The Final Word About final?

In this talk, Stuart looks at PHP's `final` keyword, it's impact on type-hinting and code re-use, and how the SOLID principles can be used to strike a balance between code re-use and modification.

Presented at PHP South West in Bristol on Wednesday 8th November 2017.

1. A presentation by @stuherbert
 for @GanbaroDigital The Final Word about Final? A SOLID Approach To Extensibility
2. @GanbaroDigital I’m here to talk to you about `final` in a type-hinting world.
3. @GanbaroDigital final class Bar { // ... }
4. @GanbaroDigital final class Foo { public function doAction( Bar $input ) { // ... } }
5. @GanbaroDigital final class Foo { public function doAction( Bar $input ) { // ... } }
6. @GanbaroDigital ?? ?? Do you use `final` in your own code?
7. @GanbaroDigital ?? ?? Why do you mark your classes as final?
8. @GanbaroDigital In This Talk 1. `final` and PHP type-hinting 2. An example of `final` in the wild 3. A SOLID approach
9. @GanbaroDigital In This Talk 1. `final` and PHP type-hinting 2. An example of `final` in the wild 3. A SOLID approach
10. @GanbaroDigital In This Talk 1. `final` and PHP type-hinting 2. An example of `final` in the wild 3. A SOLID approach
11. @GanbaroDigital Not everyone will agree that `final` causes problems.
12. @GanbaroDigital These are my experiences.
13. @GanbaroDigital Other opinions are out there.
14. @GanbaroDigital final & PHP Type-Hinting
15. @GanbaroDigital ?? ?? When was `final` introduced to the PHP language?
16. @GanbaroDigital `final` is a PHP keyword introduced in PHP 5.0.0. 13th July 2004.
17. @GanbaroDigital ?? ?? When was type-hinting introduced to the PHP language?
18. @GanbaroDigital class names as type-hints also introduced in PHP 5.0.0. 13th July 2004.
19. @GanbaroDigital Let’s look at classical inheritance first ... ... with type-hinting.
20. @GanbaroDigital class Foo { public function doAction( Bar $input ) { // ... } }
21. @GanbaroDigital class Foo { public function doAction( Bar $input ) { // ... } }
22. @GanbaroDigital Class Foo Method doAction(Bar $input)
23. @GanbaroDigital Class Bar Class Foo Method doAction(Bar $input)
24. @GanbaroDigital Class Bar Class Foo Method doAction(Bar $input) ✓
25. @GanbaroDigital Class Bar
26. @GanbaroDigital Class Bar Class C1 extends Bar
27. @GanbaroDigital Class Bar Class C1 extends Bar Class C2 extends Bar
28. @GanbaroDigital Class Bar Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar
29. @GanbaroDigital Class Bar Class Foo Method doAction(Bar $input)
30. @GanbaroDigital Class Bar Class Foo Method doAction(Bar $input) ✓
31. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar Class Foo Method doAction(Bar $input)
32. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar Class Foo Method doAction(Bar $input) ✓ ✓ ✓
33. @GanbaroDigital ?? ?? What changes if we mark class Bar (our method type-hint) as `final`?
34. @GanbaroDigital class Foo { public function doAction( Bar $input ) { // ... } }
35. @GanbaroDigital class Bar { // ... }
36. @GanbaroDigital final class Bar { // ... }
37. @GanbaroDigital Class Bar Class Foo Method doAction(Bar $input)
38. @GanbaroDigital Class Bar Class Foo Method doAction(Bar $input) ✓
39. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar Class Foo Method doAction(Bar $input)
40. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar Class Foo Method doAction(Bar $input) ✗ ✗ ✗
41. @GanbaroDigital ?? ?? Why won’t our method accept child classes of class Bar?
42. @GanbaroDigital Class Bar
43. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar ✗ ✗ ✗ Class Bar
44. @GanbaroDigital `final` classes cannot be extended via inheritance
45. @GanbaroDigital “ The behaviour of `final` classes can be consumed, not modified.
46. @GanbaroDigital “ `final` classes lock down type-hinted method parameters.
47. @GanbaroDigital “It’s unreasonable to expect package maintainers to know 100% of all possible variants.
48. @GanbaroDigital ?? ?? How do we workaround a `final` class in type-hints?
49. @GanbaroDigital ?? ?? What if ... we extend Foo to override the type-hint?
50. @GanbaroDigital class Foo { public function doAction( Bar $input ) { // ... } }
51. @GanbaroDigital class MyFoo extends Foo { public function doAction( Bar $input ) { // ... } }
52. @GanbaroDigital class MyFoo extends Foo { public function doAction( MyBar $input ) { // ... } }
53. @GanbaroDigital PHP Warning: Declaration of MyFoo::doAction(MyBar $input) should be compatible with Foo::doAction(Bar $input) in test.php on line 19
54. @GanbaroDigital The code runs today. Assume today’s warnings may become tomorrow’s fatal errors.
