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sig: Optional Type Assertions for Ruby methods. [version] [travis]

This gem adds the sig method that allows you to add signatures to Ruby methods. When you call the method, it will verify that the method's arguments/result fit to the previously defined behavior:

# On main object
sig [:to_i, :to_i], Integer,
def sum(a, b)
  a.to_i + b.to_i

sum(42, false)
# Sig::ArgumentTypeError:
# - Expected false to respond to :to_i

# In modules
class A
  sig [Numeric, Numeric], Numeric,
  def mul(a, b)
    a * b
# Sig::ArgumentTypeError:
# - Expected "3" to be a Numeric, but is a String

# Explicitely define signature for singleton_class
class B
  sig_self [:reverse],
  def self.rev(object)

B.rev 42
# Sig::ArgumentTypeError:
# - Expected 42 to respond to :reverse

The first argument is an array that defines the behavior of the method arguments, and the second one the behavior of the method result. Don't forget the trailing comma, because the method definition needs to be the last argument to the sig method.

Features & Design Goals

  • Provide an intuitive way to define signatures
  • Only do argument/result type checks, nothing else
  • Use Ruby's inheritance chain, don't redefine methods
  • Encourage duck typing
  • Should work with keyword arguments
  • Only target Ruby 2.1+

This is not static typing. Ruby is a dynamic language:

Nevertheless, nothing is wrong with ensuring specific behaviour of method arguments when you need it.

Is this better than rubype?

The rubype gem achieves similar things like sig (and inspired the creation of sig). It offers a different syntax and differs in feature & implementation details, so in the end, it is a matter of taste, which gem you prefer.


Add to your Gemfile:

gem 'sig'


See example at top for basic usage.

Supported Behavior Types

You can use the following behavior types in the signature definition:

Type Meaning
Symbol Argument must respond to a method with this name
Module Argument must be of this module
Array Argument can be of any type found in the array
true Argument must be truthy
false Argument must be falsy
nil Wildcard for any argument

Example Signatures

sig [:to_i], Numeric,              # takes any object that responds to :to_i as argument, numeric result
sig [Numeric], String,             # one numeric argument, string result
sig [Numeric, Numeric], String,    # two numeric arguments, string result
sig [:to_s, :to_s],                # two arguments that support :to_s, don't care about result
sig nil, String,                   # don't care about arguments, as long result is string
sig {keyword: Integer}             # keyword argument must be an intieger
sig [:to_f, {keyword: String}],    # mixing positional and keyword arguments is possible
sig [[Numeric, NilClass]], Float   # one argument that must nil or numeric, result must be float
sig [Numeric, nil,  Numeric],      # first and third argument must be numeric, don't care about type of second

See source( or specs( for more features.

Benchmark (Take with a Grain of Salt)

You can run rake benchmark to run it on your machine.

There is still a lot room for performance improvements. Feel free to suggest some faster implementation to do the type checks (even if it is crazy and not clean, as long it does not add too much "magic", a.k.a does not make debugging harder).

Note: Starting with 0.3.0, rubype uses a C extension, which makes it much faster. The benchmark is still run with rubype 0.2.5, because 0.3.x currently does not work on jruby & rbx.


ruby version: 2.2.2
ruby engine: ruby
ruby description: ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux]
sig version: 1.0.1
rubype version: 0.2.5
contracts version: 0.9
Calculating -------------------------------------
                pure   107.628k i/100ms
                 sig    14.425k i/100ms
              rubype    12.715k i/100ms
           contracts     7.688k i/100ms
                pure      6.856M (± 0.9%) i/s -     34.333M
                 sig    192.440k (± 1.5%) i/s -    966.475k
              rubype    164.811k (± 0.8%) i/s -    826.475k
           contracts     90.089k (± 0.7%) i/s -    453.592k

                pure:  6855615.3 i/s
                 sig:   192439.7 i/s - 35.62x slower
              rubype:   164810.5 i/s - 41.60x slower
           contracts:    90088.6 i/s - 76.10x slower

JRuby 9000

ruby version: 2.2.2
ruby engine: jruby
ruby description: jruby (2.2.2) 2015-05-04 6055b79 Java HotSpot(TM) 64-Bit Server VM 24.80-b11 on 1.7.0_80-b15 +indy +jit [linux-amd64]
sig version: 1.0.1
rubype version: 0.2.5
contracts version: 0.9
Calculating -------------------------------------
                pure    70.898k i/100ms
                 sig     5.308k i/100ms
              rubype     3.152k i/100ms
           contracts   279.000  i/100ms
                pure      8.848M (±13.7%) i/s -     42.539M
                 sig    178.169k (±10.3%) i/s -    881.128k
              rubype    119.689k (±26.5%) i/s -    444.432k
           contracts     56.780k (±16.8%) i/s -    265.887k

                pure:  8848039.4 i/s
                 sig:   178168.8 i/s - 49.66x slower
              rubype:   119689.0 i/s - 73.93x slower
           contracts:    56780.4 i/s - 155.83x slower

RBX 2.5.3

ruby version: 2.1.0
ruby engine: rbx
ruby description: rubinius 2.5.3.c25 (2.1.0 fbb3f1e4 2015-05-02 3.4 JI) [x86_64-linux-gnu]
sig version: 1.0.1
rubype version: 0.2.5
contracts version: 0.9
Calculating -------------------------------------
                pure   114.964k i/100ms
                 sig     9.654k i/100ms
              rubype     3.775k i/100ms
           contracts     3.964k i/100ms
                pure     23.585M (± 3.3%) i/s -    117.263M
                 sig    134.304k (± 3.1%) i/s -    675.780k
              rubype     56.042k (± 1.7%) i/s -    283.125k
           contracts     69.820k (± 1.8%) i/s -    348.832k

                pure: 23585373.7 i/s
                 sig:   134303.7 i/s - 175.61x slower
           contracts:    69819.9 i/s - 337.80x slower
              rubype:    56042.1 i/s - 420.85x slower

Deactivate All Signature Checking

require 'sig/none' # instead of require 'sig'

Alternatives for Type Checking and More

MIT License

Copyright (C) 2015 Jan Lelis Released under the MIT license.