added list index function

stdlib/list.go

@@ -96,6 +96,45 @@ func Len(input *ast.Token, vars ast.VarTable, funcs ast.FuncTable) *ast.Token {
     return ret
 }
 
+/* Takes two arguments
+ * An index and a list
+ * Returns element indexed by index arg
+ * can return nil if index is out of bounds
+ *
+ * Example (index 2 (1 2 3)) -> 2
+ */
+func Index(in *ast.Token, vars ast.VarTable, funcs ast.FuncTable) *ast.Token {
+    idx, err := strconv.ParseInt(in.Value(), 10, 64)
+    if err != nil || idx < 0 {
+        log.Log(log.ERR,
+            "index must be a positive integer: " + err.Error(),
+            "index")
+        return nil
+    }
+
+    series := in.Next.Expand()
+    iter := &series
+    i := int64(0)
+    for i <= idx {
+        if *iter == nil {
+            log.Log(log.ERR,
+                "index out of bounds",
+                "index")
+            return nil
+        }
+
+        if i < idx {
+            iter = &((*iter).Next)
+        }
+
+        i += 1
+    }
+
+    ret := (*iter).Copy()
+    ret.Next = nil
+    return ret
+}
+
 /* Head
  * Returns first element in the list
  * Returns nil if input is not a list or if list is empty

stdlib/stdlib.go

@@ -72,6 +72,15 @@ func GenFuncTable() ast.FuncTable {
             ArgLazy:    true,
         },
 
+        "index": &ast.Function{
+            Function:   Index,
+            Name:       "index",
+            Args:   []ast.Token_t{
+                ast.NUMBER,
+                ast.LIST,
+            },
+        },
+
         "head": &ast.Function{
             Function:   Head,
             Name:       "head",