为什么在这些julia函数中不尊重constness?

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问题描述

今天早些时候由 Lyndon的问题提示:

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，不允许为已声明为const的y分配字符串).

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

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 3 行上将x定义为const会影响 2 行上x的值?

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

尤其是，这阻止了我执行操作:

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

针对Lyndon的"counterexample"注释，但它对我适得其反，因为此功能在第2行中断，而不是我期望的那样在第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?

除了命名约定外，还有一种更可接受的方法可以防止传递给函数的参数改变其类型?

Naming conventions aside, is there a more acceptable way to prevent an argument passed into a function from having its type altered?

到目前为止，这是允许我在函数中强制执行constness的唯一变体，但仍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

但是即使那样，错误仍然抱怨从字符串到浮点的无效转换"，而不是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"

Lyndon

为什么在这些julia函数中不尊重constness?

问题描述

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