diff --git a/lib/rackstash/filter_chain.rb b/lib/rackstash/filter_chain.rb
new file mode 100644
index 0000000..abaea8b
--- /dev/null
+++ b/lib/rackstash/filter_chain.rb
@@ -0,0 +1,281 @@
+# frozen_string_literal: true
+
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'monitor'
+
+module Rackstash
+  # The FilterChain contains a list of event filters which are used by a {Flow}
+  # to mutate an event before it is envoded and sent to the adapter for writing.
+  #
+  # A filter is any object responding to `call`, e.g. one of the {Filters} or a
+  # `Proc` object. When running the filters, we call each filter iin turn,
+  # passing the event. The filter can change the event in any way desired but
+  # should make sure that it preserves the basic structure of the event:
+  #
+  # * Only use basic objects: `Hash`, `Array`, `String`, `Integer`, `Float`.
+  # * Hash keys should always be strings
+  #
+  # Objects of this class are thread-save. Each method call is locked against
+  # `self`.
+  class FilterChain
+    include MonitorMixin
+
+    def initialize(filters = [])
+      mon_initialize
+
+      @filters = []
+      Array(filters).each do |filter|
+        append(filter)
+      end
+    end
+
+    # Get an existing filter at `index`.
+    #
+    # @param index [Integer, Class, String, Object] The existing filter to
+    #   fetch. It can be described in different ways: When given an `Integer`,
+    #   we expect it to be the index number; when given a `Class`, we try to
+    #   find the first filter being of that type; when given a `String`, we try
+    #   to find the first filter being of a type named like that; when given any
+    #   other object, we assume it is a filter and search for that.
+    # @return [#call, nil] The existing filter or `nil` if no existing filter
+    #   could be found for `index`
+    def [](index)
+      synchronize do
+        index = index_at(index)
+        index ? @filters[index] : nil
+      end
+    end
+
+    # Set the new filter at the given index or at the end of the chain if the
+    # value at `index` could not be found.
+    #
+    # @param index [Integer, Class, String, Object] The existing filter which
+    #   should be overwritten with `filter`. It can be described in different
+    #   ways: When given an `Integer`, we expect it to be the index number; when
+    #   given a `Class`, we try to find the first filter being of that type;
+    #   when given a `String`, we try to find the first filter being of a type
+    #   named like that; when given any other object, we assume it is a filter
+    #   and search for that.
+    # @param filter [#call, nil] the filter to set at `index`
+    # @return [#call] the given `filter`
+    def []=(index, filter)
+      raise TypeError, 'must provide a filter' unless filter.respond_to?(:call)
+
+      synchronize do
+        id = index_at(index)
+        unless id && (0..@filters.size).include?(id)
+          raise ArgumentError, "Cannot insert at index #{index.inspect}"
+        end
+
+        @filters[id] = filter
+      end
+      filter
+    end
+
+    # Adds a new filter at the end of the filter chain. You can either give a
+    # callable object (e.g. a `Proc` or one of the {Filters}) or specify the
+    # filter with a given block.
+    #
+    # @param filter [#call, nil] a filter to add. If given, this value will
+    #   take precedence to the block. If `nil`, we expect a block to be given
+    #   which we will then take as the filter.
+    # @raise [TypeError] if the given filter is not callable
+    # @return [self]
+    def append(filter = nil, &block)
+      filter ||= block
+      raise TypeError, 'must provide a filter' unless filter.respond_to?(:call)
+
+      synchronize do
+        @filters.push filter
+      end
+      self
+    end
+    alias << append
+
+    # Filter the given event by calling each defined filter with it.
+    #
+    # Each filter will be called with the current event and can manipulate it
+    # in any way. If any of the filters returns `false`, no further filter will
+    # be applied and we also return `false`. This behavior can be used by
+    # filters to cancel the writing of an individual event.
+    #
+    # @param event [Hash] an event hash, see {Sink#write} for details
+    # @return [Hash, false] the filtered event or `false` if any of the
+    #   filters returned `false`
+    def call(event)
+      each do |filter|
+        result = filter.call(event)
+        return false if result == false
+      end
+      event
+    end
+
+    # Delete an existing filter from the filter chain.
