = stack.pop;
-	auto a = stack.pop;
-
-	stack.push(a == b);
-	return true;
-}
-
-bool integral_value_bool(bool value) {
-	stack.push(Token(value));
-	return true;
-}