Question

I’m trying to create an attribute in F# with positional arguments but failing all the time.

type ColumnAttribute(?index:int,?name:string) =
    inherit Attribute()
    let mutable index = index
    let mutable name = name
    member x.Index 
        with get() = index
        and set value = index <- value
        member x.Name 
        with get() = name
        and set value = name <- value

type Substance = {
    [<Column(Index=1)>] Name : string
    [<Column(Index=0)>] Id : int
    [<Column(Name="sequence")>] Sequence : string
}

I’ve tried several different approaches but this is the closest I ended with.

Answer 1

There are 2 issues that I see with your code

1. In order for this to work you need to define a no-argument constructor for ColumnAttribute.
2. The type of Index and Name is int option and string option respectively but you need it to be int and string.

Try the following

type ColumnAttribute(index:int option,name:string option) =     
    inherit Attribute()     
    let mutable index = index
    let mutable name = name
    new () = ColumnAttribute (None, None)
    member x.Index          
        with get() = match index with | Some i -> i | None -> 0
        and set value = index <- Some value         
    member x.Name          
        with get() = match name with | Some n -> n | None -> ""
        and set value = name <- Some value  

type Substance = {     
    [<Column(Index=1)>] Name : string     
    [<Column(Index=0)>] Id : int     
    [<Column(Name="sequence")>] Sequence : string }