[RFC FS-1331] - Allow opens in type and expression scopes

[ijklam]

• Suggestion
• Implementation

[ijklam]

I am a bit wanna to make the type-scoped opens must to be the first part of a type definition, or remove it, due to the complexity of the typechecking on types.

On current prototype, the type-scoped opens can be applied to all members and vals and interface, including the definition section of record/union/exceptions.

type ABC2() =
//    inherit Object()                     // cannot find Object here
//    let maxValue = Int32.MaxValue        // cannot find Int32 here
    [<DefaultValue>] val mutable minValue: Int32
    member _.Write x = Console.Write (x: string)
    interface IDisposable with
        member this.Dispose (): unit = raise (NotImplementedException())
    open System

type A = 
  | A of Int32            // can find Int32 here
    with
        open System

[brianrourkeboll]

Interesting. I guess that's because some parts of type definitions implicitly have recursive scope while others don't?

[ijklam]

The typechecking to fields, structural fields, interface ... with are separate, it doesn't know the order of opens when typechecking them, therefore we can only take all opens out, apply to the env, and pass it to the typecheck functions.

The opens are always putted before members because of the pre-process to auto properties. Only typechecking lets can keep the order of opens.

I don't like the current type-scoped opens, as it is not following the up-to-down order. And by having these restrictions, it seems less useful.

[T-Gro]

I support the restriction - in a type definition, allow opens right below the beginning of a type.

The examples you have shown - symbols being available even when open System comes later - breaks my natural top-to-bottom reading habit for F# code.

Even if in types it is limited at the beginning of a type only, its important that it still works for any expression body within that type - that works, right?

[ijklam]

Yes. I will make type-scoped opens must at the beginning of type definition.