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The maybe monad as a replacement for nil
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My Problem My Solution
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Everyone Stand Up
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No content
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a!/samphippen
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My Problem
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Type Checking is the Antithesis of Object Oriented Programming
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Type
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Types
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What is a type?
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What is a data type?
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A type is a set of possible values and operations
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Class
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Terms are literally interchangeable in Ruby
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Terms are literally interchangeable in Ruby Konstantin
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Fixnum
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✕
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+
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/
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—
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1.class # => Fixnum
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You know what all these things do
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1+1 # => 2
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Array
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count
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each
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In Ruby some types are interchangeable
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Typeclass
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A set of types and common operations
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There is some expectation of what the operations will do
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Duck typing
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All number types in Ruby form a typeclass
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Fixnum Float BigDecimal
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Numeric Op Numeric = Numeric
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Positive Numeric + Positive Numeric = Positive Numeric =
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Also collections
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Hash Set Array
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Type Checking is the Antithesis of Object Oriented Programming
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Type Checking is the Antithesis of Object Oriented Programming
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Type Checking
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This term has two meanings
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Compile time type checking
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public static final List seriouslyiamsoboredwh ocares
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Like in Java
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Clearly we don’t do this in Ruby
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So what do I mean?
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ActiveRecord::Base #find_by
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pony = Pony.find_by(:id => smth) pony.neigh
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pony = Pony.find_by(:id => smth) if pony pony.neigh else puts “No Pony can’t neigh” end
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The problem here is two return types
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nil Pony < AR::Base
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We’re forced to add a type check
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Also, I think this is the wrong type check
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pony = Pony.find_by(:id => smth) if !pony.nil? pony.neigh else puts “No Pony can’t neigh” end
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A more explicit type check
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But still wrong
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pony = Pony.find_by(:id => smth) if !pony.nil? pony.neigh else puts “No Pony can’t neigh” end
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pony = Pony.find_by(:id => smth) if pony.respond_to?(:neigh) pony.neigh else puts “No Pony can’t neigh” end
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This type checking adds unnecessary complexity to our app
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Type Checking is the Antithesis of Object Oriented Programming
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Type Checking is the Antithesis of Object Oriented Programming
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Antithesis
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I am using it to mean “DOING IT WRONG”
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Type Checking is the Antithesis of Object Oriented Programming
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Type Checking is the Antithesis of Object Oriented Programming
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Object oriented programming
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Konstantin Haase says:
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Data abstraction and control abstraction
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Alan Kay says:
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Everything is an object
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Objects communicate by sending and receiving messages
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def bees if :bar == a.foo else end end
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def bees a.foo nil end
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Tell don’t ask
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Objects have their own memory (in terms of objects).
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Data hiding
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Every object is an instance of a class (which must be an object).
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The class holds the shared behavior for its instances (in the form of objects in a program list)
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To eval a program list, control is passed to the first object and the remainder is treated as its message.
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Type Checking is the Antithesis of Object Oriented Programming
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Type Checking is the Antithesis of Object Oriented Programming
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My Problem
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My Problem
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My Solution
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Just always make your methods return things of a consistent type class
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Thanks!
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No obviously there’s more
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Third party APIs do this all the time
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pony = Pony.find_by(:id => smth) if pony pony.neigh else puts “No Pony can’t neigh” end
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The problem here is two return types
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As a client of this API I am forced to add a type check
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nil is such a common case
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How do we fix it?
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Null object pattern
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I think this one is quite well known
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class Pony def horse_power 0.5 end end
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Pony.find_by( :key => value ) || NullPony.new
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class NullPony def horse_power 0 end end
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NullPony quacks the same as Pony
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Solves the typing problem
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Summing over ponies will only count Pony objects
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0 might be the wrong default
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Pony * NullPony = 0
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Decided the default for horse_power when defining the class
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Change is inevitable
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Can’t predict how NullPony will be used in the future
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Maybe Typeclass
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Solves same problem
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Allows for runtime defaults
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#map(&blk) -> Maybe #value_or(a) -> a
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class Just def initialize(value) @value = value end def map(&blk) def value_or(x) Just.new(blk.call(@value)) @value end end end
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class Nothing def map(&blk) self end def value_or(x) x end end
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A consistent interface for dealing with missing values
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NoMethodError: undefined method `foo' for nil:NilClass
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NoMethodError: undefined method `foo' for nil:NilClass
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[Maybe, Nothing, Maybe]
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call map on all of them
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collapse with value_or
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To Recap:
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Null object can replace nils if you know the defaults at class definition time
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Maybe if you want defaults at run time
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Your job is not to make Alan Kay happy
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RSpec RSpec ! ! RSpec 3
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tinyurl.com/ samfr2014
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Let’s have some questions a!/samphippen
[email protected]