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Implementing Interfaces
f#compiler

I am trying to understand the implementation of interfaces. Here is a simple example

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type ITestInterface = interface
    abstract member count : int with get
end

type testobj = class
    interface ITestInterface
    with
        member x.get_count() = 0
    end
end

type ITestHolder = interface
    abstract member Data : ITestInterface with get
end

type holder_obj = class
    val data : ITestInterface
    // Constructor    
    new() = { data = new testobj()  // Here the compiler complains: The expression has type testobj
                                                  // but is used with type ITestInterface
                }
    interface ITestHolder
    with 
        member x.get_Data() = data
    end
end

Since testobj implements ITestInterface, what is the issue with the compiler?

Thanks in advance

Chris

.

What is the reason for needing an explict downcast?

By on 6/27/2006 8:19 PM ()

F# requires types to match exactly or you get a type error. You can generally get round this by placing # before your type constraint. For example the following won't compile:

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open System.Windows.Forms

let show_form (form : Form) =
    form.Show()

// PrintPreviewDialog is defined in the BCL and is 
// derived directly the Form class
let my_form = new PrintPreviewDialog() 

do show_form my_form

Where as the following will:

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let show_form_revised (form : #Form) =
    form.Show()

// ThreadExceptionDialog  is define in the BCL and is 
// directly derived type of the Form class
let another_form = new ThreadExceptionDialog(new Exception()) 
do show_form_revised another_form
By on 6/27/2006 10:43 PM ()

You need an explicit downcast in the constructor, also a constructor was missing from testobj and "member x.get_Data() = x.data" was missing the "x". The following will compile:

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type ITestInterface = interface
    abstract member count : int with get
end

type testobj = class
new() = {}
    interface ITestInterface
    with
        member x.get_count() = 0
    end
end

type ITestHolder = interface
    abstract member Data : ITestInterface with get
end


type holder_obj = class
    val data : ITestInterface
    // Constructor
    new() = { data = (new testobj()  :> ITestInterface)
                }
    interface ITestHolder
    with
        member x.get_Data() = x.data
    end
end
By on 6/23/2006 12:14 AM ()

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