朱莉娅掉期有什么问题！宏? [英] What's wrong with this Julia swap! macro?

问题描述

我正在尝试在Julia中编写一个简单的swap!宏，以了解宏系统.到目前为止，这是我的代码:

I'm trying to write a simple swap! macro in Julia, to understand the macro system. Here's my code so far:

macro swap!(x, y)
    quote
        local tmp = $(esc(y))
        $x = $(esc(y))
        $y = tmp
    end
end

a = 1
b = 2

@swap!(a, b)

# prints: a: 1, b: 2
println("a: $a, b: $b")

这运行时没有错误，但实际上并没有更改值.朱莉娅似乎没有一个仅在不执行宏的情况下扩展宏的功能(据我所知)，因此这很难调试.

This runs without error, but isn't actually changing the values. Julia doesn't seem to have a function that just expands macros without executing them (as far as I can see), so this is hard to debug.

REPL中的等效引号似乎可以正常工作:

The equivalent quote in a REPL seems to work as expected:

julia> a = 1
1

julia> a_sym = :a
:a

julia> a_sym
:a

julia> b = 2
2

julia> b_sym = :b
:b

julia> eval(quote
       tmp = $a_sym
       $a_sym = $b
       $b_sym = tmp
       end)
1

julia> a
2

julia> b
1

我在做什么错了?

推荐答案

我认为您可能想要以下内容，但

I think you probably want something like below but are getting messed up by hygiene. The trick is to escape correctly.

macro swap(x,y)
   quote
      local tmp = $(esc(x))
      $(esc(x)) = $(esc(y))
      $(esc(y)) = tmp
    end
end

这是展开后的样子

julia> macroexpand(quote @swap(x,y) end)
quote  # none, line 1:
    begin  # none, line 3:
        local #189#tmp = x # line 4:
        x = y # line 5:
        y = #189#tmp
    end
end

效果

julia> x
1

julia> y
2

julia> @swap(x,y)

julia> x
2

julia> y
1

通过对比，您的宏正确地在一个赋值中转义了y，但在其他2条语句中未转义，因此设置了引入变量的值，而不是预期的x和y.

By contrast your macro escapes y in one assignment correctly, but not in the other 2 statements and so sets the values of introduced variables rather than the intended x and y.

julia> macroexpand(quote @swap!(x,y) end)
quote  # none, line 1:
    begin  # none, line 3:
        local #208#tmp = y # line 4:
        #209#x = y # line 5:
        #210#y = #208#tmp
    end 
end

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