Sad Methods

In general, Ruby's reflection capabilities are pretty powerful, although not always logical. However, when reflecting on a method's (or proc's) usage, you are sometimes stuck with sad methods.

Sad methods only work for code, that is written in Ruby itself. And Ruby itself (official MRI) is written in C, which limits such methods' usefulness quite a lot. This is an implementation specific problem, and it naturally does not occur in implementations like Rubinius. The following three methods are affected:


The arity of a method (or proc) will return:

For example:

method(:require).arity # => 1

However: For methods written in C, returns -1 if the call takes a variable number of arguments

method(:puts).arity # => -1

Source Location

The source_location of a method (or proc) returns a two-element array containing the path to the source file and the line number, where it is defined:

# => ["/home/jan/.rvm/rubies/ruby-2.3.0/lib/.../core_ext/kernel_require.rb", 39]

However: Returns […] nil if this method was not defined in Ruby (i.e. native)

method(:puts).source_location # => nil


The Method#parameters method lets inspect what kind of parameters a method (or proc) takes:

# => [[:req, :dir], [:opt, :options], [:block, :block]]

However: Like arity, it will treat all C methods with multiple arguments as "variable number of arguments", instead of providing exact information:

#=> [[:rest]]

Also See

More Idiosyncratic Ruby