Expand conditional primitive to choose between `then` and `else` branch

i.e. `1 if true then false else` evaluates to `true`, `0 if true then
false else` evaluates to `false`.

2 files changed, 18 insertions, 8 deletions

diff --git a/src/primitives/eval.d b/src/primitives/eval.d
index 3f52e6d..533f491 100644
--- a/src/primitives/eval.d
+++ b/src/primitives/eval.d
@@ -8,10 +8,13 @@ bool evaluate(int value) {
 
 bool evaluate(string word) {
 	if ( drop_mode ) {
-		if ( word == "then" ) {
-			return conditional_end;
-		} else {
-			return true;
+		switch ( word ) {
+			case "then":
+				return conditional_then;
+			case "else":
+				return conditional_else;
+			default:
+				return true;
 		}
 	}
 
@@ -23,9 +26,11 @@ bool evaluate(string word) {
 		case "@":
 			return unary_op_variable_resolve;
 		case "if":
-			return conditional_start;
+			return conditional_if;
 		case "then":
-			return conditional_end;
+			return conditional_then;
+		case "else":
+			return conditional_else;
 		case "+":
 			return binary_op_add;
 		case "*":
diff --git a/src/primitives/impl.d b/src/primitives/impl.d
index 56ea4ce..cac3e64 100644
--- a/src/primitives/impl.d
+++ b/src/primitives/impl.d
@@ -31,7 +31,7 @@ bool unary_op_variable_resolve() {
 	return true;
 }
 
-bool conditional_start() {
+bool conditional_if() {
 	switch ( stack.pop.get!int ) {
 		case 0:
 			drop_mode = true;
@@ -44,7 +44,12 @@ bool conditional_start() {
 	}
 }
 
-bool conditional_end() {
+bool conditional_then() {
+	drop_mode = !drop_mode;
+	return true;
+}
+
+bool conditional_else() {
 	drop_mode = false;
 	return true;
 }