Implement list length

This lays groundwork for arity checks.

2 files changed, 24 insertions, 4 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
@@ -1,11 +1,17 @@
 --- lam.util
 
-local util = {}
-local unpack = table.unpack or unpack
+local m = {}
 
-function util.pop (tbl)
+function m.pop (tbl)
+        --[[ Remove the first element from TBL. ]]
         return table.remove(tbl, 1)
 end
 
+function m.arity (r, min, max)
+        --[[ Return whether R is within MIN and MAX (inclusive). ]]
+        local len = #r
+        return len >= min and len <= max
+end
+
 ---
-return util
+return m