如何更好地迭代在Clojure中的状态（monad？） [英] How to Better Iterate over State in Clojure (monad?)

问题描述

我刚写了这段代码：

 （defn parameters [transform-factory state] 
（lazy-seq （let [[r1 state]（uniform state）
 [r2 state]（uniform state）
 [t state]（transform-factory state）
（cons [t [r1 r2] ]（参数transform-factory state）））））
 
（defn repeated-transform [mosaic n transform-factory state] 
 （parameters transform-factory state））））
  

参数函数生成从 state 生成的延迟序列的值，用于参数化某物（在这种情况下为马赛克）的重复变换。 / p>

在我看来，参数显示了一个相当常见的模式，当你有一些状态（在这种情况下生成随机值）。有这个名字吗？

有没有更好的方式来写第一个函数？相关的问题通常可以用 reduce 来解决，它携带状态，但在这里我没有减少。类似地， reduction 似乎不合适。这是monad的好例子吗？ （从理论pov我没有看到你如何定义一种方式将多个实例合并为一个，但也许不改变实际应用 - 它似乎像monads解决的其他地方，其中一些状态需要的类型

（ps我提到的随机数，但我不能替换这个解决方案使用后面的可变状态 - 作为正常随机

解决方案

你可以看看状态monad，看看它是否是一个非常适合你。

使用monad的一般原则是：

• 顺序（管道操作）


• 可重复使用的模块化副作用处理（例如：错误处理/记录/
  状态）


• 保留业务逻辑清除纯函数

我发现Monads上的一些资源非常有用（对于Clojure）是

Adam Smyczek：
Monads介绍（视频）
http://www.youtube.com/watch?v=ObR3qi4Guys

和
Jim Duey：
Monads in Clojure
http://www.clojure.net/2012/02 / 02 / Monads-in-Clojure /

I just wrote this code:

(defn parameters [transform-factory state]
  (lazy-seq (let [[r1 state] (uniform state)
                  [r2 state] (uniform state)
                  [t state] (transform-factory state)]
              (cons [t [r1 r2]] (parameters transform-factory state)))))

(defn repeated-transform [mosaic n transform-factory state]
  (reduce transform-square mosaic
    (take n (parameters transform-factory state))))

the parameters function generates a lazy sequence of values generated from the state, which are used to parameterise a repeated transformation of something (a "mosaic" in this case).

it seems to me that parameters shows a fairly common pattern which surfaces when you have some state that must be carried around (in this case to generate random values). is there a name for this?

is there a better way to write the first function? related problems can often be solved with reduce, which "carries along" the state, but here i have nothing to reduce. similarly, reductions doesn't seem to fit. is this a good case for a monad? (from a theoretical pov i don't see how you define a way to combine multiple instances into one, but perhaps that doesn't change the practical application - it does seem like the kind of problem monads solve elsewhere, where some state needs to be carried around).

(ps i mentioned random numbers, but i can't replace this with a solution that uses mutable state behind the scenes - as "normal" random routines do - for reasons unrelated to the question).

解决方案

You could certainly look at the state monad to see if it is a good fit for you.

General guidelines to use monads are:

• Sequential execution (pipeline operations)
• Reusable modular side effect processing (ex: error handling/ logging/ state)
• Keep your business logic clean in pure functions

Some resources on monads that I found very useful (for Clojure) are

Adam Smyczek: Introduction to Monads (Video) http://www.youtube.com/watch?v=ObR3qi4Guys

and Jim Duey: Monads in Clojure http://www.clojure.net/2012/02/02/Monads-in-Clojure/

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