Is it possible to add DivideByInt to existing types?

Question

Given a list of TimeSpan values, I would like to get the average TimeSpan. It would be elegant if I could just write it like this:

let durations = [TimeSpan.FromSeconds 4.; TimeSpan.FromSeconds 2.]
let avg = durations |> List.average

However, the compiler complains about this:

The type 'TimeSpan' does not support the operator 'DivideByInt'

which is quite reasonable, since TimeSpan indeed doesn't define DivideByInt.

To work around this issue, I attempted to extend TimeSpan like this (before using List.average):

type TimeSpan with
    static member DivideByInt (ts : TimeSpan) (i : int) = 
        (float ts.Ticks) / (float i) |> int64 |> TimeSpan.FromTicks

While this implementation seems to work correctly when used directly, it unfortunately doesn't seem to make the compiler happy when using List.average.

While I'm very much aware that I can use List.averageBy, or just write a custom function to calculate the average of a list of TimeSpan values, I thought it'd be elegant if one could write durations |> List.average.

Is there any way to enable that syntax?

Does TimeSpan have a static member Zero? https://msdn.microsoft.com/en-us/library/ee353446.aspx?f=255&MSPPError=-2147217396 — Ricardo Rodrigues, Jun 02 '15 at 15:33
You could use a wrapper on which the static members `DivideByInt`, `(+)` and `Zero` are defined. Duplicate of this [question](http://stackoverflow.com/questions/20338670/project-new-values-from-existing-value)... — kaefer, Jun 02 '15 at 16:21

score 2 · Accepted Answer · edited May 23 '17 at 12:05

2

No, it's not possible. See this old question. In theory it might be possible to implement it in a future version of F#.

edited May 23 '17 at 12:05

Community

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answered Jun 02 '15 at 15:44

Gus

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score 0 · Answer 2 · answered Jun 02 '15 at 15:35

0

when I tried to overload the op using

type TimeSpan with
member this.op_DivideByInt (ts : TimeSpan) (i : int) = 
    (float ts.Ticks) / (float i) |> int64 |> TimeSpan.FromTicks

let durations = [TimeSpan.FromSeconds 4.; TimeSpan.FromSeconds 2.]
let avg = durations |> List.average

I get the following compiler warning: Extension members cannot provide operator overloads. Consider defining the operator as part of the type definition instead.

so it looks like we can't do this.

answered Jun 02 '15 at 15:35

Chester Husk

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According to MSDN the members must be static: https://msdn.microsoft.com/en-us/library/ee353446.aspx?f=255&MSPPError=-2147217396 – Ricardo Rodrigues Jun 02 '15 at 15:37
true, however the compiler didn't interpret my code as an operator until I tried it as an instance member. – Chester Husk Jun 02 '15 at 15:51

Is it possible to add DivideByInt to existing types?

2 Answers2