Ruby objects with a telephone

What is a callable object?

Anonymous function is the one that is not bound to an identifier.

block

{    }

do/end

BLOCKS (1/3)

BLOCKS (2/3)

Wikipedia describes blocks as anonymous functions.

Block is not a function, it is a complementary method syntax.

BLOCKs (3/3)

To execute a block from within a method, you use yield keyword.

It is not possible to #call a block!

Syntax:

• {}     - one-liners

• do/end - multi-liners

PROCs (1/3)

Proc class

procs (2/3)

• language construct for using closures

• constructors:

  • Proc.new {}

  • proc {} - preferred by Ruby style guide

procs (3/3)

• pass arguments through 'pipes'

  • proc { |arg1, arg2| p arg1, arg2 }

• will "answer" when called with:

  • #call() - OK

  • #[]     - NOT OK

  • #.()    - HORRIBLE

lambdas

• special, much stricter, instance of a Proc class

• constructors and parameters:

  • lambda { |arg1, arg2| }

  • ->(arg1, arg2) {}

• #call() it

• Proc#lambda?

#funfact -'proc' and 'lambda' are not keywords but Kernel methods.

The difference(s)

Lambdas behave like methods.

usage of '&' (1/2)

• inside a method definition:

  • denotes that identifier it binds is not an argument, but a proc object

  • def foo(a, b, &block)

• on a callers side:

  • array.map(&:map)

  • invokes #to_proc on a object passed

Still confused?! It's OK..

usage of '&' (2/2)

1. STEP foo { puts "Within a block!" }

2. STEP proc { puts "Within a block!" }

3. STEP def foo(&block)
          block.call
        end

rails usage examples (1/3)

In the wild, Ruby can even become "JQuerish".

RAILS usage examples (2/3)

Doing some caching.

RAILS usage examples (3/3)

You are probably using it already.

methods as objects

Methods can become object as well:

m = bar.method(:foo) # m is a method #foo
m.call               # callable object

resources

• David A. Black: the Well Grounded Rubyist, 2nd edition

• BLACKBYTE blog post

• Jim Weirich explains the difference between procs and lambdas (gist by Mislav Marohnic)

• Ruby Koans

• The Ruby book - callable objects