55. @GanbaroDigital
56. @GanbaroDigital class Foo { public function doAction( Bar $input ) { // ... } }
57. @GanbaroDigital final class Foo { public function doAction( Bar $input ) { // ... } }
58. @GanbaroDigital PHP Fatal error: Class MyFoo may not inherit from final class (Foo) in test.php on line 19
59. @GanbaroDigital `final` is excellent at protecting behaviour from third-party change.
60. @GanbaroDigital But when that behaviour is incomplete or outright broken ...
61. @GanbaroDigital final In The Wild
62. @GanbaroDigital UUID RFC
63. @GanbaroDigital https://wiki.php.net/rfc/uuid
64. @GanbaroDigital final class UUID { // ... }
65. @GanbaroDigital UUIDs are globally-unique, non co-ordinated values.
66. @GanbaroDigital They are useful as primary keys.
67. @GanbaroDigital function doSomething (int $id, ...) { // ... }
68. @GanbaroDigital function doSomething (UUID $id, ...) { // ... }
69. @GanbaroDigital ?? ?? What if we want to abstract away our ID scheme?
70. @GanbaroDigital function doSomething (UUID $id, ...) { // ... }
71. @GanbaroDigital function doSomething (MyID $id, ...) { // ... }
72. @GanbaroDigital class MyID extends UUID { // ... }
73. @GanbaroDigital class MyID extends UUID { // ... } ✗
74. @GanbaroDigital `final` class UUID prevents us doing so via inheritance.
75. @GanbaroDigital ?? ?? How could we work around this?
76. @GanbaroDigital “ Use the decorator pattern for abstracting underlying behaviour.
77. @GanbaroDigital class MyID extends UUID { // ... }
78. @GanbaroDigital class MyID { private $uuid; public function __toString( return (string)$this->uuid; ); }
79. @GanbaroDigital class MyID { private $uuid; public function __toString( return (string)$this->uuid; ); }
80. @GanbaroDigital class MyID { private $uuid; public function __toString( return (string)$this->uuid; ); }
81. @GanbaroDigital UUIDs can be used as primary keys.
82. @GanbaroDigital UUIDs perform poorly as primary keys in popular database engines.
83. @GanbaroDigital The solution: CombGUIDs
84. @GanbaroDigital Jimmy Nelson, The Cost of GUIDs as Primary Keys, March 8th, 2002 http://www.informit.com/articles/ article.aspx?p=25862
85. @GanbaroDigital xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
86. @GanbaroDigital xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx { -- MySQL -- }
87. @GanbaroDigital xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx { --- SQL Server --- }
88. @GanbaroDigital ?? ?? Should we use the decorator pattern to provide CombGUIDs?
89. @GanbaroDigital In this case, we are not adding an abstraction. We are adding a variant.
90. @GanbaroDigital The PHP UUID RFC did not include CombGUIDs.
91. @GanbaroDigital class MysqlCombGUID extends UUID { // ... }
92. @GanbaroDigital class SqlServerCombGUID extends UUID { // ... }
93. @GanbaroDigital final class UUID { // ... }
94. @GanbaroDigital The PHP UUID RFC did not include CombGUIDs ...
95. @GanbaroDigital ... and prevented anyone else from doing so from userland.
96. @GanbaroDigital “It’s unreasonable to expect package maintainers to know 100% of all possible variants.
97. @GanbaroDigital ?? ?? When is it reasonable to actively prevent third-party variants?
98. @GanbaroDigital ?? ?? How do we solve this?
99. @GanbaroDigital A SOLID Approach
100. @GanbaroDigital SOLID are design principles for object-oriented code
101. @GanbaroDigital SOLID Principles • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
102. @GanbaroDigital SOLID Principles • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
103. @GanbaroDigital SOLID Principles • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
104. @GanbaroDigital SOLID Principles • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
105. @GanbaroDigital SOLID Principles • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
106. @GanbaroDigital I’m going to focus on how `final` affects two SOLID principles.
107. @GanbaroDigital `final` Affects ...? • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
108. @GanbaroDigital `final` Affects ... • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
109. @GanbaroDigital Open/closed: open for extension, closed for modification.
110. @GanbaroDigital `final` ensures that a class is closed for modification!
111. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar ✗ ✗ ✗ Class Bar
112. @GanbaroDigital We’ve also seen that `final` ensures that the consuming class
 is closed for extension.
113. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar Class Foo Method doAction(Bar $input) ✗ ✗ ✗
114. @GanbaroDigital Liskov substitution principle: type T may be substituted by a sub-type S without altering the desirable properties of the program.
115. @GanbaroDigital Class Bar Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar
116. @GanbaroDigital Class Bar Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar with caveats!
117. @GanbaroDigital `final` prevents us creating sub-type S at all!
118. @GanbaroDigital final class Bar { // ... }
119. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar ✗ ✗ ✗ Class Bar
120. @GanbaroDigital ?? ?? Can `final` classes also be SOLID?
121. @GanbaroDigital SOLID Principles • Single responsibility • Open/closed • Liskov substitution • Interface segregation • Dependency inversion
122. @GanbaroDigital ?? ?? What if we used interfaces for type-hinting?
