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Lensing into a value of discriminated union contained in ListModel
websharperlistmodellens

With a ListModel of Pet and knowing the precise name of a pet:

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type Pet =
| Dog of name:string * owner:int
| Cat of name:string

How can I extract a IRef to set the owner of a Dog?

For the following example, I want to get a IRef for cupcake in order to use it with a Doc.Input to change the owner.

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type Pet with
  static member Name =
    function
    | Dog (n, _) -> n
	| Cat n -> n
                
let listModel = 
  ListModel.Create 
    Pet.Name 
    [ Dog ("muffin", 1)
      Dog ("cupcake", 1)
      Cat "muffin" ]

This is how I am doing it for the moment:

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listModel.LensInto 
  (function 
  | Dog ("cupcake", owner) -> owner 
  | _ -> failwith "Was supposed to be a dog") 
  (fun p v -> 
    match p with
    | Dog ("cupcake", _) -> Dog ("cupcake", v) 
    | _ -> failwith "Was supposed to be a dog") 
"cupcake"
.

One thing zou could do is create an IRef<int option> out of it with LensInto:

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let dogOwner name =
    listModel.LensInto
        (function
            | Dog (_, owner) -> Some owner
            | Cat _ -> None)
        (fun a s ->
            match s, a with
            | Some o, Dog (name, owner) -> Dog (name, owner)
            | _, x -> x)
        name

It would probably be nice to have a Prism function to complement Lens like in Aether, which would have getters and setters of the form ('T -> 'V option) * ('T -> 'V -> 'V).

By on 4/4/2016 1:10 AM ()

Taking a deeper look into it Prism is not really applicable here as our lenses are a bit different because of how IRefs and Var operate in general.

By on 4/4/2016 2:09 AM ()

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