[main] Add avg function, fix empty-generator problem (fixes #140).

doc/all.md

@@ -1270,6 +1270,9 @@ into a hexadecimal string.)
 `range` takes an integer and returns a generator over the integers
 from zero to that integer, minus one.
 
+`avg` takes a generator and returns the average of the values in that
+generator.
+
 `to-function` takes a callable string (e.g. a function name) and
 converts it into a function object.  Using `funcall` on the function
 object will then be quicker than using it on the original string.

lib/rt.ch

@@ -638,6 +638,25 @@
         then;
         .f until; ,,
 
+: avg
+    gen var; gen !;
+    0 total var; total !;
+    0 count var; count !;
+    begin;
+        gen @; shift; dup; is-null; if;
+            drop;
+            leave;
+        then;
+        total @; +; total !;
+        count @; 1 +; count !;
+        0 until;
+    count @; 0 =; if;
+        0
+    else;
+        total @; 0.0 +; count @; /;
+    then;
+    ,,
+
 : mlist
     depth; 1 <; if;
         "mlist requires at least one argument" error;

src/vm/vm_list.rs

@@ -182,18 +182,18 @@ impl VM {
                         }
                     }
 
-                    /* todo: need a generator-specific run function,
-                     * to avoid the stuffing around here. */
-                    let local_vars_stack = generator_object.local_vars_stack.clone();
-                    let chunk = generator_object.chunk.clone();
-                    let call_stack_chunks = &mut generator_object.call_stack_chunks;
-                    mem::swap(call_stack_chunks, &mut self.call_stack_chunks);
-
                     let current_index = index;
-                    if current_index == chunk.borrow().data.len() {
+                    if current_index == generator_object.chunk.borrow().data.len() {
                         /* At end of function: push null. */
                         self.stack.push(Value::Null);
                     } else {
+                        /* todo: need a generator-specific run function,
+                         * to avoid the stuffing around here. */
+                        let local_vars_stack = generator_object.local_vars_stack.clone();
+                        let chunk = generator_object.chunk.clone();
+                        let call_stack_chunks = &mut generator_object.call_stack_chunks;
+                        mem::swap(call_stack_chunks, &mut self.call_stack_chunks);
+
                         let plvs_stack = self.local_var_stack.clone();
                         self.local_var_stack = local_vars_stack;
                         let backup_chunk = self.chunk.clone();

tests/tests.rs

@@ -2213,3 +2213,8 @@ fn hash_literal_test() {
     basic_error_test("h(1)", "1:5: expected even number of elements for hash");
     basic_error_test("h(h(1 2) 3)", "1:13: expected string for hash key");
 }
+
+#[test]
+fn avg_test() {
+    basic_test("10 range; avg", "4.5");
+}