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A Smalltalk like Method Finder for Ruby

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This project was originally inspired by Smalltalk’s Method Finder, but additional features were added over time.


Common sense not included!

While I never had any problems with this, it’s still better to be safe than sorry, so use this with caution and maybe not on production data.

I initially wrote this for the students of the core Ruby course on RubyLearning, so Rails is not of interest to me (not saying it doesn’t work there, just that I test in plain IRB/Pry and not with the Rails console.


Ruby 1.9.3+ (also works with Rubinius in 1.9 mode). Versions of MethodFinder up to 1.2.5 will also work with Ruby 1.8.7. Note: CI only runs newer versions of Ruby.


Add this line to your application’s Gemfile:

gem 'methodfinder'

And then execute:

$ bundle

Or install it yourself as:

$ gem install methodfinder



Provided with a receiver, the desired result and possibly some arguments, MethodFinder.find will list all methods that produce the given result when called on the receiver with the provided arguments.

MethodFinder.find(10, 1, 3)
#=> ["Fixnum#%", "Fixnum#<=>", "Fixnum#>>", "Fixnum#[]", "Integer#gcd", "Fixnum#modulo", "Numeric#remainder"]
MethodFinder.find("abc", "ABC")
#=> ["String#swapcase", "String#swapcase!", "String#upcase", "String#upcase!"]
MethodFinder.find(10, 100, 2)
#=> ["Fixnum#**"]
MethodFinder.find(['a', 'b', 'c'], ['A', 'B', 'C']) { |x| x.upcase }
#=> ["Array#collect", "Array#collect!", "Enumerable#collect_concat", "Enumerable#flat_map", "Array#map", "Array#map!"]


This gem also adds Object#find_method, which besides offering an alternative interface to pretty much the same functionality as MethodFinder.find, also allows you to test for state other than the return value of the method.

%w[a b c].find_method { |a| a.unknown(1) ; a == %w[a c] }
#=> ["Array#delete_at", "Array#slice!"]
10.find_method { |n| n.unknown(3) == 1 }
#=> ["Fixnum#%", "Fixnum#<=>", "Fixnum#>>", "Fixnum#[]", "Integer#gcd", "Fixnum#modulo", "Numeric#remainder"]

Inside find_method’s block, the receiver is available as block argument and the special method unknown is used as a placeholder for the desired method.

You can also call find_method without passing a block. This is the same as calling MethodFinder.find.

10.find_method(1, 3)
#=> ["Fixnum#%", "Fixnum#<=>", "Fixnum#>>", "Fixnum#[]", "Integer#gcd", "Fixnum#modulo", "Numeric#remainder"]


You can exclude methods from being tried by editing the hashes MethodFinder::INSTANCE_METHOD_BLACKLIST and MethodFinder::CLASS_METHOD_BLACKLIST. Both use the class/module as key and an array of method names as values (note that class, module and method names have to be symbols).

For example, to blacklist the instance method shutdown of Object, you would do

MethodFinder::INSTANCE_METHOD_BLACKLIST[:Object] << :shutdown

This might come in handy when using MethodFinder together with other gems as such as interactive_editor.


A simple method to return all currently defined modules and classes.

#=> [ArgumentError, Array, BasicObject, Bignum ... ZeroDivisionError]


Searches for a given name within a class. The first parameter can either be a class object, a symbol or a string whereas the optional second parameter can be a string or a regular expression:

MethodFinder.find_in_class_or_module('Array', 'shuff')
#=> [:shuffle, :shuffle!]
MethodFinder.find_in_class_or_module(Float, /^to/)
#=> [:to_f, :to_i, :to_int, :to_r, :to_s]

If the second parameter is omitted, all methods of the class or module will be returned.

#=> [:acos, :acosh, :asin ... :tanh]


If the METHOD_FINDER_DEBUG environment variable is set, the name of each candidate method is printed to STDERR before it is invoked. This can be useful to identify (and blacklist) misbehaving methods.

It can be set on the command line e.g.:


Or you can toggle it inside IRB/Pry:

>> MethodFinder.toggle_debug!


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.