How do I declare circular dependant abstract Classes in F#

Question

Consider the two abstract classes alpha and beta:

[<AbstractClass>]  
type alpha () =
    abstract member foo: beta->beta

[<AbstractClass>] 
and beta () = //***
    abstract member bar: alpha

If I try to compile that I get an error, on the line indicated with * * *:

error FS0010: Unexpected keyword 'and' in interaction

And if I write:

[<AbstractClass>]  
type alpha () =
    abstract member foo: beta->beta

and beta () =
    abstract member bar: alpha

then I get:

error FS0365: No implementation was given for 'abstract member beta.bar : alpha'

and the hint that I should add the AbstractClass Attribute

So how do i declare circularly defined abstract classes?

Answer 1

Put the attribute after the 'and' keyword:

[<AbstractClass>]
type alpha () =
    abstract member foo : beta -> beta

and [<AbstractClass>]  beta () =
    abstract member bar : alpha