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, 34 insertions, 43 deletions

diff --git a/repl.d b/repl.d
index b68b46a..df2905d 100644
--- a/repl.d
+++ b/repl.d
@@ -1,69 +1,60 @@
 import std.stdio;
 import std.string;
-import std.conv;
 import std.variant;
 import std.typecons;
 
-import std.container : SList;
-
 import src.stack;
 
 static import definition = src.definition;
 static import primitives = src.primitives.eval;
 
-template process(T)
-if ( is(T == int) || is(T == bool) ) {
-	void process(T value) {
-		try {
-			if ( !primitives.evaluate(value) ) {
-				stack.push(value);
-			}
-		}
-		catch (Exception ex) {
-			writeln("Error: ", ex.msg);
-		}
-	}
-}
-
-void process(string word) {
+Stack!Token resolve(Token token) {
 	try {
-		if ( !primitives.evaluate(word) ) {
-			if ( word in definition.words ) {
-				foreach ( token; definition.words[word] ) {
-					process(token);
-				}
-			} else {
-				stack.push(word);
-			}
+		if ( primitives.evaluate(token) ) {
+			return primitives.result;
+		} else {
+			return token.visit!(
+				(int    value) => Stack!Token(Token(value)),
+				(bool   value) => Stack!Token(Token(value)),
+				(string word ) => definition.get(word)
+			);
 		}
 	}
 	catch (Exception ex) {
 		writeln("Error: ", ex.msg);
+		return Stack!Token();
 	}
 }
 
-void process(Token token) {
-	token.visit!(
-		(int    value) => process(value),
-		(bool   value) => process(value),
-		(string word ) => process(word)
-	);
+void process(string value) {
+	Stack!Token buffer;
+	Token token = toToken(value);
+
+	if ( !definition.handle(token) ) {
+		buffer = resolve(token);
+	}
+
+	while ( !buffer.empty ) {
+		Token current = buffer.pop;
+
+		if ( !definition.handle(current) ) {
+			Stack!Token resolved = resolve(current);
+
+			if ( !resolved.empty ) {
+				if ( resolved.front == current ) {
+					stack.push(current);
+				} else {
+					buffer.insertFront(resolved[]);
+				}
+			}
+		}
+	}
 }
 
 void main() {
 	while ( !stdin.eof ) {
 		foreach ( token; stdin.readln.split ) {
-			if ( token.isNumeric ) {
-				auto value = parse!int(token);
-
-				if ( !definition.handle(value) ) {
-					process(value);
-				}
-			} else {
-				if ( !definition.handle(token) ) {
-					process(token);
-				}
-			}
+			process(token);
 		}
 	}
 }