- add #is_a? support

- add docs for #respond_to?

lib/insist/predicates.rb

@@ -5,11 +5,21 @@ module Insist::Predicates
   include Insist::Assert
   PREDICATE_METHOD_RE = /\?$/
 
+  # Fails if the value does not respond to a method.
+  # 
+  # insist { "hurray" }.respond_to?(:size)
   def respond_to?(method)
     assert(value.respond_to?(method),
            "#{value.class} does not respond to the '#{method}' method")
   end # def respond_to?
 
+  # Fails if the value.is_a?(klass) returns false.
+  # 
+  # insist { "hurray" }.is_a?(Number)
+  def is_a?(klass)
+    assert(value.is_a?(klass), "#{value.class} is not a #{klass}")
+  end
+
   # Pass through any 'foo?' style method calls to the 'value' 
   # and fail if the the return is false.
   def method_missing(m, *args)

spec/insist/predicates_spec.rb

@@ -5,6 +5,18 @@
 # The predicates feature will delegateany predicate method calls (ones ending
 # in "?") to the block value and fail if the return is false.
 describe Insist::Predicates do
+  describe "#is_a?" do
+    it "should succeed if value#is_a? returns true" do
+      insist { "some string" }.is_a?(Object)
+      insist { "some string" }.is_a?(String)
+    end
+    it "should fail if value#is_a? returns false" do
+      reject { "some string" }.is_a?(Numeric)
+      reject { "some string" }.is_a?(Fixnum)
+      reject { "some string" }.is_a?(File)
+    end
+  end
+
   describe "#respond_to?" do
     subject do
       insist { [1, 2, 3] }