AS guide: explains acts_like?

1 files changed, 17 insertions, 0 deletions

diff --git a/railties/guides/source/active_support_overview.textile b/railties/guides/source/active_support_overview.textile
index ab9390883b..a88432dc3b 100644
--- a/railties/guides/source/active_support_overview.textile
+++ b/railties/guides/source/active_support_overview.textile
@@ -78,6 +78,23 @@ class Proc
 end
 </ruby>
 
+h4. +acts_like?(duck)+
+
+The method +acts_like+ provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as +String+ defines
+
+<ruby>
+def acts_like_string?
+end
+</ruby>
+
+which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:
+
+<ruby>
+some_klass.acts_like?(:string)
+</ruby>
+
+Rails has classes that act like +Date+ or +Time+ and follow this contract.
+
 h3. Extensions to +Module+
 
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