为什么在这些 julia 函数中不尊重 constness? [英] Why is constness not respected inside these julia functions?

问题描述

今天早些时候由 Lyndon 的问题提示:

Prompted by Lyndon's question earlier today:

一个.

julia> function f1(x::Float64) 
         const y = x;
         y = "This should throw an error since y is of constant type";
         return y;
       end
f1 (generic function with 1 method)

julia> f1(1.0)
"This should throw an error since y is of constant type"

为什么 const 关键字在这里不能按预期工作?(即，不允许将字符串分配给已声明为 const 的 y).

Why does the const keyword not work as expected here? (i.e., disallow assigning a string to y which has been declared as const).

b.

julia> function f2(x::Float64)
         show(x);
         const x = 1.;
       end
f2 (generic function with 1 method)

julia> f2(1.0)
ERROR: UndefVarError: x not defined
Stacktrace:
 [1] f2(::Float64) at ./REPL[1]:2

为什么在第3行定义x为const会影响x 行 2?

Why does defining x as const on line 3 affect the value of x on line 2?

c.

特别是，这使我无法这样做:

In particular, this prevents me from doing:

function f(x::Float64)
  const x = x; # ensure x cannot change type, to simulate "strong typing"
  x = "This should throw an error";
end

关于 Lyndon 的 counterexample"评论，但它适得其反，因为这个函数在第 2 行中断，而不是像我预期的那样在第 3 行中断.

I was going to offer this as a way to simulate "strong typing", with regard to Lyndon's "counterexample" comment, but it backfired on me, since this function breaks at line 2, rather than line 3 as I expected it to.

是什么导致了这种行为?这会被视为错误还是故意行为?

What is causing this behaviour? Would this be considered a bug, or intentional behaviour?

除了命名约定，有没有更可接受的方法来防止传递给函数的参数改变其类型?
_{(例如，是否有一种已定义且适当的方法来做到这一点:我不是在寻找变通方法，例如创建一个将 x 转换为不可变类型的包装器等)}

Naming conventions aside, is there a more acceptable way to prevent an argument passed into a function from having its type altered?
_{(as in, is there a defined and appropriate way to do this: I'm not after workarounds, e.g. creating a wrapper that converts x to an immutable type etc)}

到目前为止，这是唯一允许我在函数中强制执行 const 的变体，但它仍然需要引入新的变量名:

So far, this is the only variant that allows me to enforce constness in a function, but it still requires the introduction of a new variable name:

julia> function f(x::Float64)
         const x_const::Float64 = x;
         x_const = "helle"; # this will break, as expected
       end

但即便如此，错误只是抱怨从字符串到浮点数的无效转换"而不是对常量的无效重新定义"

but even then, the error just complains of an "invalid conversion from string to float" rather than an "invalid redefinition of a constant"

推荐答案

因为本地范围内的const还没有实现:

Because const in local scope is not yet implemented:

https://github.com/JuliaLang/julia/issues/5148

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