+    #
+    # @param index [Integer, Class, String, Object] The existing filter to
+    #   delete. It can be described in different ways: When given an `Integer`,
+    #   we expect it to be the index number; when given a `Class`, we try to
+    #   find the first filter being of that type; when given a `String`, we try
+    #   to find the first filter being of a type named like that; when given any
+    #   other object, we assume it is a filter and search for that.
+    # @raise [ArgumentError] if the existing filter could not be found
+    # @return [#call, nil] the deleted filter or `nil` if no filter for `index`
+    #   could be found
+    def delete(index)
+      synchronize do
+        index = index_at(index)
+        @filters.delete_at(index) if index
+      end
+    end
+
+    # Calls the given block once for each filter in `self`, passing that filter
+    # as a parameter. Concurrent changes to `self` do not affect the running
+    # enumeration.
+    #
+    # An `Enumerator` is returned if no block is given.
+    #
+    # @yield [filter] calls the given block once for each filter
+    # @yieldparam filter [#call] the yielded filter
+    # @return [Enumerator, self] Returns `self` if a block was given or an
+    #   `Enumerator` if no block was given.
+    def each
+      return enum_for(__method__) unless block_given?
+      synchronize { @filters.dup }.each do |filter|
+        yield filter
+      end
+      self
+    end
+
+    # Returns the index of the first filter in `self` matching
+    #
+    # @param filter [Integer, Class, String, Object] The existing filter to
+    #   find. It can be described in different ways: When given an `Integer`,
+    #   we expect it to be the index number; when given a `Class`, we try to
+    #   find the first filter being of that type; when given a `String`, we try
+    #   to find the first filter being of a type named like that; when given any
+    #   other object, we assume it is a filter and search for that.
+    # @return [Integer, nil] The index of the existing filter or `nil` if no
+    #   filter could be found for `index`
+    def index(filter)
+      synchronize { index_at(filter) }
+    end
+
+    # Insert a new filter after an existing filter in the filter chain.
+    #
+    # @param index [Integer, Class, String, Object] The existing filter after
+    #   which the new one should be inserted. It can be described in different
+    #   ways: When given an `Integer`, we expect it to be the index number; when
+    #   given a `Class`, we try to find the first filter being of that type;
+    #   when given a `String`, we try to find the first filter being of a type
+    #   named like that; when given any other object, we assume it is a filter
+    #   and search for that.
+    # @param filter [#call, nil] a filter to insert. If given, this value will
+    #   take precedence to the block. If `nil`, we expect a block to be given
+    #   which we will then take as the filter.
+    # @raise [TypeError] if the given filter is not callable
+    # @raise [ArgumentError] if the existing filter could not be found
+    # @return [self]
+    def insert_after(index, filter = nil, &block)
+      filter ||= block
+      raise TypeError, 'must provide a filter' unless filter.respond_to?(:call)
+
+      synchronize do
+        id = index_at(index)
+        unless id && (0...@filters.size).include?(id)
+          raise ArgumentError, "No such filter to insert after: #{index.inspect}"
+        end
+
+        @filters.insert(id + 1, filter)
+      end
+      self
+    end
+
+    # Insert a new filter before an existing filter in the filter chain.
+    #
+    # @param index [Integer, Class, String, Object] The existing filter before
+    #   which the new one should be inserted. It can be described in different
+    #   ways: When given an `Integer`, we expect it to be the index number; when
+    #   given a `Class`, we try to find the first filter being of that type;
+    #   when given a `String`, we try to find the first filter being of a type
+    #   named like that; when given any other object, we assume it is a filter
+    #   and search for that.
+    # @param filter [#call, nil] a filter to insert. If given, this value will
+    #   take precedence to the block. If `nil`, we expect a block to be given
+    #   which we will then take as the filter.
