Summary

methodchain - ruby helpers for method chaining: chain, tap, then, else, and, or

Easy ways to navigate around nil without creating local variables.

Initial blog post describing previous ideas: blog.thoughtfolder.com/2008-03-16-navigating-nil-method-chaining-in-ruby.html

Author and License

Copyright © 2008 Greg Weber, gregweber.info Licensed under the MIT license

Examples

##then and ##else

old way

person = nil
name = person ? person.name : nil

new way

name = person.then {|p| p.name}
# or
name = person.then {name}

not a huge savings. But sometimes the person variable is actually a function call, and then we must save it in a variable first.

old way

def find(*args)
  # do some expensive database queries
end

person = find(:first)
@phone = person && person.phone   # => nil

new way

@phone = find(:first).then {phone}   # => nil

We have reduced a line of code and removed a local variable. ##else is the opposite of #then, and the two methods can be used together

'a'.then{'b'} #=> 'b'
nil.then{'b'}.else{'c'} #=> 'c'

message sending

The normal conditional for ##then and ##else is self

if self # inside MethodChain#then
  # evaluate block
end

##then and ##else allow message sending as the conditional. See more examples of message sending with the MethodChain#chain examples below

"not empty".then(:empty?) {"N/A"} # => "not empty"
         "".then(:empty?) {"N/A"} # => "N/A"

##and, ##or

old way

Return a default value or the original value depending on whether multiple conditions are met

Person = Struct.new(:phone )
blank = Person.new('') # or {:phone => nil}
blank.phone && (not blank.phone.empty?) ? blank.phone : "N/A" # => "N/A"
p = Person.new('123')
p.phone && (not p.phone.empty?) ? p.phone : "N/A" # => "123"

new way

blank.phone.and {not empty?} || "N/A" # => "N/A"
p.phone.and {not empty?} || "N/A" # => "123"

##tap

if you don’t already know about this method, look it up on the net. The tap included here allows message sending.

old way

arr = [1]
arr.compact! # => nil
arr.first # => 1

normal ##tap (still valid)

[1].tap {|arr| arr.compact!}.first # => 1

new ##tap

[1].tap(:compact!).first # => 1

normal ##tap (still valid)

[1].tap {|arr| arr.compact!}.tap {|arr| arr * 2}.first # => 1

new ##tap

[1].tap( :compact!, [:*, 2] ).first # => 1

You can also pass Procs as arguments

[1].tap( :compact!, lambda{|arr| arr * 2} ).first # => 1

##chain

chain is like tap, but instead of always returning self, it will return the result of the method call.

[1].chain(:first) == [1].first

But there is an important difference- chain guards against certain results (by default it guards against nil and false)

old way

customer = nil
customer && customer.order && customer.order.id

new way

customer.chain(:order, :id)

note that this is equivalent to

customer.then {order}.then {id}

##chain - Custom guards, multiple arguments, and Procs

old way - guarding against zero

value = 0

result = if value == 0 then value else
  tmp = value.abs

  if tmp == 0 then tmp else
    tmp * 20
  end
end
result # => 0

new way

value.chain(:abs, [:*, 20]) {|s| s == 0 }   # => 0

Procs can be used, so this is equivalent to

value.chain(:abs, lambda {|n| n * 20 }) {|s| s == 0 } # => 0

Usage

require 'rubygems'

import all MethodChain methods into Object

require 'methodchain'

selectively import MethodChain methods

require 'methodchain/not-included'

You can then include methodchain into selected classes, or you can use the module-import gem to include only certain methods

gem install module-import

require 'module-import'
class Object
  import MethodChain, :chain  # I only want Object#chain
end

import will still load all the private methods from the module:

• yield_or_eval

• send_as_function

• send_as_functions

Implementation

There are no proxy objects and no use of method_missing- these are simply function calls, so it should be fast.

private methods:

• yield_or_eval: allows the two different block forms {|p| p.name} and name, where the first form yields self and the second form is called using instance_eval.

• send_as_function: allows symbols and arrays to be sent as messages, and calls yield_or_eval on Proc arguments

• send_as_functions:

  def send_arguments_as_functions *args

  args.each {|arg| send_as_function arg}
  self
  

  end

Install

gem install methodchain

Source

browser

github.com/gregwebs/methodchain/tree/master

repository

git clone git://github.com/gregwebs/methodchain.git

Homepage

gregweber.info/projects/methodchain.html

RDoc documentation

included with gem