diff --git a/middleman-core/lib/middleman-core/application.rb b/middleman-core/lib/middleman-core/application.rb
index fd072d57..1121bf23 100644
--- a/middleman-core/lib/middleman-core/application.rb
+++ b/middleman-core/lib/middleman-core/application.rb
@@ -15,6 +15,8 @@ require 'hooks'
 # Our custom logger
 require 'middleman-core/logger'
 
+require 'middleman-core/contracts'
+
 require 'middleman-core/sitemap/store'
 
 require 'middleman-core/configuration'
@@ -30,6 +32,7 @@ require 'middleman-core/template_renderer'
 module Middleman
   class Application
     extend Forwardable
+    include Contracts
 
     class << self
       # Global configuration for the whole Middleman project.
@@ -165,7 +168,11 @@ module Middleman
     attr_reader :config
     attr_reader :generic_template_context
     attr_reader :extensions
+
+    Contract None => SetOf['Middleman::Application::MiddlewareDescriptor']
     attr_reader :middleware
+
+    Contract None => SetOf['Middleman::Application::MapDescriptor']
     attr_reader :mappings
 
     # Reference to Logger singleton
@@ -179,8 +186,8 @@ module Middleman
       # Search the root of the project for required files
       $LOAD_PATH.unshift(root) unless $LOAD_PATH.include?(root)
 
-      @middleware = []
-      @mappings = []
+      @middleware = Set.new
+      @mappings = Set.new
 
       @template_context_class = Class.new(Middleman::TemplateContext)
       @generic_template_context = @template_context_class.new(self)
@@ -296,20 +303,26 @@ module Middleman
       File.join(root, config[:source])
     end
 
+    MiddlewareDescriptor = Struct.new(:class, :options, :block)
+
     # Use Rack middleware
     #
     # @param [Class] middleware Middleware module
     # @return [void]
+    Contract Any, Args[Any], Proc => Any
     def use(middleware, *args, &block)
-      @middleware << [middleware, args, block]
+      @middleware << MiddlewareDescriptor.new(middleware, args, block)
     end
 
+    MapDescriptor = Struct.new(:path, :block)
+
     # Add Rack App mapped to specific path
     #
     # @param [String] map Path to map
     # @return [void]
+    Contract String, Proc => Any
     def map(map, &block)
-      @mappings << [map, block]
+      @mappings << MapDescriptor.new(map, block)
     end
 
     # Work around this bug: http://bugs.ruby-lang.org/issues/4521
diff --git a/middleman-core/lib/middleman-core/contracts.rb b/middleman-core/lib/middleman-core/contracts.rb
index 3eec15d2..2f13bc1c 100644
--- a/middleman-core/lib/middleman-core/contracts.rb
+++ b/middleman-core/lib/middleman-core/contracts.rb
@@ -1,22 +1,56 @@
 if ENV['TEST'] || ENV['CONTRACTS'] == 'true'
   require 'contracts'
 
-  class IsA
-    def self.[](val)
-      @lookup ||= {}
-      @lookup[val] ||= new(val)
+  module Contracts
+    class IsA
+      def self.[](val)
+        @lookup ||= {}
+        @lookup[val] ||= new(val)
+      end
+
+      def initialize(val)
+        @val = val
+      end
+
+      def valid?(val)
+        val.is_a? @val.constantize
+      end
     end
 
-    def initialize(val)
-      @val = val
+    class ArrayOf
+      def initialize(contract)
+        @contract = contract.is_a?(String) ? IsA[contract] : contract
+      end
     end
 
-    def valid?(val)
-      val.is_a? @val.constantize
+    class SetOf < CallableClass
+      def initialize(contract)
+        @contract = contract.is_a?(String) ? IsA[contract] : contract
+      end
+
+      def valid?(vals)
+        return false unless vals.is_a?(Set)
+        vals.all? do |val|
+          res, _ = Contract.valid?(val, @contract)
+          res
+        end
+      end
+
+      def to_s
+        "a set of #{@contract}"
+      end
+
+      def testable?
+        Testable.testable? @contract
+      end
+
+      def test_data
+        Set.new([], [Testable.test_data(@contract)], [Testable.test_data(@contract), Testable.test_data(@contract)])
+      end
     end
+
+    ResourceList = Contracts::ArrayOf[IsA['Middleman::Sitemap::Resource']]
   end
-
-  ResourceList = Contracts::ArrayOf[IsA['Middleman::Sitemap::Resource']]
 else
   module Contracts
     def self.included(base)
@@ -91,5 +125,12 @@ else
 