+    # @raise [TypeError] if the given filter is not callable
+    # @raise [ArgumentError] if the existing filter could not be found
+    # @return [self]
+    def insert_before(index, filter = nil, &block)
+      filter ||= block
+      raise TypeError, 'must provide a filter' unless filter.respond_to?(:call)
+
+      synchronize do
+        id = index_at(index)
+        unless id && (0...@filters.size).include?(id)
+          raise ArgumentError, "No such filter to insert before: #{index.inspect}"
+        end
+
+        @filters.insert(id, filter)
+      end
+      self
+    end
+    alias insert insert_before
+
+    # @return [String] a string representation of `self`
+    def inspect
+      id_str = Object.instance_method(:to_s).bind(self).call[2..-2]
+      "#<#{id_str} #{self}>"
+    end
+
+    # @return [Integer] the number of elements in `self`. May be zero.
+    def length
+      synchronize { @filters.length }
+    end
+    alias count length
+    alias size length
+
+    # Prepends a new filter at the beginning of the filter chain. You can either
+    # give a callable object (e.g. a `Proc` or one of the {Filters}) or specify
+    # the filter with a given block.
+    #
+    # @param filter [#call, nil] a filter to prepend. If given, this value will
+    #   take precedence to the block. If `nil`, we expect a block to be given
+    #   which we will then take as the filter.
+    # @raise [TypeError] if the given filter is not callable
+    # @return [self]
+    def unshift(filter = nil, &block)
+      filter ||= block
+      raise TypeError, 'must provide a filter' unless filter.respond_to?(:call)
+
+      synchronize do
+        @filters.unshift filter
+      end
+      self
+    end
+
+    # @return [String] an Array-compatible string representation of `self`
+    def to_s
+      @filters.to_s
+    end
+
+    private
+
+    def index_at(index)
+      case index
+      when Integer, ->(o) { o.respond_to?(:to_int) }
+        index.to_int
+      when Class
+        @filters.index { |filter| filter.is_a?(index) }
+      when String, ->(o) { o.respond_to?(:to_str) }
+        index = index.to_str
+        @filters.index { |filter| filter.class.ancestors.map(&:name).include?(index) }
+      else
+        @filters.index { |filter| filter == index }
+      end
+    end
+  end
+end
diff --git a/spec/rackstash/filter_chain_spec.rb b/spec/rackstash/filter_chain_spec.rb
new file mode 100644
index 0000000..418fe27
--- /dev/null
+++ b/spec/rackstash/filter_chain_spec.rb
@@ -0,0 +1,443 @@
+# frozen_string_literal: true
+
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'spec_helper'
+
+require 'rackstash/filter_chain'
+
+describe Rackstash::FilterChain do
+  let(:filter_chain) { described_class.new }
+
+  Struct.new('MyFilter') do
+    def call(event)
+      event
+    end
+  end
+
+  def a_filter
+    Struct::MyFilter.new
+  end
+
+  let(:filter) { a_filter }
+
+  describe '#initialize' do
+    it 'accepts a single filter' do
+      filter_chain = described_class.new(-> {})
+      expect(filter_chain.length).to eql 1
+    end
+
+    it 'accepts a list of filters' do
+      chain = described_class.new([-> {}, -> {}, -> {}])
+      expect(chain.length).to eql 3
+    end
+  end
+
+  describe '#[]' do
+    it 'returns the filter by index' do
+      filter_chain << filter
+
+      expect(filter_chain[0]).to equal filter
+      expect(filter_chain[-1]).to equal filter
+
+      expect(filter_chain[1]).to be_nil
+      expect(filter_chain[-2]).to be_nil
+    end
+
+    it 'returns the filter by class or ancestor' do
+      filter_chain << filter
+
+      expect(filter_chain[Struct::MyFilter]).to equal filter
+      expect(filter_chain[Struct]).to equal filter
+      expect(filter_chain[Integer]).to be_nil
+    end
+
+    it 'returns the filter by class or ancestor name' do
+      filter_chain << filter
+
+      expect(filter_chain['Struct::MyFilter']).to equal filter
+      expect(filter_chain['Struct']).to equal filter
+      expect(filter_chain['Integer']).to be_nil
+    end
+
+    it 'returns the filter by equivalence' do
+      filter_chain << filter
