My Problem, My Solution

A talk about typing I gave at Frozen Rails 2014

Transcript

1. The maybe monad as a replacement for nil
2. My Problem My Solution
3. Everyone Stand Up
5. a!/samphippen
6. My Problem
7. Type Checking is the Antithesis of Object Oriented Programming
8. Type
9. Types
10. What is a type?
11. What is a data type?
12. A type is a set of possible values and operations
13. Class
14. Terms are literally interchangeable in Ruby
15. Terms are literally interchangeable in Ruby Konstantin
16. Fixnum
17. ✕
18. +
19. /
20. —
21. 1.class # => Fixnum
22. You know what all these things do
23. 1+1 # => 2
24. Array
25. count
26. each
27. In Ruby some types are interchangeable
28. Typeclass
29. A set of types and common operations
30. There is some expectation of what the operations will do
31. Duck typing
32. All number types in Ruby form a typeclass
33. Fixnum Float BigDecimal
34. Numeric Op Numeric = Numeric
35. Positive Numeric + Positive Numeric = Positive Numeric =
36. Also collections
37. Hash Set Array
38. Type Checking is the Antithesis of Object Oriented Programming
39. Type Checking is the Antithesis of Object Oriented Programming
40. Type Checking
41. This term has two meanings
42. Compile time type checking
43. public static final List<string> seriouslyiamsoboredwh ocares
44. Like in Java
45. Clearly we don’t do this in Ruby
46. So what do I mean?
47. ActiveRecord::Base #find_by
48. pony = Pony.find_by(:id => smth) pony.neigh
49. pony = Pony.find_by(:id => smth) if pony pony.neigh else puts “No Pony can’t neigh” end
50. The problem here is two return types
51. nil Pony < AR::Base
52. We’re forced to add a type check
53. Also, I think this is the wrong type check
54. pony = Pony.find_by(:id => smth) if !pony.nil? pony.neigh else puts “No Pony can’t neigh” end
55. A more explicit type check
56. But still wrong
57. pony = Pony.find_by(:id => smth) if !pony.nil? pony.neigh else puts “No Pony can’t neigh” end
58. pony = Pony.find_by(:id => smth) if pony.respond_to?(:neigh) pony.neigh else puts “No Pony can’t neigh” end
59. This type checking adds unnecessary complexity to our app
60. Type Checking is the Antithesis of Object Oriented Programming
61. Type Checking is the Antithesis of Object Oriented Programming
62. Antithesis
63. I am using it to mean “DOING IT WRONG”
64. Type Checking is the Antithesis of Object Oriented Programming
65. Type Checking is the Antithesis of Object Oriented Programming
66. Object oriented programming
67. Konstantin Haase says:
68. Data abstraction and control abstraction
69. Alan Kay says:
70. Everything is an object
71. Objects communicate by sending and receiving messages
72. def bees if :bar == a.foo else end end
73. def bees a.foo nil end
74. Tell don’t ask
75. Objects have their own memory (in terms of objects).
76. Data hiding
77. Every object is an instance of a class (which must be an object).
78. The class holds the shared behavior for its instances (in the form of objects in a program list)
79. To eval a program list, control is passed to the first object and the remainder is treated as its message.
80. Type Checking is the Antithesis of Object Oriented Programming
81. Type Checking is the Antithesis of Object Oriented Programming
82. My Problem
83. My Problem
84. My Solution
85. Just always make your methods return things of a consistent type class
86. Thanks!
87. No obviously there’s more
88. Third party APIs do this all the time
89. pony = Pony.find_by(:id => smth) if pony pony.neigh else puts “No Pony can’t neigh” end
90. The problem here is two return types
91. As a client of this API I am forced to add a type check
92. nil is such a common case
93. How do we fix it?
94. Null object pattern
95. I think this one is quite well known
96. class Pony def horse_power 0.5 end end
97. Pony.find_by( :key => value ) || NullPony.new
98. class NullPony def horse_power 0 end end
99. NullPony quacks the same as Pony
100. Solves the typing problem
101. Summing over ponies will only count Pony objects
102. 0 might be the wrong default
103. Pony * NullPony = 0
104. Decided the default for horse_power when defining the class
105. Change is inevitable
106. Can’t predict how NullPony will be used in the future
107. Maybe Typeclass
108. Solves same problem
109. Allows for runtime defaults
110. #map(&blk) -> Maybe #value_or(a) -> a
111. class Just def initialize(value) @value = value end def map(&blk) def value_or(x) Just.new(blk.call(@value)) @value end end end
112. class Nothing def map(&blk) self end def value_or(x) x end end
113. A consistent interface for dealing with missing values
114. NoMethodError: undefined method `foo' for nil:NilClass
115. NoMethodError: undefined method `foo' for nil:NilClass
116. [Maybe, Nothing, Maybe]
117. call map on all of them
118. collapse with value_or
119. To Recap:
120. Null object can replace nils if you know the defaults at class definition time
121. Maybe if you want defaults at run time
122. Your job is not to make Alan Kay happy
123. RSpec RSpec ! ! RSpec 3
124. tinyurl.com/ samfr2014
125. Let’s have some questions a!/samphippen sam@funandplausible.com