Implement not

Also fix a silly bunch in Environment: looking up a symbol with a
falsy binding was treated as an error. Whoops!

lib/environment.rb

@@ -11,7 +11,12 @@ def initialize
     end
 
     def [](symbol)
-      lookup(symbol) || raise("symbol \"#{symbol}\" is unbound")
+      frame = lookup_frame_with(symbol)
+      if frame && frame.has_key?(symbol)
+        frame[symbol]
+      else
+        raise("symbol \"#{symbol}\" is unbound")
+      end
     end
 
     def []=(symbol, value)
@@ -54,11 +59,6 @@ def to_s
 
     attr_accessor :stack
 
-    def lookup(symbol)
-      frame = lookup_frame_with(symbol)
-      frame && frame[symbol]
-    end
-
     def lookup_frame_with(symbol)
       stack.select { |frame| frame.has_key?(symbol) }.compact.last
     end

lib/standard-library.blu

@@ -31,6 +31,11 @@
       (cons (f (first seq))
             (map f (rest seq)))))
 
+(define (not value)
+  (if value
+      false
+      true))
+
 (define (null? exp)
   (== exp (quote ())))
 

spec/standard_library_spec.rb

@@ -25,9 +25,8 @@
 
   describe "if" do
     it "expands into the equivalent cond expression" do
-      expect_eval(
-        "(if (== 1 2) 3 4)"
-      ).to eq(4)
+      expect_eval("(if (== 1 1) 3 4)").to eq(3)
+      expect_eval("(if (== 1 2) 3 4)").to eq(4)
     end
 
     it "doesn't inadvertently execute the alternative branch" do
@@ -82,6 +81,13 @@
     end
   end
 
+  describe "not" do
+    it "flips the result of a statement" do
+      expect_eval("(not false)").to eq(true)
+      expect_eval("(not true)").to eq(false)
+    end
+  end
+
   describe "null?" do
     it "returns true when its argument is the empty list" do
       expect_eval("(null? (quote ()))").to eq(true)