if

src/main.cpp

@@ -1,6 +1,7 @@
 #include <cassert>
 #include <iostream>
 #include <istream>
+#include <memory>
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
@@ -50,6 +51,11 @@ struct Number : SelfEval {
     }
 };
 
+struct Nil : SelfEval {
+	void Get() {}
+};
+
+
 struct Fork : Data {
     size_t data;
     size_t Get() {
@@ -65,7 +71,7 @@ struct Lambda : Data {
 
 enum class locs { EVAL_START, EVAL_OP_FIN, EVAL_TOP_ARG, APPLY };
 
-enum class special_indef_eval {NONE, PROGN};
+enum class special_indef_eval {NONE, PROGN, IF_BEGIN, FORCE_RETURN_NEXT_ARG};
 
 
 struct Registers {
@@ -171,6 +177,8 @@ struct Machine {
 					}
 					if(c->data == "progn") {
 						reg.screen = special_indef_eval::PROGN;
+					} else if (c->data == "if") {
+						reg.screen = special_indef_eval::IF_BEGIN;
 					}
 				}
                 reg.args_left = mem[((Fork *)std::get<Data *>(reg.Exp))->data].cdr;
@@ -191,6 +199,23 @@ struct Machine {
                         reg.args_current = temp;
                     }
                     mem[reg.args_current].d = std::get<Data*>(reg.R);
+
+					// is in an 'if' statement but got first arg back
+					if (reg.screen == special_indef_eval::IF_BEGIN) {
+						Nil * v = dynamic_cast<Nil*>(mem[reg.args_current].d);
+						if(v != nullptr) {
+							// if conditional returned false
+							// skip first argument
+
+							reg.args_left = mem[reg.args_left].cdr;
+							reg.screen = special_indef_eval::FORCE_RETURN_NEXT_ARG;
+						} else {
+							// if conditional returned true
+							reg.screen = special_indef_eval::FORCE_RETURN_NEXT_ARG;
+							
+						}
+					}
+					
                 }
                 if (reg.args_left != 0) { //args left
 					auto onlyoneleft = mem[reg.args_left].cdr == 0;
@@ -198,7 +223,8 @@ struct Machine {
 
 					bool newframe = false;
                     if (reg.screen == special_indef_eval::NONE
-						|| (reg.screen == special_indef_eval::PROGN && !onlyoneleft)) {
+						|| (reg.screen == special_indef_eval::PROGN && !onlyoneleft)
+						|| (reg.screen == special_indef_eval::IF_BEGIN)) {
 						stack.push_back(reg);
 						newframe = true;
 
@@ -253,6 +279,8 @@ struct Machine {
 					throw "err";
 				}
             }
+
+
             }
         }
     }
@@ -328,10 +356,30 @@ struct Machine {
 				std::cout << n->data << std::endl;
 			} else if (String * s = dynamic_cast<String*>(mem[cdr].d)) {
 				std::cout << s->data << std::endl;
+			} else if(Nil * n = dynamic_cast<Nil*>(mem[cdr].d)) {
+				std::cout << "!nil!" << std::endl;
 			} else {
 				throw "invalid print";
 			}
 			return cdr;
+		} else if (fname == "<") {
+			Number * f = dynamic_cast<Number*>(mem[cdr].d);
+			if (!f) throw "Non Number applied to <";
+			cdr = mem[cdr].cdr;
+			Number * l = dynamic_cast<Number*>(mem[cdr].d);
+			if (!l) throw "Non Number applied to <";
+				
+          
+			if (f->Get() < l->Get()) {
+				Number *n = new Number;
+				n->data = 1;
+				return newCell(n);
+			} else {
+				Nil * n = new Nil;
+				return newCell(n);
+			}
+
+
 		}
         return 0;
     }
@@ -466,7 +514,7 @@ struct Machine {
 };
 
 int main() {
-    auto m = Machine(10000);
+    auto m = Machine(1000);
 
 
     Cell c;
@@ -487,6 +535,8 @@ int main() {
     m.reg.E.builtin.insert("lambda");
     m.reg.E.builtin.insert("progn");
     m.reg.E.builtin.insert("print");
+    m.reg.E.builtin.insert("<");
+    m.reg.E.builtin.insert("if");
 
     //m.ListPrint(m.mem[exp]);
     std::cout << std::endl;