int list is not a union type in Fable

[njlr]

Description

int list (or any other list) is not a union type in Fable.

Repro code

open System
open FSharp.Reflection

let isUnionType (ty : Type) =
  FSharpType.IsUnion(ty, true)

let isListType (ty : Type) =
  ty.IsGenericType && ty.GetGenericTypeDefinition() = typedefof<list<_>>

let ty = typeof<int list>

printfn $"%b{isUnionType ty}"
printfn $"%b{isListType ty}"

Expected and actual results

Expect this in .NET and Fable:

true
true

But Fable gives:

false
true

Related information

• Fable version: 4.1.4
• Operating system: Ubuntu 22.04

[github-actions]

🤖 This is an automated response from Repo Assist.

Root cause identified.

In Fable's runtime reflection library (src/fable-library-ts/Reflection.ts), the isUnion check is:

export function isUnion(t: any): boolean {
  return t instanceof TypeInfo ? t.cases != null : t instanceof Union;
}

It returns true only if the TypeInfo has a cases property set. However, list_type creates a plain TypeInfo with no cases:

export function list_type(generic: TypeInfo): TypeInfo {
  return new TypeInfo("Microsoft.FSharp.Collections.FSharpList`1", [generic]);
}

The same issue likely affects option_type:

export function option_type(generic: TypeInfo): TypeInfo {
  return new TypeInfo("Microsoft.FSharp.Core.FSharpOption`1", [generic]);
}

Why it's wrong: In .NET, list<'T> and option<'T> are implemented as discriminated unions ([]/:: and None/Some respectively), so FSharpType.IsUnion correctly returns true for them.

Fix approach: Both list_type and option_type should have cases populated to match their .NET DU structure:

• option_type cases: None (tag 0, no fields), Some (tag 1, field ["Value", generic])
• list_type cases: Empty (tag 0, no fields), Cons (tag 1, fields ["Head", generic], ["Tail", listTypeRef])

This would also fix FSharpType.GetUnionCases for these types. Note that all per-target runtime libraries (fable-library-py, Rust, Dart, Beam) may have the same gap and should be checked for consistency.

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