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Types & Refactoring

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a!/samphippen

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Types

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I think type systems are great!

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I think dynamic languages are great!

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I think thinking about types in dynamic languages is great!

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No type systems here

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NoMethodError: undefined method `foo' for nil:NilClass

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No explicit type systems here

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What is a type?

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Concrete implementation of some data and behaviour

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Class

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1.class # => FixNum

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(1.0).class # => Float

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What is a typeclass?

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A set of methods satisfied by many types

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Interface

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(1+1.0).class # => Float (1.0+1).class # => Float

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But also things like

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Positive Int + Positive Int # => Positive Int

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some_collection.count

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String Hash Array

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Form a typeclass (or interface) under count with no args

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Methods that return consistent interfaces are great

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The problem

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We’re not forced to return a single interface

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If interface is inconsistent, code must check types further down

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For example returning nil

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nil is not type classed the same as your object!

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pony = Pony.find_by(:key => value) if pony pony.neigh else puts “pony was nil” end

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The if is a subtle form of type checking.

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Type checking is the antithesis of OO

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Can we get a consistent interface?

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Refactoring

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Null object

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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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Decides default for horse_power at define time

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May or may not be what you want

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Maybe

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Steal ideas from functional programming!

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Decide defaults at run time

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Maybe is a type class

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#map(&blk) -> maybe #value_or(x) -> Object

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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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Map until done with transforms

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use maybe.value_or(“something”)

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Recap

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Methods that return nil are almost always inconsistently typed

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Use a null object to replace a nil if you know defaults at define time

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Use maybe if you want to set defaults at run time

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RSpec RSpec ! ! RSpec 3

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tinyurl.com/ sambr2014

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Let’s have some questions a!/samphippen [email protected]