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Polymorphic signature
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Hi,

I'm trying to implement a generic UI (a DialogBox), which allows to choose elements among a list.

This list can a whatever (a string list, or any object list).

The display name of the elements is generated by the ToString() call in the listbox, so typically I'm just overriding the ToString() method if needed.

Here is the code : 

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#light
open System.Drawing
open System.Windows.Forms
open System
type ui_selection_multiple = 
    class
        inherit Form
        val mutable names : array<_>
        val mutable lb : CheckedListBox
        val mutable bt : Button
        new(names) as this = {names = names ;lb=null;bt=null}
            then
                let bt = new Button()
                bt.Text <- "Go"
                bt.DialogResult <- DialogResult.OK
                let lb = new CheckedListBox()
                lb.CheckOnClick <- true 
                this.Controls.Add(lb)
                this.Controls.Add(bt)
                lb.Dock <- DockStyle.Fill
                bt.Dock <- DockStyle.Bottom
                let tmp = Array.map (fun name -> lb.Items.Add(name)) names
                this.bt <- bt
                this.lb <- lb
                ()
        member this.selected_list() =
            let selected =  IEnumerable.untyped_to_typed this.lb.CheckedItems
            IEnumerable.to_array selected
end

When including this routine within a larger library, with a function that calls it : no problem

But the names member gets typed, and so it looses its genericity (and so I cannot reuse it with a different kind of array)

When trying to compile this snippet alone, I get this error log :

<i>ui_utilities.fs(11,8): error: FS0030: Value restriction. Type inference has inferred the polymorphic signature<i> <i> val ( .ctor ) : 'a array -> ui_selection_multiple (in type ui_selection_multiple) but '( .ctor )' is constructor or property getter/setter, which cannot be more generic than their enclosing ty pe. Add type constraints to indicate the exact types involved.<i> Any idea to keep this CheckedListBox as generic as it should be? Many thanks in advance Regards Julien

.

We'll look into why the error message is poor here, but here's the fix - just declare the type variable. Type variables are not automatically generalized for type definitions - you must declare them.

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#light
open System.Drawing
open System.Windows.Forms
open System
type ui_selection_multiple<'a> = 
    class
        inherit Form
        val mutable names : array<'a>
        val mutable lb : CheckedListBox
        val mutable bt : Button
        new(names) as this = 
             {names = names ;lb=null;bt=null}
            then
                let bt = new Button()
                bt.Text <- "Go"
                bt.DialogResult <- DialogResult.OK
                let lb = new CheckedListBox()
                lb.CheckOnClick <- true 
                this.Controls.Add(lb)
                this.Controls.Add(bt)
                lb.Dock <- DockStyle.Fill
                bt.Dock <- DockStyle.Bottom
                let tmp = Array.map (fun name -> lb.Items.Add(name)) names
                this.bt <- bt
                this.lb <- lb
                ()
        member this.selected_list() =
            let selected =  IEnumerable.untyped_to_typed this.lb.CheckedItems
            IEnumerable.to_array selected
end
By on 10/31/2006 5:38 PM ()

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