{||} cannot be used as a type

[Happypig375]

{||} cannot be used as a type.

Expected behavior

Microsoft (R) F# Interactive version 10.4.0 for F# 4.6
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> {||};;
val it : {||} = {}

> let x = {||};;
val x : {||} = {}

> let x : {||} = {||};;
val x : {||} = {}

Repro steps and actual behavior

Microsoft (R) F# Interactive version 10.4.0 for F# 4.6
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> {||};;
val it : {||} = {}

> let x = {||};;
val x : {||} = {}

> let x : {||} = {||};;

  let x : {||} = {||};;
  ----------^^

stdin(3,11): error FS0010: Unexpected symbol '|}' in field declaration. Expected identifier or other token.

Known workarounds

Let the compiler infer the type.

Related information

Microsoft Visual Studio Community 2019 Version 16.1.1 VisualStudio.16.Release/16.1.1+28922.388 Microsoft .NET Framework Version 4.7.03056

Installed Version: Community

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[auduchinok]

What type do you expect it to be?

[Happypig375]

> let x = {||};;
val x : {||} = {}

[Happypig375]

For reference, this works.

> let x : {| x:int |} = {| x  = 3 |};;
val x : {|x : int|} = {x = 3;}

[cartermp]

I consider it a bug that {||} is allowed as a type, since you cannot specify an empty record type:

type R = {} // error FS0010: Unexpected symbol '}' in field declaration. Expected identifier or other token.

@dsyme thoughts?

[Happypig375]

{} should not be allowed as a type because there would be no way to disambiguate type R1 = {} and type R2 = {}.
However, all {||}s in an assembly are the same type and value. There is no ambiguity. Therefore, it should be allowed.

[matthid]

@Happypig375 Not sure about that argument. That argument holds for all records with the same fields. Regular "solutions" for this are type annotations and declaration order (as the compiler prefers the last declaration). Both would work for empty records just as well.

Therefore I'm with @cartermp: Either we don't allow it or we allow both...

[Happypig375]

You can disambiguate regular records:

> type R1 = { x:int }
type R2 = { x:int }
{ R1.x = 3 };;
type R1 =
  {x: int;}
type R2 =
  {x: int;}
val it : R1 = {x = 3;}

Not possible with empty records.

[matthid]

@Happypig375 Ah, I wasn't even thinking about this particular syntax as it either makes you repeat the typename or adds some asymmetry which I don't like.

But the following would work:

type R1 = { x:int }
type R2 = { x:int }
{ x = 3 };;
//type R1 =
//  {x: int;}
//type R2 =
//  {x: int;}
//val it : R2 = {x = 3;}

let r1 : R1 = { x = 3 };;
//val r1 : R1 = {x = 3;}
({ x = 2 } : R1);;
//val it : R1 = {x = 2;}

So I hope this clarifies my last comment. What I mean is that you can always specify which one you want to use in any expression via type annotations. The (impossible) syntax you used is not needed to solve the ambiguity:

type R1 = { }
type R2 = { }
{ };; // R2
//type R1 = {}
//type R2 = {}
//val it : R2 = {}
let r1 : R1 = {};; // R1
//val r1 : R1 = {}
({ } : R1);; // R1
//val it : R1 = {}

[cartermp]

Just to clarify my point: Anonymous Records are intended to just be records without a name. This lack of symmetry feels more like a mistake than an intentional design point. I think that is the case, otherwise it would be allowable to specify {||} in a type signature.

[dsyme]

I consider it a bug that {||} is allowed as a type, since you cannot specify an empty record type:

I agree with this

[cartermp]

Okay. Resolution:

• It's a bug that {||} can be specified as a value
• The fix would be to disallow its use in that fashion