     class IsA < Callable
     end
+
+    class SetOf < Callable
+    end
   end
 end
+
+module Contracts
+  PATH_MATCHER = Or[String, RespondTo[:match], RespondTo[:call], RespondTo[:to_s]]
+end
diff --git a/middleman-core/lib/middleman-core/core_extensions/default_helpers.rb b/middleman-core/lib/middleman-core/core_extensions/default_helpers.rb
index 99120528..fa359a77 100644
--- a/middleman-core/lib/middleman-core/core_extensions/default_helpers.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/default_helpers.rb
@@ -140,7 +140,7 @@ class Middleman::CoreExtensions::DefaultHelpers < ::Middleman::Extension
       path << index_file if path.end_with?('/')
       path = ::Middleman::Util.strip_leading_slash(path)
 
-      classes = []
+      classes = Set.new
       parts = path.split('.').first.split('/')
       parts.each_with_index { |_, i| classes << parts.first(i + 1).join('_') }
 
diff --git a/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb b/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb
index 68a83280..fca753ab 100644
--- a/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/file_watcher.rb
@@ -55,28 +55,30 @@ module Middleman
           @app = app
           @known_paths = Set.new
 
-          @_changed = []
-          @_deleted = []
+          @on_change_callbacks = Set.new
+          @on_delete_callbacks = Set.new
         end
 
+        CallbackDescriptor = Struct.new(:proc, :matcher)
+
         # Add callback to be run on file change
         #
         # @param [nil,Regexp] matcher A Regexp to match the change path against
         # @return [Array<Proc>]
-        Contract Or[Regexp, Proc] => ArrayOf[ArrayOf[Or[Proc, Regexp, nil]]]
+        Contract Or[Regexp, Proc] => SetOf['Middleman::CoreExtensions::FileWatcher::API::CallbackDescriptor']
         def changed(matcher=nil, &block)
-          @_changed << [block, matcher] if block_given?
-          @_changed
+          @on_change_callbacks << CallbackDescriptor.new(block, matcher) if block_given?
+          @on_change_callbacks
         end
 
         # Add callback to be run on file deletion
         #
         # @param [nil,Regexp] matcher A Regexp to match the deleted path against
         # @return [Array<Proc>]
-        Contract Or[Regexp, Proc] => ArrayOf[ArrayOf[Or[Proc, Regexp, nil]]]
+        Contract Or[Regexp, Proc] => SetOf['Middleman::CoreExtensions::FileWatcher::API::CallbackDescriptor']
         def deleted(matcher=nil, &block)
-          @_deleted << [block, matcher] if block_given?
-          @_deleted
+          @on_delete_callbacks << CallbackDescriptor.new(block, matcher) if block_given?
+          @on_delete_callbacks
         end
 
         # Notify callbacks that a file changed
@@ -159,9 +161,9 @@ module Middleman
         # @return [void]
         def run_callbacks(path, callbacks_name)
           path = path.to_s
-          send(callbacks_name).each do |callback, matcher|
-            next unless matcher.nil? || path.match(matcher)
-            @app.instance_exec(path, &callback)
+          send(callbacks_name).each do |callback|
+            next unless callback[:matcher].nil? || path.match(callback[:matcher])
+            @app.instance_exec(path, &callback[:proc])
           end
         end
       end
diff --git a/middleman-core/lib/middleman-core/core_extensions/routing.rb b/middleman-core/lib/middleman-core/core_extensions/routing.rb
index 61e80028..2bf11eea 100644
--- a/middleman-core/lib/middleman-core/core_extensions/routing.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/routing.rb
@@ -9,7 +9,7 @@ module Middleman
       def initialize(app, options_hash={}, &block)
         super
 
-        @page_configs = []
+        @page_configs = Set.new
       end
 
       def before_configuration
@@ -20,12 +20,14 @@ module Middleman
       Contract ResourceList => ResourceList
       def manipulate_resource_list(resources)
         resources.each do |resource|
-          @page_configs.each do |matcher, metadata|
-            resource.add_metadata(metadata) if Middleman::Util.path_match(matcher, "/#{resource.path}")
+          @page_configs.each do |p|
+            resource.add_metadata(p[:metadata]) if Middleman::Util.path_match(p[:path], "/#{resource.path}")
           end
         end
       end
 
