Partly done with implementation of LABELS

It is not yet adding the new binding to the existing bindings.

lib/yarlisp.rb

@@ -34,6 +34,9 @@ def equal(a, b)
                 false
             end
         end
+        def null(val)
+            (equal val, :NIL)
+        end
         def assoc(x, a)
             if (equal (CAR (CAR a)), x)
                 (CAR a)
@@ -42,31 +45,27 @@ def assoc(x, a)
             end
         end
         def cond(x, a)
-            def is_true?(val)
-                !(val.eql? :NIL)
-            end
             condition = (EVAL (CAR (CAR x)), a)
-            if (is_true?(condition))
+            if (!(null condition))
                 (EVAL (CAR (CDR (CAR x))), a)
             else
                 (cond (CDR x), a)
             end
         end
 
         def evlis(m, a)
-            def null(val)
-                (equal val, :NIL)
-            end
             if (null m) 
                 :NIL
             else
                 (CONS (EVAL (CAR m), a), (evlis (CDR m), a))
             end
         end
 
+        puts "(EVAL #{expr} #{env})"
+
         if (ATOM expr)
             (CDR (assoc expr, env))
-        else
+        elsif (ATOM (CAR expr))
             fn=(CAR expr)
             args=(CDR expr)
 
@@ -89,6 +88,18 @@ def null(val)
             else
                 (EVAL (CONS (CDR (assoc fn, env)), (evlis args, env)), env)
             end
+        else
+            if (EQ (CAR (CAR expr)), :LABEL)
+                (EVAL (CONS (CAR (CDR (CDR (CAR expr)))), (CDR expr)),
+                    env)
+            end
         end
     end
 end
+
+#(eval ((label, f, E), e1, ... en), a)
+#(eval (E e1...en) (append ((f (label f E))) a))
+#
+#The evaluation of ((LABEL, f, E), e1, · · · , en) is accomplished by eval-
+#           uating (E, e1, · · · , en) with the pairing (f, (LABEL, f, E)) put on the front of
+#       the previous list a of pairs.

spec/yarlisp_spec.rb

@@ -47,6 +47,7 @@
 
         it "handles QUOTE function" do
             (EVAL [:QUOTE, [:x, :NIL]], []).should eq :x
+            (EVAL [:QUOTE, [[:x, :y], :NIL]], []).should eq [:x, :y]
         end
 
         it "handles ATOM function" do
@@ -81,5 +82,11 @@
             (EVAL [:x, [:y, [:z, :NIL]]],
              [[:x, :CONS], [[:y, [:QUOTE, [:b, :NIL]]], [[:z, [:QUOTE, [:c, :NIL]]], :NIL]]]).should eq [:b, :c]
         end
+
+        it "handles LABELS function (used to create a binding))" do
+            (EVAL [[:LABEL, [:x, [:CONS, :NIL]]], 
+                   [[:QUOTE, [:a, :NIL]], [[:QUOTE, [:b, :NIL]], :NIL]]],
+                   []).should eq [:a, :b]
+        end
     end
 end