这个do-monad是否可以替换为let块？ [英] Could this do-monad be replaced by a let block?

问题描述

这里的作者提供以下内容do-monad组合测试生成器的示例用法：

 （require'[clojure.test.check.generators：as gen ]）
（require'[clojure.algo.monads：as m]）
（m / defmonad gen-m 
 [m-bind gen / bind 
 m-result gen / return]）
 
（def vector-and-elem 
（m / domonad gen-m 
 [n（gen / choose 1 10）
v vector gen / int n）
e（gen / element v）] 
 [v，e]））
 
（gen / sample vector-and-elem）
 （[[0 -1 1 0 -1 0 -1 1] 0] 
 [[1 1 3 3 3 -1 0 -2 2] 3] 
 [[8 4] 8] .. 。
  

有这里的评论员断言，这是一个非常好的例子，不仅仅是为了自己，而是为了提供一个真正的增值。

<对我来说，这似乎没有什么不同于一个让块正在做什么。确实 - Brian Marick 这里将 do-monad与let块进行比较。

我的问题是：这个do-monad是否可以替换为let块？

c> c> c> c> c> c> do-monad 不能被 let 块替换。但您可以像下面这样手动使用 gen / bind 和 gen / return ：

 （def vector-and-elem 
（gen / bind（gen / choose 1 10）
（fn [n] 
（gen / bind（gen / vector gen / int n）
（fn [v] 
（gen / bind（gen / elements v）
（fn [e] 
 （gen / return [ve]））））））））
  

c $ c> Monad 正在为你做掩护。

尝试以 let 写：

 （def vector-and-elem-let 
（let [n（gen / choose 1 10）
v（gen / vector gen / int n）
e（gen / elements v）] 
 [ve]））
  

因为函数：选择，向量和元素返回a 因此，例如 gen / vector 期望一个 Integer 作为第二个参数而不是 code>并且 let 甚至不编译。

The author here provides the following example usage of a do-monad to combine test generators:

(require '[clojure.test.check.generators :as gen])
(require '[clojure.algo.monads :as m])
(m/defmonad gen-m 
  [m-bind gen/bind 
   m-result gen/return])

(def vector-and-elem
  (m/domonad gen-m
    [n (gen/choose 1 10)
     v (gen/vector gen/int n)
     e (gen/element v)]
    [v, e]))

(gen/sample vector-and-elem)
    ([[0 -1 1 0 -1 0 -1 1] 0] 
    [[1 1 3 3 3 -1 0 -2 2] 3]
    [[8 4] 8]...

There commentator here asserts that this is a great example of monads not just for their own sake, but for providing a genuine value-add.

To me this doesn't seem that different from what a let-block is doing. Indeed - Brian Marick here compares the do-monad to a let block.

My question is: Could this do-monad be replaced by a let block?

解决方案

In the context of test.check the answer is no, this do-monad can't be replaced by a let block. But you can use the gen/bind and gen/return manually like this:

(def vector-and-elem
  (gen/bind (gen/choose 1 10)
            (fn [n]
              (gen/bind (gen/vector gen/int n)
                        (fn [v]
                          (gen/bind (gen/elements v)
                                    (fn [e]
                                      (gen/return [v e]))))))))

This is what the Monad is doing under the covers for you.

Trying to write this as a let:

(def vector-and-elem-let
  (let [n (gen/choose 1 10)
        v (gen/vector gen/int n)
        e (gen/elements v)]
    [v e]))

Doesn't work because the functions: choose, vector, and elements returns a generator not the result of the generator. So for example gen/vector expects a Integer as the second argument not a generator and this let doesn't even compile.

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