+
+      expect(filter_chain[filter]).to equal filter
+      expect(filter_chain[false]).to be_nil
+      expect(filter_chain[true]).to be_nil
+    end
+  end
+
+  describe '#[]=' do
+    before(:each) do
+      filter_chain << filter
+      filter_chain << -> {}
+    end
+
+    let(:new_filter) { -> {} }
+
+    it 'sets a filter by index' do
+      filter_chain[0] = new_filter
+      expect(filter_chain[0]).to equal new_filter
+
+      filter_chain[2] = new_filter
+      expect(filter_chain[2]).to equal new_filter
+    end
+
+    it 'sets a filter by class or ancestor' do
+      filter_chain[Proc] = new_filter
+      expect(filter_chain[1]).to equal new_filter
+
+      filter_chain[Struct] = new_filter
+      expect(filter_chain[0]).to equal new_filter
+    end
+
+    it 'sets a filter by class or ancestor name' do
+      filter_chain['Proc'] = new_filter
+      expect(filter_chain[1]).to equal new_filter
+
+      filter_chain['Struct'] = new_filter
+      expect(filter_chain[0]).to equal new_filter
+    end
+
+    it 'sets a filter by equivalence' do
+      filter_chain[filter] = new_filter
+      expect(filter_chain[0]).to equal new_filter
+    end
+
+    it 'raises an ArgumentError if the filter was not found' do
+      expect { filter_chain[false] = new_filter }.to raise_error ArgumentError
+      expect { filter_chain[nil] = new_filter }.to raise_error ArgumentError
+      expect { filter_chain['foo'] = new_filter }.to raise_error ArgumentError
+      expect { filter_chain[Class.new] = new_filter }.to raise_error ArgumentError
+      expect { filter_chain[34] = new_filter }.to raise_error ArgumentError
+    end
+
+    it 'raises a TypeError if the object is not a filter' do
+      expect { filter_chain[0] = :foo }.to raise_error TypeError
+      expect { filter_chain[0] = nil }.to raise_error TypeError
+      expect { filter_chain[0] = false }.to raise_error TypeError
+      expect { filter_chain[0] = 42 }.to raise_error TypeError
+      expect { filter_chain[0] = 'Foo' }.to raise_error TypeError
+    end
+  end
+
+  describe '#append' do
+    it 'appends a filter' do
+      filter_chain.append filter
+      expect(filter_chain[0]).to eql filter
+    end
+
+    it 'appends a block as the filter' do
+      filter_chain.append { :foo }
+      expect(filter_chain[0]).to be_instance_of(Proc)
+    end
+
+    it 'raises a TypeError if the object is not a filter' do
+      expect { filter_chain.append(:foo) }.to raise_error TypeError
+      expect { filter_chain.append(nil) }.to raise_error TypeError
+      expect { filter_chain.append(false) }.to raise_error TypeError
+      expect { filter_chain.append(42) }.to raise_error TypeError
+      expect { filter_chain.append('Foo') }.to raise_error TypeError
+    end
+
+    it 'can use #<< alias' do
+      filter_chain << filter
+      expect(filter_chain[0]).to eql filter
+    end
+  end
+
+  describe '#call' do
+    it 'calls all the filters' do
+      event = {}
+      filters = [a_filter, a_filter, a_filter]
+      filters.each do |filter|
+        filter_chain << filter
+      end
+
+      expect(filters).to all receive(:call).with(event)
+      filter_chain.call({})
+    end
+
+    it 'returns the event' do
+      event = {}
+
+      expect(filter_chain.call(event)).to equal event
+
+      filter_chain << filter
+      expect(filter_chain.call(event)).to equal event
+    end
+
+    it 'stops once a filter returns false' do
+      filters = [a_filter, a_filter, a_filter]
+      filters.each do |filter|
+        filter_chain << filter
+      end
+
+      expect(filters[0]).to receive(:call)
+      expect(filters[1]).to receive(:call).and_return(false)
+      expect(filters[2]).not_to receive(:call)
+
+      expect(filter_chain.call({})).to be false
+    end
+  end
+
+  describe '#delete' do
+    before(:each) do
+      filter_chain << -> {}
+      filter_chain << filter
+    end
+
+    it 'deletes by index' do
+      expect(filter_chain.delete(1)).to equal filter
+      expect(filter_chain.count).to eql 1
+    end
+
+    it 'deletes by class' do
+      expect(filter_chain.delete(Struct)).to equal filter
