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.

7 files changed, 19 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