从 F# 调用 DateTime.TryParse 的正确方法是什么？

Question

What is the correct way to call DateTime.TryParse from F#? I am trying to test some code from F# interactive and I can't figure out how to pass a mutable DateTime into the second argument by ref. What is the in/out/ref syntax in F#?

This is the method signature I'm looking at: http://msdn.microsoft.com/en-us/library/ch92fbc1.aspx?cs-save-lang=1&cs-lang=fsharp#code-snippet-1

Answer 1

Chris's answer is correct if you really need to pass a mutable DateTime by reference. However, it is much more idiomatic in F# to use the compiler's ability to treat trailing out parameters as tupled return values:

let couldParse, parsedDate = System.DateTime.TryParse("11/27/2012")

Here, the first value is the bool return value, while the second is the assigned out parameter.

Answer 2

Here's how to execute DateTime.TryParse in F#:

let mutable dt2 = System.DateTime.Now
let b2 = System.DateTime.TryParse("12-20-04 12:21:00", &dt2)

Where the & operator finds the memory address of dt2 in order to modify the reference.

Here's some additional information on F# parameter syntaxt.

Answer 3

Just for the sake of completeness, yet another option is to use ref cells, eg

let d = ref System.DateTime.MinValue
if (System.DateTime.TryParse("1/1/1", d)) then
   // ...

Answer 4

I've found one more way, it seems more functional style (from https://stackoverflow.com/a/4950763/1349649 )

match System.DateTime.TryParse "1-1-2011" with
| true, date -> printfn "Success: %A" date
| false, _ -> printfn "Failed!"

Unfirtunately, I can't find any information about how it works.

Answer 5

I have a helper module I like to include in all my projects. I collect things like this, putting all the imperative -> functional conversions in one place for reuse:

module helper = 
    
    //if some, unwrap input and call fun
    let inline ( |>- ) x f  = Option.bind f x //bind f x
    //if some, unwrap input, call fun, and wrap output 
    let inline ( |>-+ ) x f = Option.map f x 

    module dateTime =

      let tryParse (input: string) : DateTime option =
        let mutable dt = DateTime.Now
        if DateTime.TryParse(input, &dt) then Some dt else None