解释器模式 Interpreter Pattern

## 问题 Problem 有人需要在限制重重的地方运行部分你的代码，或者你的语言无法简洁地表达其他领域的问题。 Someone else needs to run parts of your code in a controlled fashion. Alternately, your language of choice cannot express the problem domain in a concise fashion. ## 方案 Solution 使用解释器模式创建一个领域语言，翻译成特殊的代码。 Use the Interpreter pattern to create a domain-specific language that you translate into specific code. 例如，有用户想在你的程序中执行数学计算。你可以让他们把代码交给_eval_运行，但是他们有可能会运行各种各样的代码。更好的方案，你可以提供一个小型的“栈计算器”语言，单独解析，这样可以只允许运行数学操作，同时还可以反馈更有用的错误信息。 Assume, for example, that the user wants to perform math inside of your application. You could let them forward code to _eval_ but that would let them run arbitrary code. Instead, you can provide a miniature “stack calculator” language that you parse separately in order to only run mathematical operations while reporting more useful error messages. ~~~ class StackCalculator parseString: (string) -> @stack = [ ] for token in string.split /\s+/ @parseToken token if @stack.length > 1 throw "Not enough operators: numbers left over" else @stack[0] parseToken: (token, lastNumber) -> if isNaN parseFloat(token) # Assume that anything other than a number is an operator @parseOperator token else @stack.push parseFloat(token) parseOperator: (operator) -> if @stack.length < 2 throw "Can't operate on a stack without at least 2 items" right = @stack.pop() left = @stack.pop() result = switch operator when "+" then left + right when "-" then left - right when "*" then left * right when "/" if right is 0 throw "Can't divide by 0" else left / right else throw "Unrecognized operator: #{operator}" @stack.push result calc = new StackCalculator calc.parseString "5 5 +" # => { result: 10 } calc.parseString "4.0 5.5 +" # => { result: 9.5 } calc.parseString "5 5 + 5 5 + *" # => { result: 100 } try calc.parseString "5 0 /" catch error error # => "Can't divide by 0" try calc.parseString "5 -" catch error error # => "Can't operate on a stack without at least 2 items" try calc.parseString "5 5 5 -" catch error error # => "Not enough operators: numbers left over" try calc.parseString "5 5 5 foo" catch error error # => "Unrecognized operator: foo" ~~~ ## 讨论 Discussion 如果不自己写解释器， 你可以结合目前有的CoffeeScrtipt解释器，以某种方式，这种方式使用它平常的语法创建算法的更加自然的（因此更易于理解）表达方式。 As an alternative to writing our own interpreter, you can co-op the existing CoffeeScript interpreter in a such a way that its normal syntax makes for more natural (and therefore more comprehensible) expressions of your algorithm. ~~~ class Sandwich constructor: (@customer, @bread='white', @toppings=[], @toasted=false)-> white = (sw) -> sw.bread = 'white' sw wheat = (sw) -> sw.bread = 'wheat' sw turkey = (sw) -> sw.toppings.push 'turkey' sw ham = (sw) -> sw.toppings.push 'ham' sw swiss = (sw) -> sw.toppings.push 'swiss' sw mayo = (sw) -> sw.toppings.push 'mayo' sw toasted = (sw) -> sw.toasted = true sw sandwich = (customer) -> new Sandwich customer to = (customer) -> customer send = (sw) -> toastedState = sw.toasted and 'a toasted' or 'an untoasted' toppingState = '' if sw.toppings.length > 0 if sw.toppings.length > 1 toppingState = " with #{sw.toppings[0..sw.toppings.length-2].join ', '} and #{sw.toppings[sw.toppings.length-1]}" else toppingState = " with #{sw.toppings[0]}" "#{sw.customer} requested #{toastedState}, #{sw.bread} bread sandwich#{toppingState}" send sandwich to 'Charlie' # => "Charlie requested an untoasted, white bread sandwich" send turkey sandwich to 'Judy' # => "Judy requested an untoasted, white bread sandwich with turkey" send toasted ham turkey sandwich to 'Rachel' # => "Rachel requested a toasted, white bread sandwich with turkey and ham" send toasted turkey ham swiss sandwich to 'Matt' # => "Matt requested a toasted, white bread sandwich with swiss, ham and turkey" ~~~ 上面这个例子，允许连续调用多个函数，因为每个函数都会返回修改后的对象，这样外围的函数就可以轮流地修改该对象。借用一个一个非常小的_to_，该例子给于构造过程了一个更加自然的语法，并且如果使用正确的话，读起来就像爱那个一个自然的句子。这样的话，你的CoffeeScript技术和你在使用的语言技术都能帮助你找到代码的问题。 This example allows for layers of functions by how it returns the modified object so that outer functions can modify it in turn. By borrowing a very and the particle _to_, the example lends natural grammar to the construction and ends up reading like an actual sentence when used correctly. This way, both your CoffeeScript skills and your existing language skills can help catch code problems.