callables in specs

callables_spec.rb

require 'spec_helper'

module CallableSubject
  def callable_subject(&_block)
    subject { -> { yield } }
  end
end

RSpec.configure do |c|
  c.extend(CallableSubject)
end

RSpec.describe 'callable subject' do
  class Callable
    def initialize(shall_raise = false)
      @shall_raise = shall_raise
    end

    def call
      shall_raise? ? raise('oops') : 'okay'
    end

    private def shall_raise?
      @shall_raise
    end
  end

  let(:callable) { Callable.new(true) }

  context 'official approach (lambda)' do
    subject { -> { callable.call } }

    it { is_expected.to raise_error(RuntimeError) }
  end

  context 'callable.method(:call)' do
    subject { callable.method(:call).to_proc }

    it { is_expected.to raise_error(RuntimeError) }
  end

  context 'custom callable_subject helper' do
    # needs to be tweaked to get the `let` vars reachable
    callable_subject { Callable.new(true).call }

    it { is_expected.to raise_error(RuntimeError) }
  end
end