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Allow interfaces with no fields, unions with no members #1229

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benjie merged 2 commits into from
Nov 5, 2026
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Allow interfaces with no fields, unions with no members #1229

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Interfaces with no fields
benjie May 23, 2026
fa2fb46
Allow a union to be empty
benjie May 23, 2026
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10 changes: 4 additions & 6 deletions spec/Section 3 -- Type System.md
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Expand Up @@ -1275,8 +1275,7 @@ Interfaces are never valid inputs.

Interface types have the potential to be invalid if incorrectly defined.

1. An Interface type must define one or more fields.
2. For each field of an Interface type:
1. For each field of an Interface type:

@DaleSeo DaleSeo Oct 29, 2026

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Same as my note on #1228: replace the rule rather than delete it, so interface Empty {} is valid and a braceless interface Empty stays invalid.

1. An Interface type must define a set of fields. The set may be empty.

1. The field must have a unique name within that Interface type; no two
fields may share the same name.
2. The field must not have a name which begins with the characters {"\_\_"}
Expand All @@ -1290,9 +1289,9 @@ Interface types have the potential to be invalid if incorrectly defined.
arguments may share the same name.
3. The argument must accept a type where {IsInputType(argumentType)}
returns {true}.
3. An interface type may declare that it implements one or more unique
2. An interface type may declare that it implements one or more unique
interfaces, but may not implement itself.
4. An interface type must be a super-set of all interfaces it implements:
3. An interface type must be a super-set of all interfaces it implements:
1. Let this interface type be {implementingType}.
2. For each interface declared implemented as {implementedType},
{IsValidImplementation(implementingType, implementedType)} must be {true}.
Expand Down Expand Up @@ -1450,8 +1449,7 @@ Unions are never valid inputs.

Union types have the potential to be invalid if incorrectly defined.

1. A Union type must include one or more unique member types.

@benjie benjie Oct 15, 2026

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Let's not lose the fact they are unique - ensure that it's declared as a "set" somewhere maybe?

@DaleSeo DaleSeo Oct 29, 2026

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I wonder how the direction suggested in #1249 (comment) applies here. Unions have no braces, so there is no way to tell an empty union from an incomplete one: union Empty would be the only form, and it becomes valid once this rule is removed. Is that fine for unions, or did you have something like union Empty = in mind?

2. The member types of a Union type must all be Object base types; Scalar,
1. The member types of a Union type must all be Object base types; Scalar,
Interface and Union types must not be member types of a Union. Similarly,
wrapping types must not be member types of a Union.

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