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Project Notes

Proc v Lambda

Exploring differences and similarities between procs and lambdas in Ruby.


Ruby gives you all these ways of creating closures: Proc, proc, lambda ->.


  • these are all basically the same, but with some subtle differences that one is best advised to avoid making code dependent upon.
  • when in doubt, just use lambda (multi-line) or stabby -> (single line).

Language Feature Definitions

  • Proc - A Proc object is an encapsulation of a block of code, which can be stored in a local variable, passed to a method or another Proc, and can be called
  • proc - kernel method Equivalent to
  • lambda - Equivalent to, except the resulting Proc objects check the number of parameters passed when called.
  • -> - A proc can be created with -> (it is in fact a lambda-style proc)

Technical Diffferences

Procs and lambdas both return Proc objects, but they have different “lambda-ness”!! The lambda-ness affects argument handling and the behavior of return and break.

proc_handle = proc { puts "i'm a proc" }
lambda_handle = lambda { puts "i'm a lambda-style proc" }
assert_equal proc_handle.class, lambda_handle.class
assert_equal false, proc_handle.lambda?
assert_equal true, lambda_handle.lambda?

Difference 1: arguments

Lambdas check the number of arguments, while procs do not

Difference 2: return context

Procs return from the current method, while lambdas return from the lambda itself:

  • return inside a lambda returns to the calling context where the lambda was executed
  • return inside a proc return from the calling context where the proc was executed

NB: break has the same effect is applicable

Best Practices in a Nutshell

Stabby Lambda Definition with Parameters Use parens:

l = ->(x, y) { something(x, y) }

Stabby Lambda Definition without Parameters Don’t use parens:

l = -> { something }

Multi-line Lambda Definition Use lambda method instead of stabby lambda ->:

l = lambda do |a, b|
  tmp = a * 7
  tmp * b / 50

proc vs Prefer proc over

p = proc { |n| puts n }

Proc Call Prefer over proc[] or proc.() for both lambdas and procs.

l = ->(v) { puts v }

Example Code

The examples.rb file wraps up demonstrations of all these features in a set of tests. Not very exciting to run!

$ ruby examples.rb
Run options: --seed 46371

# Running:


Finished in 0.001421s, 8444.7566 runs/s, 16185.7834 assertions/s.

12 runs, 23 assertions, 0 failures, 0 errors, 0 skips

Credits and References

About LCK#169 rubyclosure
Project Source on GitHub Return to the Project Catalog

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