为什么这个 F# 计算表达式会发出警告？
此代码：
type Result = Success of string
type Tracer() =
member x.Bind(p: Result, rest: (string -> Result)) =
match p with
| Success s -> rest s
let tracer = new Tracer()
let t = tracer {
let! x = Success "yes!"
let! y = Success "waste of time"
return! Success x
}
printfn "%A" t
打印 Success "yes!"
但给出警告，暗示它不应该工作：
File1.fs(19,3): warning FS0708: This control Construction may only be如果计算表达式构建器定义了“ReturnFrom”方法，则使用
似乎是一个奇怪的警告：如果它是正确的，那么代码不应该工作。难道真的只是说构建器必须合成 ReturnFrom 吗？
（F# 版本 1.9.7.4，针对 .NET Framework 版本 v4.0.21006 进行编译）
This code:
type Result = Success of string
type Tracer() =
member x.Bind(p: Result, rest: (string -> Result)) =
match p with
| Success s -> rest s
let tracer = new Tracer()
let t = tracer {
let! x = Success "yes!"
let! y = Success "waste of time"
return! Success x
}
printfn "%A" t
prints Success "yes!"
But gives a warning that implies that it shouldn't work:
File1.fs(19,3): warning FS0708: This control construct may only be used if the computation expression builder defines a 'ReturnFrom' method
Seems like a strange warning: if it's right, then the code shouldn't work. Is it really just saying that the builder had to synthesize ReturnFrom?
(F# Version 1.9.7.4, compiling for .NET Framework Version v4.0.21006)
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评论（2）
我已将问题发送至[电子邮件受保护]，这是一个错误。他们说这将是下一个版本中的错误。
（他们几乎立即回复，在感恩节也不例外 - 我只是花了一段时间才将信息放在这里。）
I sent the question to [email protected], and it's a bug. They said it'll be an error in the next release.
(They responded almost immediately, on Thanksgiving no less - it just took me a while to put the info here.)
我很惊讶这有效。 规范第 6.10 节 在未指定时没有提及任何有关合成
ReturnFrom的内容。是否有理由不将member x.ReturnFrom v = v放在构建器上？或者一个member x.Return(v) = Success v，这样你就可以用return x结束你的tracer块，这会更有效传统的？I'm surprised that this works. Section 6.10 of the spec doesn't mention anything about synthesizing
ReturnFromwhen it's not specified. Is there a reason not to just put amember x.ReturnFrom v = von the builder? Or amember x.Return(v) = Success v, so that you could end yourtracerblock withreturn x, which would be more traditional?