add If stmt

bltin.c

@@ -12,6 +12,7 @@ Object Cons	= (Object){.type=OBLTIN, .sym="cons"};
 Object Define= (Object){.type=OBLTIN, .sym="define"};
 Object Setq	= (Object){.type=OBLTIN, .sym="setq"};
 Object Eq	= (Object){.type=OBLTIN, .sym="eq"};
+Object If	= (Object){.type=OBLTIN, .sym="if"};
 
 extern Object* fnplus(Object *, Object *);
 extern Object* fnlambda(Object *, Object *);
@@ -23,6 +24,7 @@ extern Object* fncar(Object *, Object *);
 extern Object* fncdr(Object *, Object *);
 extern Object* fncons(Object *, Object *);
 extern Object* fneq(Object *, Object *);
+extern Object* fnif(Object *, Object *);
 /*extern Object* fnminus(Object *, Object *);*/
 
 Bltinfn
@@ -42,6 +44,7 @@ bltinlookup(Object *obj)
 		{&Cdr ,fncdr},
 		{&Cons ,fncons},
         {&Eq, fneq},
+        {&If, fnif},
 		{&Minus ,0},
         {0},
 	};

dat.h

@@ -92,3 +92,4 @@ extern Object Cons;
 extern Object Define;
 extern Object Setq;
 extern Object Eq;
+extern Object If;

eval.c

@@ -14,6 +14,12 @@ exprlen(Object *expr)
 	return l;
 }
 
+int
+istrue(Object *o)
+{
+    return o && o->type==OINT && o->num!=0;
+}
+
 static int
 islist(Object *obj)
 {
@@ -176,6 +182,21 @@ fneq(Object *env, Object *list)
     return newint(gc, eq(env, a, b));
 }
 
+/*
+ * (if ((cond) val)* )
+ * (if ((eq x y) 0) ((eq x z ) 1) (default))
+ */
+Object*
+fnif(Object *env, Object *list)
+{
+    if(list == 0 || list == &Nil)
+        return newint(gc, 0);
+    Object *t = eval(env, list->car->car);
+    if(istrue(t))
+        return eval(env, list->car->cdr->car);
+    return fnif(env, list->cdr);
+}
+
 static Object*
 evallist(Object *env, Object *list)
 {

obj.c

@@ -52,7 +52,7 @@ newsymbol(GC *gc, char *str, int len)
 {
 	static Object *syms[] = {
 		&Nil,  &Minus, &Plus, &Lambda, &Car, &Cdr, &Quote, &Cons,
-        &Define, &Setq, &Eq,
+        &Define, &Setq, &Eq, &If,
 	};
 	for(int i = 0; i < sizeof(syms)/sizeof(syms[0]); ++i){
 		Object *c = syms[i];