为什么 when-let 和 if-let 默认不支持多重绑定? [英] Why don't when-let and if-let support multiple bindings by default?

问题描述

为什么when-let和if-let默认不支持多重绑定?

所以:

(when-let [a ...乙...](+ a b))

...而不是:

(when-let [a ...(当-让 [b ...(+ a b)))

我知道我可以编写自己的宏或使用 monad(如此处所述:http://inclojurewetrust.blogspot.com/2010/12/when-let-maybe.html).

解决方案

因为(至少对于 if-let 而言)处理else"情况并不明显.

至少，受到 更好的嵌套 if-let 方式的启发clojure 我开始写一个宏来做到这一点.给定

(if-let* [a ...乙...]行动其他)

它会产生

(if-let [a ...](如果让 [b ...]行动?))

我不清楚如何继续(else"有两个地方).

你可以说任何失败都应该有一个替代方案，或者 when-let 没有替代方案，但是如果任何测试改变了状态，那么事情仍然会变得一团糟.

简而言之，它比我预期的要复杂一些，所以我想当前的方法避免了必须调用解决方案.

另一种说法:您假设 if-let 应该像 let 一样嵌套.更好的模型可能是 cond，它不是嵌套的 if"，而是替代的 if"，因此不太适合范围......或者，另一种说法它:if 不能更好地处理这种情况.

Why don't when-let and if-let support multiple bindings by default?

So:

(when-let [a ...
           b ...]
  (+ a b))

...instead of:

(when-let [a ...
  (when-let [b ...
    (+ a b)))

I am aware that I can write my own macro or use a monad (as described here: http://inclojurewetrust.blogspot.com/2010/12/when-let-maybe.html).

解决方案

Because (for if-let, at least) it's not obvious what to do with the "else" cases.

At least, motivated by Better way to nest if-let in clojure I started to write a macro that did this. Given

(if-let* [a ...
          b ...]
  action
  other)

it would generate

(if-let [a ...]
  (if-let [b ...]
    action
    ?))

and it wasn't clear to me how to continue (there are two places for "else").

You can say that there should be a single alternative for any failure, or none for when-let, but if any of the tests mutate state then things are still going to get messy.

In short, it's a little more complicated than I expected, and so I guess the current approach avoids having to make a call on what the solution should be.

Another way of saying the same thing: you're assuming if-let should nest like let. A better model might be cond, which isn't a "nested if" but more an "alternative if", and so doesn't fit well with scopes... or, yet another way of saying it: if doesn't handle this case any better.

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