Demand (Ruby Gem)

Return a variable if it's present (and optionally of the right type), otherwise, a default or nil. Adds a top level demand() method, which replaces long lines of repetitive code to check for nil?/present?/empty?, etc., hard-to-read ternary operators (?:) and chunky if statements. Instead you can make a simple method call:

require 'demand'

x = false
y = '   '

demand(x) #=> false (that is, x)
demand(y) #=> nil
demand(y, 'Not present') #=> 'Not present'

By present here we mean that:

• The variable is not equal to nil nor responds truthfully to nil?
• If it is an Array or Hash, it isn't empty; nor does it respond truthfully to empty?
• If it is a String, it isn't empty or just whitespace, i.e. if it is .striped, it won't just be empty

If you actually want your variable to be nil (i.e. you want the default value when the variable is not nil), specify the class you're looking for as NilClass):

expected_to_be_nil = nil

demand(expected_to_be_nil, 'Not nil') #=> 'Not nil'
demand(expected_to_be_nil, 'Not nil', NilClass) #=> nil (that is, expected_to_be_nil)

If you want an Array or Hash and don't mind if an empty one is passed, just specify an empty Array or Hash as the default value.

If you specify a Class or Module in the third parameter, the variable must be of this Class or include this Module.

x = 'Hello world'
y = false

demand(x, 'Not the right type', String) #=> 'Hello world' (that is, x)
demand(y, 'Not the right type', String) #=> 'Not the right type'
demand(y, 'Not the right type', Boolean) #=> false (that is, y)

The type Boolean is also made available when using this gem (via the Boolean gem). This has the effect that true and false include Boolean, so we can check if something is_a?(Boolean) which will pass just for true and false values.

If a block is specified, this will run only if the variable passes all the conditions. The variable is yielded to the block and also still returned by the method.

x = 5

demand(x, nil, Integer) {|x| puts x * 2 } #=> returns: 5; puts: 10
demand(x, nil, String) {|x| puts 'Hello' } #=> returns nil; puts is not run

Not really recommended, but you can adjust how the demand() method works by setting Demand::OPTIONS[:yield_default] = true and Demand::OPTIONS[:return_yield] = true.

Once set, these switches change how all further calls to demand() behave. The switches are included for flexibility to developer preferences, but use with caution: things could get confusing quickly. Probably if you feel like you need these, demand() may not be the right tool.