f# - F# 模块初始化的不同行为

Question

我有以下 F# 程序:

open MyModule

printfn "%d" test

MyModule 为:

module MyModule

printfn "foo"

let test = 
  printfn "bar"
  42

这会产生以下输出:

foo
bar
42

当我将 MyModule 更改为:

module MyModule

printfn "foo"

let test = 
  // printfn "bar" <-- note the comment!
  42

...结果是:

42

为什么不再打印“foo”？

Answer 1

我认为规范第 12.5.1 节 Execution of Static Initializers ，有你的答案。引用相关位:

the static initializer for the file is executed on first access of a value that has observable initialization

和

All definitions have observable initialization except for the following definitions in modules:

以下列表包括:

Non-mutable, non-thread-local values that are bound to a simple constant expression

注释掉第一行test后，就变成了常量表达式。因此，它不再触发静态初始化。

编辑

规范没有提供此行为的基本原理，但它与 C# 类似。例如，在此代码中，静态初始化永远不会发生:

class Program {
    static void Main(string[] args) {
        Console.WriteLine(T.Integer);
        Console.WriteLine(T.Null);
        Console.WriteLine(T.Enum);
        Console.Read();
    }
}

static class T {
    static T() {
        Console.WriteLine("You won't see this.");
    }
    public const int Integer = 1;
    public const string Null = null;
    public const ConsoleKey Enum = ConsoleKey.Escape;
}