+      PageDescriptor = Struct.new(:path, :metadata)
+
       # The page method allows options to be set for a given source path, regex, or glob.
       # Options that may be set include layout, locals, proxy, andx ignore.
       #
@@ -43,6 +45,7 @@ module Middleman
       # @option opts [Hash] locals Local variables for the template. These will be available when the template renders.
       # @option opts [Hash] data Extra metadata to add to the page. This is the same as frontmatter, though frontmatter will take precedence over metadata defined here. Available via {Resource#data}.
       # @return [void]
+      Contract String, Hash => Any
       def page(path, opts={})
         options = opts.dup
 
@@ -63,7 +66,7 @@ module Middleman
 
         path = '/' + Util.strip_leading_slash(path) if path.is_a?(String)
 
-        @page_configs << [path, metadata]
+        @page_configs << PageDescriptor.new(path, metadata)
       end
     end
   end
diff --git a/middleman-core/lib/middleman-core/extensions.rb b/middleman-core/lib/middleman-core/extensions.rb
index 37690957..77c99a64 100644
--- a/middleman-core/lib/middleman-core/extensions.rb
+++ b/middleman-core/lib/middleman-core/extensions.rb
@@ -9,9 +9,9 @@ module Middleman
     @registered = {}
     @auto_activate = {
       # Activate before the Sitemap is instantiated
-      before_sitemap: [],
+      before_sitemap: Set.new,
       # Activate the extension before `config.rb` and the `:before_configuration` hook.
-      before_configuration: []
+      before_configuration: Set.new
     }
 
     AutoActivation = Struct.new(:name, :modes)
diff --git a/middleman-core/lib/middleman-core/extensions/minify_css.rb b/middleman-core/lib/middleman-core/extensions/minify_css.rb
index 08a28d95..75b2ca4b 100644
--- a/middleman-core/lib/middleman-core/extensions/minify_css.rb
+++ b/middleman-core/lib/middleman-core/extensions/minify_css.rb
@@ -32,6 +32,11 @@ class Middleman::Extensions::MinifyCss < ::Middleman::Extension
     # Init
     # @param [Class] app
     # @param [Hash] options
+    Contract RespondTo[:call], ({
+      ignore: ArrayOf[PATH_MATCHER],
+      inline: Bool,
+      compressor: Or[Proc, RespondTo[:to_proc], RespondTo[:compress]]
+    }) => Any
     def initialize(app, options={})
       @app = app
       @ignore = options.fetch(:ignore)
diff --git a/middleman-core/lib/middleman-core/extensions/minify_javascript.rb b/middleman-core/lib/middleman-core/extensions/minify_javascript.rb
index 208b4fda..5c90bd13 100644
--- a/middleman-core/lib/middleman-core/extensions/minify_javascript.rb
+++ b/middleman-core/lib/middleman-core/extensions/minify_javascript.rb
@@ -23,6 +23,11 @@ class Middleman::Extensions::MinifyJavascript < ::Middleman::Extension
     # Init
     # @param [Class] app
     # @param [Hash] options
+    Contract RespondTo[:call], ({
+      ignore: ArrayOf[PATH_MATCHER],
+      inline: Bool,
+      compressor: Or[Proc, RespondTo[:to_proc], RespondTo[:compress]]
+    }) => Any
     def initialize(app, options={})
       @app = app
       @ignore = options.fetch(:ignore)
diff --git a/middleman-core/lib/middleman-core/middleware/inline_url_rewriter.rb b/middleman-core/lib/middleman-core/middleware/inline_url_rewriter.rb
index 094a9d9c..1151800c 100644
--- a/middleman-core/lib/middleman-core/middleware/inline_url_rewriter.rb
+++ b/middleman-core/lib/middleman-core/middleware/inline_url_rewriter.rb
@@ -8,21 +8,31 @@ module Middleman
     class InlineURLRewriter
       include Contracts
 
+      IGNORE_DESCRIPTOR = Or[Regexp, RespondTo[:call], String]
+
+      Contract RespondTo[:call], ({
+        middleman_app: IsA['Middleman::Application'],
+        id: Maybe[Symbol],
+        proc: Or[Proc, Method],
+        url_extensions: ArrayOf[String],
+        source_extensions: ArrayOf[String],
+        ignore: ArrayOf[IGNORE_DESCRIPTOR]
+      }) => Any
       def initialize(app, options={})
         @rack_app = app
-        @middleman_app = options[:middleman_app]
+        @middleman_app = options.fetch(:middleman_app)
 
