使用元编程在Julia中声明顶级变量

问题描述

我想使用元编程来回答这个问题，但是对于循环导致我的变量未在最高(REPL)范围内定义:

I want to answer this question using metaprogramming, but the scoping rules of for loops are causing my variables to not be defined in the upper-most (REPL) scope:

for x = [:A1, :A2]
   @eval x = rand(2,2)
end

我知道可能有一个简单的解决方法，但是我周五晚上的脑子想不起来.你们其中一个元编程迷可以帮助我找到一个简洁的解决方案吗? (我意识到宏可能可以转义，但我想考虑更短的内容)

I know there's probably an easy way around this, but my Friday night brain can't think of one. Can one of you metaprogramming junkies help me find a succinct solution? (I realize a macro might work with escaping, but I'm trying to think of something shorter)

推荐答案

如果只想在全局范围内定义变量，则只缺少$:

If you only want to define the variables in the global scope, you're just missing a $:

for x = [:A1, :A2]
    @eval $x = rand(2,2)
end

但是，即使将@eval放在函数中，也总是在顶级对其进行评估.如果要在函数作用域中定义变量，则需要将整个函数放在@eval中，构造代码块，并将其插值到函数中:

But @eval is always evaluated at top level even if you put it inside a function. If you want to define the variables in a function scope, you need to put the entire function inside @eval, construct the code block, and interpolate it into the function:

@eval function f()
    ...
    $([:($x = rand(2, 2)) for x in [:A1, :A2]]...)
    ...
end

此代码也可以微不足道地改编成宏(但随后确实需要esc).

This code can also be trivially adapted into a macro (but then it does need esc).

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