Add nicer API for be_sorted

Tests are added for this matcher, using the fast_spec_helper

spec/support/matchers/be_sorted.rb

@@ -4,18 +4,75 @@
 #
 # By default, this checks that the collection is sorted ascending
 # but you can check order by specific field and order by passing
-# them, eg:
+# them, either as arguments, or using the fluent interface, eg:
 #
 # ```
+# # Usage examples:
+# expect(collection).to be_sorted
+# expect(collection).to be_sorted(:field)
 # expect(collection).to be_sorted(:field, :desc)
+# expect(collection).to be_sorted.asc
+# expect(collection).to be_sorted.desc.by(&:field)
+# expect(collection).to be_sorted.by(&:field).desc
+# expect(collection).to be_sorted.by { |x| [x.foo, x.bar] }
 # ```
-RSpec::Matchers.define :be_sorted do |by, order = :asc|
+RSpec::Matchers.define :be_sorted do |on = :itself, order = :asc|
+  def by(&block)
+    @comparator = block
+    self
+  end
+
+  def asc
+    @direction = :asc
+    self
+  end
+
+  def desc
+    @direction = :desc
+    self
+  end
+
+  def format_with(proc)
+    @format_with = proc
+    self
+  end
+
+  define_method :comparator do
+    @comparator || on
+  end
+
+  define_method :descending? do
+    (@direction || order.to_sym) == :desc
+  end
+
+  def order(items)
+    descending? ? items.reverse : items
+  end
+
+  def sort(items)
+    items.sort_by(&comparator)
+  end
+
   match do |actual|
-    next true unless actual.present? # emtpy collection is sorted
+    next true unless actual.present? # empty collection is sorted
+
+    actual = actual.to_a if actual.respond_to?(:to_a) && !actual.respond_to?(:sort_by)
+
+    @got = actual
+    @expected = order(sort(actual))
 
-    actual
-      .then { |collection| by ? collection.sort_by(&by) : collection.sort }
-      .then { |sorted_collection| order.to_sym == :desc ? sorted_collection.reverse : sorted_collection }
-      .then { |sorted_collection| sorted_collection == actual }
+    @expected == actual
+  end
+
+  def failure_message
+    "Expected #{show(@expected)}, got #{show(@got)}"
+  end
+
+  def show(things)
+    if @format_with
+      things.map(&@format_with)
+    else
+      things
+    end
   end
 end

spec/support_specs/matchers/be_sorted_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+load File.expand_path('../../../spec/support/matchers/be_sorted.rb', __dir__)
+
+RSpec.describe 'be_sorted' do
+  it 'matches empty collections, regardless of arguments' do
+    expect([])
+      .to be_sorted
+      .and be_sorted.asc
+      .and be_sorted.desc
+      .and be_sorted(:foo)
+      .and be_sorted(:bar)
+
+    expect([].to_set).to be_sorted
+    expect({}).to be_sorted
+  end
+
+  it 'matches in both directions' do
+    expect([1, 2, 3]).to be_sorted.asc
+    expect([3, 2, 1]).to be_sorted.desc
+  end
+
+  it 'can match on a projection' do
+    xs = [['a', 10], ['b', 7], ['c', 4]]
+
+    expect(xs).to be_sorted.asc.by(&:first)
+    expect(xs).to be_sorted(:first, :asc)
+    expect(xs).to be_sorted.desc.by(&:second)
+    expect(xs).to be_sorted(:second, :desc)
+  end
+end