1 files changed, 14 insertions, 0 deletions

@@ -97,6 +97,20 @@ function m.cons (a, b)
                                 end
                                 return "(" .. table.concat(out, " ") .. ")"
                         end,
+                __len =
+                        function (self)
+                                local function go (lis, acc)
+                                        -- improper lists don't have lengths
+                                        -- ... but don't error here.
+                                        if not m.isa(lis, "cons") then
+                                                return nil
+                                        end
+                                        if lis[2] == m.null then return acc
+                                        else return go(lis[2], acc+1)
+                                        end
+                                end
+                                return go(self, 1)
+                        end,
         }
         return setmetatable(t, mt)
 end