Add custom matchers for rspec

[alessandro-molari]

This is a basic example for Failure and Success monads.

RSpec::Matchers.define :be_success do |expected|
  if expected.nil?
    match { |actual| actual.success? }
    failure_message { |actual| "Actual `#{actual}` isn't a success." }
    failure_message_when_negated { |actual| "Actual `#{actual}` is a success." }
  elsif expected.is_a?(Class)
    match do |actual|
      if actual.value.is_a?(Class)
        actual.success? && actual.value == expected
      else
        actual.success? && actual.value.is_a?(expected)
      end
    end
    failure_message do |actual|
      if actual.value.is_a?(Class)
        "The success value `#{actual.value}` isn't same class of `#{expected}`."
      else
        "The success value `#{actual.value}` isn't instance of `#{expected}`."
      end
    end
    failure_message_when_negated do |actual|
      if actual.value.is_a?(Class)
        "The success value `#{actual.value}` is same class of `#{expected}`."
      else
        "The success value `#{actual.value}` is instance of `#{expected}`."
      end
    end
  elsif expected.is_a?(Regexp)
    match { |actual| actual.success? && actual.value =~ expected }
    failure_message do |actual|
      "The success value `#{actual.value}` doesn't match regex `#{expected}`."
    end
    failure_message_when_negated do |actual|
      "The success value `#{actual.value}` matches regex `#{expected}`."
    end
  elsif expected.respond_to?(:==)
    match { |actual| actual.success? && actual.value == expected }
    failure_message do |actual|
      "The success value `#{actual.value}` isn't equal to `#{expected}`."
    end
    failure_message_when_negated do |actual|
      "The success value `#{actual.value}` is equal to `#{expected}`."
    end
  else
    fail "Cannot handle expected: `#{expected}`."
  end
end

RSpec::Matchers.define :be_failure do |expected|
  if expected.nil?
    match { |actual| actual.failure? }
    failure_message { |actual| "Actual `#{actual}` isn't a failure." }
    failure_message_when_negated { |actual| "Actual `#{actual}` is a failure." }
  elsif expected.is_a?(Class)
    match do |actual|
      if actual.value.is_a?(Class)
        actual.success? && actual.value == expected
      else
        actual.failure? && actual.value.is_a?(expected)
      end
    end
    failure_message do |actual|
      if actual.value.is_a?(Class)
        "The failure value `#{actual.value}` isn't same class of `#{expected}`."
      else
        "The failure value `#{actual.value}` isn't instance of `#{expected}`."
      end
    end
    failure_message_when_negated do |actual|
      if actual.value.is_a?(Class)
        "The failure value `#{actual.value}` is same class of `#{expected}`."
      else
        "The failure value `#{actual.value}` is instance of `#{expected}`."
      end
    end
  elsif expected.is_a?(Regexp)
    match { |actual| actual.failure? && actual.value =~ expected }
    failure_message do |actual|
      "The failure value `#{actual.value}` doesn't match regex `#{expected}`."
    end
    failure_message_when_negated do |actual|
      "The failure value `#{actual.value}` matches regex `#{expected}`."
    end
  elsif expected.respond_to?(:==)
    match { |actual| actual.failure? && actual.value == expected }
    failure_message do |actual|
      "The failure value `#{actual.value}` isn't equal to `#{expected}`."
    end
    failure_message_when_negated do |actual|
      "The failure value `#{actual.value}` is equal to `#{expected}`."
    end
  else
    fail "Cannot handle expected: `#{expected}`."
  end
end

[pawelniewie]

@alem0lars thanks for sharing!