lab19: init

lab19/eval/eval.rb

@@ -0,0 +1,67 @@
+# Ruby has rich metaprogramming tools, often bowered from Smalltalk.
+# Like JavaScript, it has an eval feature.
+prog = "puts 3 + 4"
+eval prog
+
+# Eval is one of the most powerful metaprogramming features,
+# but it is also one of the most dangerous.
+print "Please enter your method name: "
+m = gets.chomp
+eval "def #{m}; puts 'Hi!'; end"
+eval m
+
+=begin
+Thomass-MacBook-Pro-3:lab15 taustin$ ruby eval.rb 
+Please enter your method name: abc; end; puts "Mwah, hah, hah!"; #
+Mwah, hah, hah!
+eval.rb:11: (eval):1: compile error (SyntaxError)
+(eval):1: syntax error, unexpected kEND, expecting $end
+abc; end; puts "Mwah, hah, hah!"; #
+-----------------------
+Eval is horribly abused (Richards et al. 2011, See the Eval that men do,), but it is useful at times.
+For instance, in JavaScript, it served as an early (but unsafe) version of JSON.parse.
+Similar to goto, it is a powerful but dangerous construct, and is often used in places
+where the language is missing a key feature.
+"A design pattern is the sincerest form of feature request".
+
+However, it does not tend to show up as often in Ruby.
+In part, Ruby has some safer alternatives that are nearly as powerful.
+They take blocks rather than expressions.
+=end
+
+# instance_eval -- used for prying open objects to get at their private data.
+# This can be handy for things like writing a debugger.
+class Person
+  attr_reader :name
+  def initialize name
+    @name = name
+  end
+end
+
+bob = Person.new "Robert"
+puts bob.name
+#bob.name = "Bobby" # Error
+
+bob.instance_eval do
+  @name = "Bobby"
+end
+puts bob.name
+
+# And finally class_eval/module_eval, which serve as an alternate way of opening up a class.
+favorite_song = "Streets of Laredo"
+class Person
+  #puts favorite_song # error
+end
+
+Person.class_eval do
+  puts favorite_song
+  #def sing
+  #  puts "When #{@name} went out in the #{favorite_song}..." # Will not see favorite_song
+  #end
+  define_method "sing" do
+    puts "When #{@name} went out in the #{favorite_song}..."
+  end
+end
+
+bob.sing
+