-        @uid = options[:id]
-        @proc = options[:proc]
+        @uid = options.fetch(:id, nil)
+        @proc = options.fetch(:proc)
 
         raise 'InlineURLRewriter requires a :proc to call with inline URL results' unless @proc
 
-        @exts = options[:url_extensions]
+        @exts = options.fetch(:url_extensions)
 
-        @source_exts = options[:source_extensions]
+        @source_exts = options.fetch(:source_extensions)
         @source_exts_regex_text = Regexp.union(@source_exts).to_s
 
-        @ignore = options[:ignore]
+        @ignore = options.fetch(:ignore)
       end
 
       def call(env)
@@ -66,7 +76,7 @@ module Middleman
         [status, headers, response]
       end
 
-      Contract Or[Regexp, RespondTo[:call], String] => Bool
+      Contract IGNORE_DESCRIPTOR => Bool
       def should_ignore?(validator, value)
         if validator.is_a? Regexp
           # Treat as Regexp
diff --git a/middleman-core/lib/middleman-core/rack.rb b/middleman-core/lib/middleman-core/rack.rb
index 3d540685..cd80f61b 100644
--- a/middleman-core/lib/middleman-core/rack.rb
+++ b/middleman-core/lib/middleman-core/rack.rb
@@ -23,15 +23,15 @@ module Middleman
       app.use ::Rack::Lint
       app.use ::Rack::Head
 
-      @middleman.middleware.each do |klass, options, middleware_block|
-        app.use(klass, *options, &middleware_block)
+      @middleman.middleware.each do |middleware|
+        app.use(middleware[:class], *middleware[:options], &middleware[:block])
       end
 
       inner_app = self
       app.map('/') { run inner_app }
 
-      @middleman.mappings.each do |path, map_block|
-        app.map(path, &map_block)
+      @middleman.mappings.each do |mapping|
+        app.map(mapping[:path], &mapping[:block])
       end
 
       app
diff --git a/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb b/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb
index 7ac26615..565c3151 100644
--- a/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb
+++ b/middleman-core/lib/middleman-core/sitemap/extensions/ignores.rb
@@ -10,7 +10,7 @@ module Middleman
           @app.define_singleton_method :ignore, &method(:create_ignore)
 
           # Array of callbacks which can ass ignored
-          @ignored_callbacks = []
+          @ignored_callbacks = Set.new
 
           @app.sitemap.define_singleton_method :ignored?, &method(:ignored?)
         end
diff --git a/middleman-core/lib/middleman-core/sitemap/resource.rb b/middleman-core/lib/middleman-core/sitemap/resource.rb
index eee2068a..add3919d 100644
--- a/middleman-core/lib/middleman-core/sitemap/resource.rb
+++ b/middleman-core/lib/middleman-core/sitemap/resource.rb
@@ -30,8 +30,11 @@ module Middleman
       # @return [String]
       alias_method :request_path, :destination_path
 
+      METADATA_HASH = ({ options: Maybe[Hash], locals: Maybe[Hash], page: Maybe[Hash] })
+
       # The metadata for this resource
       # @return [Hash]
+      Contract None => METADATA_HASH
       attr_reader :metadata
 
       # Initialize resource with parent store and URL
@@ -66,7 +69,7 @@ module Middleman
       #   Locals are local variables for rendering this resource's template
       #   Page are data that is exposed through this resource's data member.
       #   Note: It is named 'page' for backwards compatibility with older MM.
-      Contract Hash => Hash
+      Contract METADATA_HASH => METADATA_HASH
       def add_metadata(meta={})
         @metadata.deep_merge!(meta)
       end
diff --git a/middleman-core/lib/middleman-core/util.rb b/middleman-core/lib/middleman-core/util.rb
index b98994d0..36dd881d 100644
--- a/middleman-core/lib/middleman-core/util.rb
+++ b/middleman-core/lib/middleman-core/util.rb
@@ -124,7 +124,7 @@ module Middleman
     # @param [String, #match, #call] matcher A matcher String, RegExp, Proc, etc.
     # @param [String] path A path as a string
     # @return [Boolean] Whether the path matches the matcher
-    Contract Or[String, RespondTo[:match], RespondTo[:call], RespondTo[:to_s]], String => Bool
+    Contract PATH_MATCHER, String => Bool
     def self.path_match(matcher, path)
       case
       when matcher.is_a?(String)