Question

I just read the information on this page, and while a new ? operator is mentioned, it’s quite unclear to me what would its usage be.
Could anyone please provide a quick explanation, post a code snipped of how would this operator be used and possibly mention a use case?
Edit: this is really awkward, I’ve noticed that the ? operator is no longer mentioned in Don’s release notes. Any idea of why is that?

Answer 1

There are two new “special” operators in this F# release, (?) and (?<-). They are not defined, but they are available for overloading, so you can define them yourself. The special bit is how they treat their 2nd operand: they require it to be a valid F# identifier, but pass it to function implementing the operator as a string. In other words:

a?b

is desugared to:

(?) a "b"

and:

a?b <- c

is desugared to:

 (?<-) a "b" c

A very simple definition of those operators could be:

let inline (?) (obj: 'a) (propName: string) : 'b =
    let propInfo = typeof<'a>.GetProperty(propName)
    propInfo.GetValue(obj, null) :?> 'b

let inline (?<-) (obj: 'a) (propName: string) (value: 'b) =
    let propInfo = typeof<'a>.GetProperty(propName)
    propInfo.SetValue(obj, value, null)

Note that since the return type for the gettor is generic, you’ll have to specify it at use site in most cases, i.e.:

let name = foo?Name : string

though you can still chain-call (?) (since first argument of (?) is also generic):

let len = foo?Name?Length : int

Another, more interesting, implementation is to re-use CallByName method provided by VB:

open Microsoft.VisualBasic    

let inline (?) (obj: 'a) (propName: string) : 'b =
    Interaction.CallByName(obj, propName, CallType.Get, null) :?> 'b //'

let inline (?<-) (obj: 'a) (propName: string) (value: 'b) =
    Interaction.CallByName(obj, propName, CallType.Set, [| (value :> obj) |])
    |> ignore

The advantage of that is that it will handle both properties and fields correctly, work with IDispatch COM objects, etc.