Add a predicate method to each node

This allows the AST to be searched via a predicate method rather than
hardcoding the class name and doing is_a? checks.  For example, rather
than:

```
ast.grep(Psych::Nodes::Scalar).each do |node|
  # .. do something
end
```

Now you can do:

```
ast.find_all(&:scalar?).each do |node|
  # .. do something
end
```

Your code no longer needs to know the exact class used in the AST.

8 files changed, 35 insertions, 0 deletions

diff --git a/lib/psych/nodes/alias.rb b/lib/psych/nodes/alias.rb
index 8131a4b..6da655f 100644
--- a/lib/psych/nodes/alias.rb
+++ b/lib/psych/nodes/alias.rb
@@ -14,6 +14,8 @@ module Psych
       def initialize anchor
         @anchor = anchor
       end
+
+      def alias?; true; end
     end
   end
 end
diff --git a/lib/psych/nodes/document.rb b/lib/psych/nodes/document.rb
index 3cd418e..f57410d 100644
--- a/lib/psych/nodes/document.rb
+++ b/lib/psych/nodes/document.rb
@@ -56,6 +56,8 @@ module Psych
       def root
         children.first
       end
+
+      def document?; true; end
     end
   end
 end
diff --git a/lib/psych/nodes/mapping.rb b/lib/psych/nodes/mapping.rb
index b921ddc..d49678c 100644
--- a/lib/psych/nodes/mapping.rb
+++ b/lib/psych/nodes/mapping.rb
@@ -52,6 +52,8 @@ module Psych
         @implicit = implicit
         @style    = style
       end
+
+      def mapping?; true; end
     end
   end
 end
diff --git a/lib/psych/nodes/node.rb b/lib/psych/nodes/node.rb
index 6d86669..f59fb89 100644
--- a/lib/psych/nodes/node.rb
+++ b/lib/psych/nodes/node.rb
@@ -63,6 +63,13 @@ module Psych
         io
       end
       alias :to_yaml :yaml
+
+      def alias?;    false; end
+      def document?; false; end
+      def mapping?;  false; end
+      def scalar?;   false; end
+      def sequence?; false; end
+      def stream?;   false; end
     end
   end
 end
diff --git a/lib/psych/nodes/scalar.rb b/lib/psych/nodes/scalar.rb
index b448858..e2616b6 100644
--- a/lib/psych/nodes/scalar.rb
+++ b/lib/psych/nodes/scalar.rb
@@ -63,6 +63,8 @@ module Psych
         @quoted = quoted
         @style  = style
       end
+
+      def scalar?; true; end
     end
   end
 end
diff --git a/lib/psych/nodes/sequence.rb b/lib/psych/nodes/sequence.rb
index 77c2c60..740f193 100644
--- a/lib/psych/nodes/sequence.rb
+++ b/lib/psych/nodes/sequence.rb
@@ -77,6 +77,8 @@ module Psych
         @implicit = implicit
         @style    = style
       end
+
+      def sequence?; true; end
     end
   end
 end
diff --git a/lib/psych/nodes/stream.rb b/lib/psych/nodes/stream.rb
index 2474fe6..b525217 100644
--- a/lib/psych/nodes/stream.rb
+++ b/lib/psych/nodes/stream.rb
@@ -33,6 +33,8 @@ module Psych
         super()
         @encoding = encoding
       end
+
+      def stream?; true; end
     end
   end
 end
diff --git a/test/psych/test_stream.rb b/test/psych/test_stream.rb
index 3bd557c..9b71c6d 100644
--- a/test/psych/test_stream.rb
+++ b/test/psych/test_stream.rb
@@ -3,6 +3,22 @@ require_relative 'helper'
 
 module Psych
   class TestStream < TestCase
+    [
+      [Psych::Nodes::Alias, :alias?],
+      [Psych::Nodes::Document, :document?],
+      [Psych::Nodes::Mapping, :mapping?],
+      [Psych::Nodes::Scalar, :scalar?],
+      [Psych::Nodes::Sequence, :sequence?],
+      [Psych::Nodes::Stream, :stream?],
+    ].each do |klass, block|
+      define_method :"test_predicate_#{block}" do
+        rb = Psych.parse_stream("---\n- foo: bar\n- &a !!str Anchored\n- *a")
+        nodes = rb.grep(klass)
+        assert_operator nodes.length, :>, 0
+        assert_equal nodes, rb.find_all(&block)
+      end
+    end
+
     def test_parse_partial
       rb = Psych.parse("--- foo\n...\n--- `").to_ruby
       assert_equal 'foo', rb