Lua DSL 还是巧妙的闭包？

Question

我正在尝试使用 Lua 并尝试实现一些列表处理逻辑。我不确定我是否真正掌握了协程和匿名函数的力量，但我正在尝试。我知道这些东西在其他脚本语言（如 Ruby/Groovy/Javascript）中是如何工作的，并且我想在 Lua 中做一些同样聪明的事情。这是我想出的例子：

model = { { player = "Cliff", age = 35, gender = "male" }, { player = "Ally", age = 36, gender = "female" }, { player = "Jasmine", age = 13, gender = "female" }, { player = "Lauren", age = 6.5, gender = "female" } }

function allplayers()
    return coroutine.create(function()
        for idx, each in ipairs(model) do
            coroutine.yield(idx, each)
        end
    end)
end

function handlePlayers(source)
    local status, idx, each = coroutine.resume(source)
    while each do
        print(idx, each.player)
        status, idx, each = coroutine.resume(source)
    end
end

function having(source, predicate)
    return coroutine.create(function()
        local status, idx, each = coroutine.resume(source)
        while each do
            if predicate(each)  then
                coroutine.yield(idx, each)
            end
            status, idx, each = coroutine.resume(source)
        end
    end)
end

handlePlayers(having(allplayers(), function(each) return each.age < 30 end))

理想情况下，我希望能够编写如下代码：

allplayers(having(function(each) return each.age < 30 end))

甚至更好：

allplayers(having({each.age < 30 }))

产生相同的输出，但我不太清楚如何或即使这可以完成吧。我上面的内容对于所有迭代和循环等等来说似乎太多余了。有没有更绝妙的方法来做到这一点？ （天哪，我怀念用 Groovy 编码，因为总有一种更绝妙的方式来做某事......）

Answer 1

如果不需要重用过滤后的结果，请考虑迭代器：

model = {
  { player = "Cliff", age = 35, gender = "male" },
  { player = "Ally", age = 36, gender = "female" },
  { player = "Jasmine", age = 13, gender = "female" },
  { player = "Lauren", age = 6.5, gender = "female" }
}

function model:having(predicate)
    local index = 0 
    return function()
        while true do
            index = index + 1
            if index > #self then break end
            if predicate(self[index]) then return self[index] end
        end
    end
end

for item in model:having(function(m) return m.age < 30 end) do
    print(item.player)
end

Answer 2

这是极其复杂和过度设计的。您想通过谓词过滤一个列表。那么就这样做吧；这是一个简单的循环。就写吧。

即使您绝对必须以函数式风格执行此操作（Lua 不是函数式语言。它可以以这种方式工作，但它不是函数式的），协程也不合适。

观察：

model = {
  { player = "Cliff", age = 35, gender = "male" },
  { player = "Ally", age = 36, gender = "female" },
  { player = "Jasmine", age = 13, gender = "female" },
  { player = "Lauren", age = 6.5, gender = "female" }
}

function for_each_array(list, operation)
  for key, value in ipairs(list) do
    operation(value)
  end
end

function filter_if(list, predicate)
  return function(value)
    if(predicate(value)) then
      list[#list + 1] = value
    end
  end
end

local list = {}
for_each_array(model, filter_if(list, function(each) return each.age < 30 end))

for_each_array(list, function(each) print(each.player) end)

看到了吗？根本不需要协程。

Answer 3

您可能想尝试模仿 Underscore.js JavaScript 库。

您的示例如下所示：

_ = require 'underscore'
model = {
  { player = "Cliff", age = 35, gender = "male" },
  { player = "Ally", age = 36, gender = "female" },
  { player = "Jasmine", age = 13, gender = "female" },
  { player = "Lauren", age = 6.5, gender = "female" }
}
result1 = _.select(model, function(p) return p.age < 30 end) -- traditional way
result2 = _(model):select(function(p) return p.age < 30 end) -- object-oriented style, calls can be chained
assert(_.is_equal(result1, result2))

请注意，Lua 不会为您提供任何其他语法糖（除了 : 运算符）。如果您确实想要更短的语法，您应该查看 Metalua，它允许您更改语言，并提供以下开箱即用的短函数语法：

result = _.select(model, |p| p.age < 30)