How to implement an interface member that returns void in F#

Question

Imagine the following interface in C#:

interface IFoo {
    void Bar();
}

How can I implement this in F#? All the examples I've found during 30 minutes of searching online show only examples that have return types which I suppose is more common in a functional style, but something I can't avoid in this instance.

Here's what I have so far:

type Bar() =
    interface IFoo with
        member this.Bar() =
            void

Fails with _FS0010:

Unexpected keyword 'void' in expression_.

ChaosPandion · Accepted Answer · 2010-06-18T17:29:53.190

22

The equivalent is unit which is syntactically defined as ().

type Bar() =
    interface IFoo with
        member this.Bar () = ()

edited Jun 18 '10 at 17:29

answered Jun 18 '10 at 06:27

ChaosPandion

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@Drew - It is an awesome language. – ChaosPandion Jun 18 '10 at 13:30
The C# code was a method, not a property. To reproduce this in F#, I believe you need a member with signature unit -> unit. This would be written "member this.Bar () = ()". Without the unit parameter, the F# could would be a property with a getter. – Jason Jun 18 '10 at 17:18
@Jason - Thanks, it is safe to assume that @Drew implicitly corrected my mistake. – ChaosPandion Jun 18 '10 at 17:30
@Jason, you're right. Actually my method took arguments (and surprisingly wasn't called Bar!) so I'd messed up transcribing it in the original question. Thanks for pointing this out. I'll edit the question. – Drew Noakes Jun 18 '10 at 18:19
@Drew - Wait your method wasn't named Bar? That is a production quality method name... :) – ChaosPandion Jun 19 '10 at 16:16

score 7 · Answer 2 · edited Jun 18 '10 at 10:04

7

For general info on F# types, see

The basic syntax of F# - types

From that page:

The unit type has only one value, written "()". It is a little bit like "void", in the sense that if you have a function that you only call for side-effects (e.g. printf), such a function will have a return type of "unit". Every function takes an argument and returns a result, so you use "unit" to signify that the argument/result is uninteresting/meaningless.

edited Jun 18 '10 at 10:04

Drew Noakes

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answered Jun 18 '10 at 09:00

Brian

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Thanks for the link. I included a quote from the page in your answer. – Drew Noakes Jun 18 '10 at 10:05

score 6 · Answer 3 · answered Jun 18 '10 at 06:30

6

The return type needs to be (), so something like member this.Bar = () should do the trick

answered Jun 18 '10 at 06:30

corvuscorax

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7

To be pedantic, the return _type_ is `unit`, the only _value_ of that type is spelled `()`. – Brian Jun 18 '10 at 08:58

score 3 · Answer 4 · answered Apr 12 '16 at 15:46

3

The equivalent in F# is:

type IFoo =
    abstract member Bar: unit -> unit

answered Apr 12 '16 at 15:46

Márcio Azevedo

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How to implement an interface member that returns void in F#

4 Answers4

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