# Elixir: 多太(Polymorphism)

developerworks

## 概述

# 开尔文
defmodule Kelvin do
defstruct name: "Kelvin", symbol: "K", degree: 0
end
# 华氏温度
defmodule Fahrenheit do
defstruct name: "Fahrenheit", symbol: "°F", degree: 0
end
# 摄氏温度
defmodule Celsius do
defstruct name: "Celsius", symbol: "°C", degree: 0
end
# 温度协议(接口, 只有函数定义, 没有函数实现)
defprotocol Temperature do
@doc """
Convert Kelvin and Fahrenheit to Celsius degree
"""
def to_celsius(degree)
end

# 开尔文, 实现了Temperature 协议(接口)
defimpl Temperature, for: Kelvin do
@doc """
Deduct 273.15
"""
def to_celsius(kelvin) do
celsius_degree = kelvin.degree - 273.15
%Celsius{degree: celsius_degree}
end
end
# 华氏摄氏度实现
defimpl Temperature, for: Fahrenheit do
@doc """
Deduct 32, then multiply by 5, then divide by 9
"""
def to_celsius(fahrenheit) do
celsius_degree = (fahrenheit.degree - 32) * 5 / 9
%Celsius{degree: celsius_degree}
end
end

iex> fahrenheit = %Fahrenheit{degree: 45}
%Fahrenheit{degree: 45, name: "Fahrenheit", symbol: "°F"}
iex> celsius = Temperature.to_celsius(fahrenheit)
%Celsius{degree: 7.22, name: "Celsius", symbol: "°C"}
iex> kelvin = %Kelvin{degree: 300}
%Kelvin{degree: 300, name: "Kelvin", symbol: "K"}
iex> celsius = Temperature.to_celsius(kelvin)
%Celsius{degree: 26.85, name: "Celsius", symbol: "°C"}

iex> Temperature.to_celsius(%{degree: 12})
** (Protocol.UndefinedError) protocol Temperature not implemented for %{degree: 12}
iex:11: Temperature.impl_for!/1
iex:15: Temperature.to_celsius/1

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