+      expect(filter_chain.count).to eql 1
+    end
+
+    it 'deletes by class name' do
+      expect(filter_chain.delete('Struct')).to equal filter
+      expect(filter_chain.count).to eql 1
+    end
+
+    it 'deletes by reference' do
+      expect(filter_chain.delete(filter)).to equal filter
+      expect(filter_chain.count).to eql 1
+    end
+
+    it 'returns nil if the filter was not found' do
+      expect(filter_chain.delete(nil)).to be_nil
+      expect(filter_chain.delete(true)).to be_nil
+      expect(filter_chain.delete(false)).to be_nil
+      expect(filter_chain.delete('Blar')).to be_nil
+      expect(filter_chain.delete(Object.new)).to be_nil
+      expect(filter_chain.delete(Class.new)).to be_nil
+
+      # at the end, all filters are still present
+      expect(filter_chain.count).to eql 2
+    end
+  end
+
+  describe '#each' do
+    it 'yields each filter' do
+      filter_chain << -> {}
+      filter_chain << filter
+
+      expect { |b| filter_chain.each(&b) }
+        .to yield_successive_args(instance_of(Proc), filter)
+    end
+
+    it 'returns the filter chain if a block was provided' do
+      filter_chain << -> {}
+      expect(filter_chain.each {}).to equal filter_chain
+    end
+
+    it 'returns an Enumerator if no block was provided' do
+      filter_chain << -> {}
+      expect(filter_chain.each).to be_instance_of Enumerator
+    end
+
+    it 'operators on a copy of the internal data' do
+      yielded = 0
+      filter_chain << -> {}
+
+      filter_chain.each do |flow|
+        yielded += 1
+        filter_chain[1] = flow
+      end
+
+      expect(yielded).to eql 1
+    end
+  end
+
+  describe '#index' do
+    it 'finds the filter index' do
+      filter_chain << filter
+
+      expect(filter_chain.index(0)).to eql 0
+      expect(filter_chain.index(Struct)).to eql 0
+      expect(filter_chain.index('Struct')).to eql 0
+      expect(filter_chain.index(filter)).to eql 0
+    end
+
+    it 'returns nil if the filter was not found' do
+      expect(filter_chain.index(nil)).to be_nil
+      expect(filter_chain.index(true)).to be_nil
+      expect(filter_chain.index(false)).to be_nil
+      expect(filter_chain.index('Blar')).to be_nil
+      expect(filter_chain.index(Object.new)).to be_nil
+      expect(filter_chain.index(Class.new)).to be_nil
+    end
+  end
+
+  describe '#insert_before' do
+    before(:each) do
+      filter_chain << -> {}
+      filter_chain << filter
+    end
+
+    let(:inserted) { -> {} }
+
+    it 'inserts before index' do
+      expect(filter_chain.insert_before(1, inserted)).to equal filter_chain
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to equal filter
+    end
+
+    it 'inserts before class' do
+      expect(filter_chain.insert_before(Struct, inserted)).to equal filter_chain
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to equal filter
+    end
+
+    it 'inserts before class name' do
+      expect(filter_chain.insert_before('Struct', inserted)).to equal filter_chain
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to equal filter
+    end
+
+    it 'inserts before reference' do
+      expect(filter_chain.insert_before(filter, inserted)).to equal filter_chain
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to equal filter
+    end
+
+    it 'raises ArgumentError if the filter was not found' do
+      expect { filter_chain.insert_before(nil, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_before(true, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_before(false, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_before('Blar', inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_before(Object.new, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_before(Class.new, inserted) }.to raise_error ArgumentError
+    end
+
+    it 'raises a TypeError if the object is not a filter' do
+      expect { filter_chain.insert_before(1, :foo) }.to raise_error TypeError
+      expect { filter_chain.insert_before(1, nil) }.to raise_error TypeError
+      expect { filter_chain.insert_before(1, false) }.to raise_error TypeError
