在一些关于F#成员约束的功能和写函数之后,像这样:
After some playing around F# member constraints feature and writing function like this:
let inline parse< ^a when ^a : (static member Parse: string -> ^a) > s = (^a: (static member Parse: string -> ^a) s)这完全正常:
let xs = [ "123"; "456"; "999" ] |> List.map parse<int>我试图写其他函数 tryParse ,它使用静态方法 TryParse 并将解析结果包装到'一个选项类型中,以便更好地支持F#。这样的东西不编译:
I'm trying to write other func tryParse, that uses static method TryParse and wraps the parse result into 'a option type for better support in F#. Something like this doesn't compiles:
let inline tryParse s = let mutable x = Unchecked.defaultof< ^a> if (^a: (static member TryParse: string * ^a byref -> bool) (s, &x)) then Some x else None错误是:
错误FS0001:表达式为,预期类型为 byref ,但这里有类型'a ref
F# ref --cells不起作用:
F# ref-cells doesn't work too:
let inline tryParse s = let x = ref Unchecked.defaultof< ^a> if (^a: (static member TryParse: string * ^a byref -> bool) (s, x)) then Some x else None我做错了什么?
推荐答案strong> UPDATE
UPDATE
这似乎在F#3.0中已修复。
This appears to be fixed in F# 3.0.
answer:
我同意Stephen的评论,这很可能是一个错误。对byref类型有很多限制,所以对我来说,它并不特别令人惊讶,他们不能很好地与成员约束。这里是一个使用反射的(丑陋的)解决方法:
I agree with Stephen's comment that it's most likely a bug. There are many limitations on byref types, so it's not particularly surprising to me that they don't play well with member constraints. Here's an (ugly) workaround using reflection:
type parseDel<'a> = delegate of string * 'a byref -> bool type Parser< ^a when ^a : (static member TryParse: string * ^a byref -> bool)> private ()= static let parser = System.Delegate.CreateDelegate(typeof<parseDel<'a>>, typeof<'a>.GetMethod("TryParse", [|typeof<string>; typeof<'a>.MakeByRefType()|])) :?> parseDel<'a> static member inline ParseDel = parser let inline tryParse (s:string) = let mutable x = Unchecked.defaultof< ^a> if Parser<_>.ParseDel.Invoke(s, &x) then Some x else None let one : int option = tryParse "1"更多推荐
F#成员约束+ ^ a byref参数
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