[refactor] Change the internal representation for is-a-list

This update locks us into a late ECMA-5 implementation (no ECMA-3 for
you!) but it improves the way this representation works and is presented
to users.  It also corresponds (unsurprisingly) to the format used by
my experimental LiSP implementation, thus making my life easier.

Also: Included unit tests to ensure that nilp, listp, pairp, and cellp
all detect the correct matrix of... things.

src/lists.coffee

@@ -1,6 +1,6 @@
 vectorp = (a) -> toString.call(a) == '[object Array]'
 
-cellp = (a) -> vectorp(a) and a.__list == true
+cellp = (a) -> vectorp(a) and a.__type == 'list'
 
 pairp = (a) -> cellp(a) and (a.length == 2)
 
@@ -10,12 +10,19 @@ recordp = (a) -> Object.prototype.toString.call(a) == '[object Object]'
 
 nilp = (a) -> cellp(a) and a.length == 0
 
-nil = (-> l = []; l.__list = true; l)()
+makeAsCell = (l) ->
+  Object.defineProperty l, '__type',
+    value: 'list'
+    configurable: false
+    enumerable: false
+    writable: false
+  l
+
+nil = (-> makeAsCell([]))()
 
 cons = (a, b = nil) ->
-  l = if (not (a?)) then b else [a, b]
-  l.__list = true
-  l
+  return nil if (nilp a) and (nilp b)
+  makeAsCell if (not (a?)) then b else [a, b]
 
 car = (a) -> a[0]
 
@@ -57,6 +64,7 @@ module.exports =
   cdr: cdr
   list: list
   nilp: nilp
+  cellp: cellp
   pairp: pairp
   listp: listp
   vectorToList: vectorToList

test/lists.coffee

@@ -3,6 +3,7 @@ chai.should()
 expect = chai.expect
 
 {listToVector, vectorToList, cons, list, nil,
+  cellp, pairp, listp, nilp,
   metacadr, car, cdr, cadr} = require '../src/lists'
 
 describe "Basic list building", ->
@@ -27,6 +28,20 @@ describe "Basic list traversing", ->
   it "should car cdr cdr", ->
     expect(car cdr cdr mcsimple).to.equal("c")
 
+describe "Basic list typing", ->
+  it "Should recognize an empty cell", ->
+    expect(cellp(cons())).to.equal(true)
+  it "Should recognize a nil cell", ->
+    expect(nilp(cons())).to.equal(true)
+  it "Should recognize a paired cell", ->
+    expect(cellp(cons("a", "b"))).to.equal(true)
+    expect(pairp(cons("a", "b"))).to.equal(true)
+    expect(listp(cons("a", "b"))).to.equal(false)
+  it "Should recognize a list", ->
+    expect(cellp(cons("a"))).to.equal(true)
+    expect(pairp(cons("a"))).to.equal(true)
+    expect(listp(cons("a", cons("b")))).to.equal(true)
+
 describe 'Round trip equivalence', ->
   for [t, v] in [
     [[], []]
@@ -40,9 +55,10 @@ describe 'Round trip equivalence', ->
 describe 'List Building', ->
   for [t, v] in [
     [cons(), []]
-    [cons(nil), [nil]]
+    [cons(nil), []]
     [cons('a'), ['a']]
     [cons('a', cons('b')), ['a', 'b']]
+    [cons('a', cons('b', cons(nil))), ['a', 'b']]
     [cons('a', cons('b', cons('c'))), ['a', 'b', 'c']]
     [cons('a', cons('b', cons('c'), nil)), ['a', 'b', 'c']]]
     do (t, v) ->