first

diff --git a/implementations/uvm-ncc/joy.c b/implementations/uvm-ncc/joy.c
index d4e3796..2dc706f 100644 (file)
--- a/implementations/uvm-ncc/joy.c
+++ b/implementations/uvm-ncc/joy.c
@@ -45,7 +45,8 @@ u64 error = 0;
 #define STRING_HEAP_OOM 5
 #define NOT_ENOUGH_VALUES_ON_STACK 6
 #define NOT_A_LIST 7
-#define CANNOT_TAKE_REST_OF_EMPTY_LIST 8
+#define CANNOT_TAKE_FIRST_OF_EMPTY_LIST 8
+#define CANNOT_TAKE_REST_OF_EMPTY_LIST 9
 
 
 #define CHECK_ERROR if (error != NO_ERROR) return 0;
@@ -585,11 +586,12 @@ u64
 joy_eval(char *symbol, u32 stack, u32 expression)
 {
        MATCH("clear") return (u64)expression;
-       MATCH("swaack") { stack = swaack(stack); }
+       MATCH("dup") { stack = dup(stack); }
+       else MATCH("first") { stack = first(stack); }
        else MATCH("pop") { stack = pop(stack); }
-       else MATCH("dup") { stack = dup(stack); }
-       else MATCH("stack") { stack = cons(stack, stack); }
        else MATCH("rest") { stack = rest(stack); }
+       else MATCH("stack") { stack = cons(stack, stack); }
+       else MATCH("swaack") { stack = swaack(stack); }
        else MATCH("swap") { stack = swap(stack); }
        // first ...
        //else MATCH("") { stack = (stack); }
@@ -611,6 +613,21 @@ swaack(u32 stack)
 
 
 u32
+first(u32 stack)
+{
+       u32 list = pop_list(stack);
+       CHECK_ERROR
+       if (!list) {
+               error = CANNOT_TAKE_FIRST_OF_EMPTY_LIST;
+               return 0;
+       }
+       stack = cons(head(list), tail(stack));
+       CHECK_ERROR
+       return stack;
+}
+
+
+u32
 rest(u32 stack)
 {
        u32 list = pop_list(stack);
@@ -619,9 +636,7 @@ rest(u32 stack)
                error = CANNOT_TAKE_REST_OF_EMPTY_LIST;
                return 0;
        }
-       stack = tail(stack);
-       list = tail(list);
-       stack = cons(list, stack);
+       stack = cons(tail(list), tail(stack));
        CHECK_ERROR
        return stack;
 }
@@ -718,7 +733,7 @@ main()
        print_endl();
        */
 
-       u32 expression = text_to_expression("1 2 3 stack rest");
+       u32 expression = text_to_expression("1 2 3 stack rest first");
        //u32 expression = text_to_expression("1 2 3 clear 4 5 6");
        //u32 expression = text_to_expression(" 1[2[true 3][aa[aa bb] aa bb cc]bob]false[]bob 3[4] ga[]ry");
        print_joy_list(expression);