123. @GanbaroDigital final class Bar { // ... }
124. @GanbaroDigital final class Bar implements Interface1 { // ... }
125. @GanbaroDigital class Foo { public function doAction( Bar $input ) { // ... } }
126. @GanbaroDigital class Foo { public function doAction( Interface1 $input ) { // ... } }
127. @GanbaroDigital Class Foo Method doAction(Bar $input)
128. @GanbaroDigital Class Foo Method doAction(Interface1 $input)
129. @GanbaroDigital Class Bar Class Foo Method doAction(Interface1 $input)
130. @GanbaroDigital Class Bar Class Foo Method doAction(Interface1 $input) ✗
131. @GanbaroDigital Class Bar (Interface1) Class Foo Method doAction(Interface1 $input)
132. @GanbaroDigital Class Bar (Interface1) Class Foo Method doAction(Interface1 $input) ✓
133. @GanbaroDigital Class Bar (Interface1)
134. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar ✗ ✗ ✗ Class Bar (Interface1)
135. @GanbaroDigital Class Bar is still closed for modification. ✓
136. @GanbaroDigital Class Foo is open to new variants. ✓
137. @GanbaroDigital Class Bar (Interface1) Class C1 (Interface1) Class C2 (Interface1) Class C3 (Interface1)
138. @GanbaroDigital Class C1 (Interface1) Class C2 (Interface1) Class C3 (Interface1) Class Foo Method doAction(Interface1 $input)
139. @GanbaroDigital Class C1 (Interface1) Class C2 (Interface1) Class C3 (Interface1) Class Foo Method doAction(Interface1 $input) ✓ ✓ ✓
140. @GanbaroDigital ?? ?? What about sub-types and Liskov’s substitution principle?
141. @GanbaroDigital Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar ✗ ✗ ✗ Class Bar (Interface1)
142. @GanbaroDigital Class Bar (Interface1) Class C1 (Interface1) Class C2 (Interface1) Class C3 (Interface1)
143. @GanbaroDigital final class Bar implements Interface1 { // ... }
144. @GanbaroDigital interface SubType1 extends Interface1 { // ... }
145. @GanbaroDigital Class Bar Class C1 extends Bar Class C2 extends Bar Class C3 extends Bar with caveats!
146. @GanbaroDigital Interface1 SubType 2 SubType1 SubType3
147. @GanbaroDigital
148. @GanbaroDigital final class Foo { public function doAction( Interface1 $input ) { // ... } }
149. @GanbaroDigital
150. @GanbaroDigital final class Foo { public function doAction( Interface1 $input ) { // ... } }
151. @GanbaroDigital final class Foo implements Interface2 { public function doAction( Interface1 $input ) { // ... } }
152. @GanbaroDigital “ Behaviour is closed to modification. The system is open for extension (via variants).
153. @GanbaroDigital UUID RFC
154. @GanbaroDigital final class UUID { // ... }
155. @GanbaroDigital function doSomething (UUID $id, ...) { // ... }
156. @GanbaroDigital
157. @GanbaroDigital interface UUID { // ... }
158. @GanbaroDigital class MysqlCombGUID extends UUID { // ... }
159. @GanbaroDigital class MysqlCombGUID extends UUID { // ... } ✗
160. @GanbaroDigital class MysqlCombGUID implements UUID { // ... }
161. @GanbaroDigital class SqlServerCombGUID implements UUID
 { // ... }
162. @GanbaroDigital Userland UUID variants would be fully supported.
163. @GanbaroDigital class MyID extends UUID { // ... }
164. @GanbaroDigital class MyID extends UUID { // ... } ✗
165. @GanbaroDigital class MyID { private $uuid; public function __toString( return (string)$this->uuid; ); }
166. @GanbaroDigital Abstractions should still be done via aggregation & the decorator pattern.
167. @GanbaroDigital Abstractions would benefit from some interfaces too!
168. @GanbaroDigital In Summary
169. @GanbaroDigital “ Use the decorator pattern for abstracting underlying behaviour.
170. @GanbaroDigital “ The behaviour of `final` classes can be consumed, not modified.
171. @GanbaroDigital “ `final` classes lock down type-hinted method parameters.
172. @GanbaroDigital “It’s unreasonable to expect package maintainers to know 100% of all possible variants.
173. @GanbaroDigital “ Use interface inheritance for new variants.
174. @GanbaroDigital “ `final` classes that don’t implement interfaces restrict package re-use.
175. @GanbaroDigital “ `final` classes that don’t implement interfaces are not SOLID.
176. @GanbaroDigital “ `final` classes that don’t implement interfaces are a code smell.
177. @GanbaroDigital “Don’t use `final` classes as type-hints. Use interfaces.
178. @GanbaroDigital
179. @GanbaroDigital “ Behaviour is closed to modification. The system is open for extension (via variants).
180. Thank You Any Questions? A presentation by @stuherbert
 for @GanbaroDigital