Groovy学习笔记——实现Ruby的case .. when表达式

今天看到有人在Groovy的邮件列表上问Groovy能不能支持Ruby的case ... when表达式：

car = "Patriot"

manufacturer = case car
  when "Focus": "Ford"
  when "Navigator": "Lincoln"
  when "Camry": "Toyota"
  when "Civic": "Honda"
  when "Patriot": "Jeep"
  when "Jetta": "VW"
  when "Ceyene": "Porsche"
  when "Outback": "Subaru"
  when "520i": "BMW"
  when "Tundra": "Nissan"
  else "Unknown"
end

puts "The " + car  + " is made by "  + manufacturer

然后Guillaume给出了这么一段代码：

def car = "Patriot"

def manufacturer = match(car) {
  when "Focus",     "Ford"
  when "Navigator", "Lincoln"
  when "Camry",     "Toyota"
  when "Civic",     "Honda"
  when "Patriot",   "Jeep"
  when "Jetta",     "VW"
  when "Ceyene",    "Porsche"
  when "Outback",   "Subaru"
  when "520i",      "BMW"
  when "Tundra",    "Nissan"
  otherwise         "Unknown"
}

println "The $car is made by $manufacturer"

def match(obj, closure) {
    closure.subject = obj
    closure.when = { value, result ->
        if (value == subject)
            throw new MatchResultException(result: result)
    }
    closure.otherwise = { return it }
    closure.resolveStrategy = Closure.DELEGATE_FIRST
    try {
        closure()
        closure.otherwise()
    } catch (MatchResultException r) {
        r.result
    }
}

class MatchResultException extends RuntimeException {
    def result
}

我不是很喜欢里面用异常来控制程序的流程，而且觉得“when "Focus", "Ford"”中间的逗号不够直观，因此就在上面的代码的基础上做了一些修改：

def match(subject, closure) {
    def whenMap = [:], otherwise = null
    closure.when = { map -> whenMap.putAll(map) }
    closure.otherwise = { otherwise = it }
    closure.resolveStrategy = Closure.DELEGATE_FIRST
    closure()
    def result = whenMap.find { condition, value -> subject in condition }
    return result ? result.value : otherwise
}

def manufacturer(car) {
    match(car) {
        when "Focus":     "Ford"
        when "Navigator": "Lincoln"
        when "Camry":     "Toyota"
        when "Civic":     "Honda"
        when "Patriot":   "Jeep"
        when "Jetta":     "VW"
        when "Ceyene":    "Porsche"
        when "Outback":   "Subaru"
        when "520i":      "BMW"
        when "Tundra":    "Nissan"
        otherwise         "Unknown"
    }
}

println "The Patriot is made by ${manufacturer('Patriot')}"
println "The QQ is made by ${manufacturer('QQ')}"

以上代码在Groovy 1.6下编译通过。