avdi · December 5, 2011 22:17 · Dec 5, 2011 · Dec 5, 2011
diff --git a/when.rb b/when.rb
@@ -76,3 +76,21 @@ def gross?
 result # => "yum"
 
 # Man for some reason I'm hungry for a sandwich now.
+
+# P.S. The number of keywords Ruby will let you also use as method
+# names is frankly a little shocking. Here's Object#if:
+
+class Object
+  def if(predicate)
+    if predicate.to_proc.(self)
+      yield(self)
+    else
+      self
+    end
+  end
+end
+
+result = sandwich.if(:gross?){|s| 
+  s.contents.delete(:liver); s
+}.get_into_my_belly!
+result # => "yum"
diff --git a/when.rb b/when.rb
@@ -0,0 +1,78 @@
+def maybe_a_sandwich
+  nil
+end
+
+# Methods that might return nil are annoying. We want to write a nice
+# confident chain:
+
+result = nil
+result = maybe_a_sandwich.add_horseradish.get_into_my_belly! rescue $! 
+result # => #<NoMethodError: undefined method `add_horseradish' for nil:NilClass>
+
+# But then we find out it might return nil and have to rewrite it with
+# special-case handling:
+
+class NoSandwich
+  def method_missing(*)
+    self
+  end
+
+  def to_s
+    "I'm so hungry!"
+  end
+end
+
+sandwich = (maybe_a_sandwich || NoSandwich.new)
+result = sandwich.add_horseradish.get_into_my_belly! 
+result # => I'm so hungry!
+
+# It would be cool if we could just insert the special case into the
+# chain. 
+
+class Object
+  def when(matcher)
+    if matcher === self then yield(self) else self end
+  end
+end
+
+result = maybe_a_sandwich.when(nil){NoSandwich.new}.add_horseradish.get_into_my_belly! 
+result # => I'm so hungry!
+
+# I can think of lots of other uses for this. Like a special case for
+# an empty collection:
+def sandwiches
+  []
+end
+
+result = begin
+  sandwiches.when(->(c){c.empty?}){raise "Out of sandwiches!"}.each do |s|
+    s.get_into_my_belly!
+  end
+rescue
+  $!
+end
+
+result # => #<RuntimeError: Out of sandwiches!>
+
+# Or a modifying filter:
+class Sandwich
+  attr_accessor :contents  
+
+  def get_into_my_belly!
+    "yum"
+  end
+
+  def gross?
+    contents.include?(:liver)
+  end
+end
+
+sandwich = Sandwich.new
+sandwich.contents = [:roast_beef, :swiss_chees, :liver]
+
+result = sandwich.when(->(s){s.gross?}){|s| 
+  s.contents.delete(:liver); s
+}.get_into_my_belly!
+result # => "yum"
+
+# Man for some reason I'm hungry for a sandwich now.
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