Should we allow to disable reflection on a specific type?

[MangelMaxime]

With the code below,

type SubType<'T> =
    {
        SubValue: 'T
    }

[<Erase>]
type TypeA<'T> = TypeA of (unit -> SubType<'T>)

type Updates =
    {
        Func : TypeA<unit>
    }

Fable generates this error:

error FABLE: Cannot get type info of generic parameter T. Fable erases generics at runtime, try inlining the functions so generics can be resolved at compile time.

This is because, here Func : TypeA<unit> has a concrete type giving.

However, for a library I am working on I kind of need this type of code supported.

A workaround, is to enable --noReflection flag in Fable but it means not reflection can happen anywhere, so perhaps it could be nice to introduce a [<NoReflection>] attribute to disable it only for specific types.

Another workaround is to use an anonymous record:

type Updates =
    {|
        VarView : TypeA<unit>
    |}

I will see if I can rewrite my library to use the anonymous record, as it would be better to not introduce an attribute for a really specific usage.

[MangelMaxime]

Note, in my situation I Updates have additional properties who are mark as mutable which is invalid for anonymous records

[github-actions]

🤖 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.

Note

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