f# - 这是 F# Builder 错误还是我的误解？

Question

当我尝试控制台编程时，我收到了意想不到的结果。

open System

let printSomeMessage =        
    printfn "Is this the F# BUG?"    

[<EntryPoint>]
let main args =    
    if args.Length = 2 then
        printSomeMessage
    else        
        printfn "Args.Length is not two."
    0

printSomeMessage 函数包含在 .cctor() 函数中。这是 IL DASM 结果。

.method private specialname rtspecialname static 
        void  .cctor() cil managed
{
  // Code size       24 (0x18)
  .maxstack  4
  IL_0000:  nop
  IL_0001:  ldstr      "Is this the F# BUG\?"
  IL_0006:  newobj     instance void class [FSharp.Core]Microsoft.FSharp.Core.PrintfFormat`5<class [FSharp.Core]Microsoft.FSharp.Core.Unit,class [mscorlib]System.IO.TextWriter,class [FSharp.Core]Microsoft.FSharp.Core.Unit,class [FSharp.Core]Microsoft.FSharp.Core.Unit,class [FSharp.Core]Microsoft.FSharp.Core.Unit>::.ctor(string)
  IL_000b:  call       !!0 [FSharp.Core]Microsoft.FSharp.Core.ExtraTopLevelOperators::PrintFormatLine<class [FSharp.Core]Microsoft.FSharp.Core.Unit>(class [FSharp.Core]Microsoft.FSharp.Core.PrintfFormat`4<!!0,class [mscorlib]System.IO.TextWriter,class [FSharp.Core]Microsoft.FSharp.Core.Unit,class [FSharp.Core]Microsoft.FSharp.Core.Unit>)
  IL_0010:  dup
  IL_0011:  stsfld     class [FSharp.Core]Microsoft.FSharp.Core.Unit '<StartupCode$FSharpBugTest>'.$Program::printSomeMessage@3
  IL_0016:  pop
  IL_0017:  ret
} // end of method $Program::.cctor

所以，它的执行结果是这样的。

Is this the F# BUG?
Args.Length is not two.

我是否缺少一些语法或 F# 特征？还是 F# builder 的 BUG？

Answer 1

不，这是您代码中的错误。您需要在“printSomeMessage”之后添加括号，否则 printSomeMessage 是一个简单的值而不是一个函数。

open System

let printSomeMessage() =        
    printfn "Is this the F# BUG?"    

[<EntryPoint>]
let main args =    
    if args.Length = 2 then
        printSomeMessage()
    else        
        printfn "Args.Length is not two."
    0

简单值在模块的构造函数中初始化，因此您会看到在模块初始化时调用了您的代码。仔细想想这是合乎逻辑的，简单值的正常情况是将字符串、整数或其他文字值绑定(bind)到标识符。您会期望这会在启动时发生。即以下将在模块启动时绑定(bind):

let x = 1
let y = "my string"