add some pair functions

include/picrin.h

@@ -13,4 +13,8 @@ void pic_free(pic_state *, void *);
 pic_state *pic_open();
 void pic_close(pic_state *);
 
+pic_value pic_cons(pic_state *pic, pic_value, pic_value);
+pic_value pic_car(pic_state *pic, pic_value);
+pic_value pic_cdr(pic_state *pic, pic_value);
+
 #endif

src/pair.c

@@ -0,0 +1,33 @@
+#include "picrin.h"
+
+pic_value
+pic_cons(pic_state *pic, pic_value car, pic_value cdr)
+{
+  struct pic_pair *pair;
+
+  pair = (struct pic_pair *)pic_gc_alloc(pic, sizeof(struct pic_pair), PIC_TT_PAIR);
+  pair->car = car;
+  pair->cdr = cdr;
+
+  return pic_obj_value(pair);
+}
+
+pic_value
+pic_car(pic_state *pic, pic_value obj)
+{
+  struct pic_pair *pair;
+
+  pair = (struct pic_pair *)obj->u->data;
+
+  return pair->car;
+}
+
+pic_value
+pic_cdr(pic_state *pic, pic_value obj)
+{
+  struct pic_pair *pair;
+
+  pair = (struct pic_pair *)obj->u->data;
+
+  return pair->cdr;
+}