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Question

I am trying to build a generic function to manipulate a record my code looks like:

type Status = Active | Inactive

type IStatus =
    abstract member Status: Status

type User = 
    { 
        Username : string;
        Status : Status
    }
    interface IStatus with
        member x.Status = x.Status


let ChangeStatus<'T when 'T :> IStatus> newStatus (t:'T) =
    {t with Status = newStatus}

Now I get the following error:

expression was expected to have type
    'T    
but here has type
    User    

Obviously I just want to create a type constraint for Records which implement IStatus. Am I thinking too OO? Or is there merit to this approach and how do I create this ChangeStatus function?

Thank you for reading.

Answer 1

I don't think it's possible what you're trying to do, because it would need a "generic record cloner" I mean a generic record expression and that's not supported at the moment.

You can create a clone method for each subclass, that should work but you will have to repeat the code to clone the record. It might be a generic solution but involving reflection.

However if you change your design you can get the desired functionality. For instance you can use a generic nested record:

type Status = Active | Inactive

type StatusRecord<'T> =
    { 
        Item   : 'T
        Status : Status
    }

let changeStatus newStatus t = {t with Status = newStatus}

// TEST

type User  = {Username  : string}
type Group = {Groupname : string; members : User list}

let user  = {Status = Active; Item = {Username = "User1"}}
let group = {Status = Active; Item = {Groupname = "Group1"; members = []}}

This is a very lightweight solution, you will write less code but it will change your design which depending on the rest of your code will make sense or not.