+      expect { filter_chain.insert_before(1, 42) }.to raise_error TypeError
+      expect { filter_chain.insert_before(1, 'Foo') }.to raise_error TypeError
+    end
+  end
+
+  describe '#insert_after' do
+    before(:each) do
+      filter_chain << filter
+      filter_chain << -> {}
+    end
+
+    let(:inserted) { -> {} }
+
+    it 'inserts after index' do
+      expect(filter_chain.insert_after(0, inserted)).to equal filter_chain
+      expect(filter_chain[0]).to equal filter
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to be_instance_of(Proc)
+    end
+
+    it 'inserts after class' do
+      expect(filter_chain.insert_after(Struct, inserted)).to equal filter_chain
+      expect(filter_chain[0]).to equal filter
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to be_instance_of(Proc)
+    end
+
+    it 'inserts after class name' do
+      expect(filter_chain.insert_after('Struct', inserted)).to equal filter_chain
+      expect(filter_chain[0]).to equal filter
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to be_instance_of(Proc)
+    end
+
+    it 'inserts after reference' do
+      expect(filter_chain.insert_after(filter, inserted)).to equal filter_chain
+      expect(filter_chain[0]).to equal filter
+      expect(filter_chain[1]).to equal inserted
+      expect(filter_chain[2]).to be_instance_of(Proc)
+    end
+
+    it 'raises ArgumentError if the filter was not found' do
+      expect { filter_chain.insert_after(nil, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_after(true, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_after(false, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_after('Blar', inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_after(Object.new, inserted) }.to raise_error ArgumentError
+      expect { filter_chain.insert_after(Class.new, inserted) }.to raise_error ArgumentError
+    end
+
+    it 'raises a TypeError if the object is not a filter' do
+      expect { filter_chain.insert_after(1, :foo) }.to raise_error TypeError
+      expect { filter_chain.insert_after(1, nil) }.to raise_error TypeError
+      expect { filter_chain.insert_after(1, false) }.to raise_error TypeError
+      expect { filter_chain.insert_after(1, 42) }.to raise_error TypeError
+      expect { filter_chain.insert_after(1, 'Foo') }.to raise_error TypeError
+    end
+  end
+
+  describe '#inspect' do
+    it 'formats the object' do
+      expect(filter_chain).to receive(:to_s).and_return('["<filter>"]')
+      expect(filter_chain.inspect).to(
+        match %r{\A#<Rackstash::FilterChain:0x[a-f0-9]+ \["<filter>"\]>\z}
+      )
+    end
+  end
+
+  describe '#length' do
+    it 'returns the number of flows' do
+      expect { filter_chain << -> {} }
+        .to change { filter_chain.length }.from(0).to(1)
+    end
+
+    it 'can use size alias' do
+      expect { filter_chain << -> {} }
+        .to change { filter_chain.size }.from(0).to(1)
+    end
+
+    it 'can use count alias' do
+      expect { filter_chain << -> {} }
+        .to change { filter_chain.count }.from(0).to(1)
+    end
+  end
+
+  describe '#unshift' do
+    before(:each) do
+      filter_chain << -> {}
+    end
+
+    it 'prepends a filter' do
+      filter_chain.unshift filter
+      expect(filter_chain[0]).to eql filter
+      expect(filter_chain.size).to eql 2
+    end
+
+    it 'prepends a block as the filter' do
+      filter_chain.unshift { :foo }
+      expect(filter_chain[0]).to be_instance_of(Proc)
+      expect(filter_chain.size).to eql 2
+    end
+
+    it 'raises a TypeError if the object is not a filter' do
+      expect { filter_chain.unshift(:foo) }.to raise_error TypeError
+      expect { filter_chain.unshift(nil) }.to raise_error TypeError
+      expect { filter_chain.unshift(false) }.to raise_error TypeError
+      expect { filter_chain.unshift(42) }.to raise_error TypeError
+      expect { filter_chain.unshift('Foo') }.to raise_error TypeError
+    end
+  end
+
+  describe '#to_s' do
+    it 'returns the array representation' do
+      filter_chain << -> {}
+
+      expect(filter_chain.to_s).to eql filter_chain.each.to_a.to_s
+    end
+  end
+end