Should we allow to disable reflection on a specific type? #4241

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1. MangelMaxime commented on Oct 23, 2025

   

   MangelMaxime

   on Oct 23, 2025

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   Note, in my situation I Updates have additional properties who are mark as mutable which is invalid for anonymous records
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   on Mar 17, 2026
3. github-actions commented on Apr 5, 2026

   

   github-actionsBot

   on Apr 5, 2026 – with GitHub Actions

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   🤖 This is an automated response from Repo Assist.

   Here's a technical analysis of the options for this request.

   Current behaviour

   Fable's reflection system emits type info for every type that might need it at runtime (used by typeof<T>, typedefof<T>, and serialisation libraries). When a generic erased-union type is stored in a record field with a concrete type parameter (like Func: TypeA<string>), reflection tries to resolve the generic and fails.

   Options

   Option 1: Per-type [<NoReflection>] attribute

   This would be a new attribute in Fable.Core that suppresses reflection emission for the decorated type. The main places to change:

   • src/Fable.Core/Fable.Core.fs — add [<AttributeUsage(AttributeTargets.Class ||| AttributeTargets.Struct)>] NoReflectionAttribute
   • src/Fable.Transforms/Transforms.Util.fs — in getEntityInfoHelper (or wherever type info is generated), check for the attribute and return a simpler/empty info
   • src/Fable.Transforms/Replacements.Util.fs — in typeInfo and related helpers, skip reflection for types with the attribute

   This is feasible but touches Fable.Core (distributed on NuGet) and shared transform code. It would need careful design: if reflection is disabled for TypeA<'T>, what happens when code calls typeof<TypeA<string>>? It should either return a dummy type or raise a compile-time warning.

   Option 2: Use --noReflection with selective opt-in

   The existing --noReflection flag disables reflection globally. A --noReflection global flag combined with a [<EnableReflection>] attribute to opt specific types back in would be the inverse approach. This might be simpler since the default path (no reflection) is already implemented.

   Option 3: Use a type wrapper without generics for the field

   As you noted, anonymous records work around this. Another pattern is to wrap in a non-generic interface:

   type ITypeA = interface end
   [<Erase>] type TypeA<'T> = TypeA of (unit -> SubType<'T>) interface ITypeA
   type Updates = { Func: ITypeA }

   This loses type safety but avoids the generic reflection issue.

   Recommendation

   Option 1 seems most user-friendly. The implementation should:

   1. Accept the attribute on type declarations
   2. When the type has [<NoReflection>], emit a placeholder type info (e.g., { fullname: "...", generics: () => [] }) or no type info at all
   3. Issue a compile-time warning if typeof<T> is used on such a type

   This is a good candidate for a focused PR in Fable.Core and Replacements.Util.fs. If you'd like to pursue it, the key entry point to look at is typeInfo in src/Fable.Transforms/Replacements.Util.fs and the attribute check pattern already used for [<Erase>] types.

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