From f28763f3afcd2c51e9b69318e3749b6bb0a4f1ea Mon Sep 17 00:00:00 2001
From: Case Duckworth
Date: Mon, 1 Apr 2024 22:16:50 -0500
Subject: Implement list length

This lays groundwork for arity checks.
---
 type.lua | 14 ++++++++++++++
 util.lua | 14 ++++++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/type.lua b/type.lua
index 3c26188..07f78f1 100644
--- a/type.lua
+++ b/type.lua
@@ -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
diff --git a/util.lua b/util.lua
index 938848c..b5a57b1 100644
--- a/util.lua
+++ b/util.lua
@@ -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
-- 
cgit 1.4.1-21-gabe81