Error: FSharp.Reflection: Microsoft.FSharp.Core.FSharpOption`1 is not an F# union type #4082

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1. mlaily

   changed the title [-]Error: FSharp.Reflection: `Microsoft.FSharp.Core.FSharpOption`1` is not an F# union type[/-] [+]Error: FSharp.Reflection: Microsoft.FSharp.Core.FSharpOption`1 is not an F# union type[/+] on Mar 16, 2025
2. mlaily commented on Mar 16, 2025

   

   mlaily

   on Mar 16, 2025

   Last edited by mlaily

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   Ah, actually the issue might not be where I though it was...

   I'm still not sure whether it's normal that an option is not seen as a DU in Fable compiled code, but the root cause of my initial error seems to be a discrepancy in the behaviour of Microsoft.FSharp.Reflection.FSharpType methods vs Fable.Core.Reflection methods, probably due to the erasure of options.

   The following code:

   type MyUnion = | A of int | B of string
   
   let testValue = Some (MyUnion.A 42)
   
   Browser.Dom.console.log ({| TestValue = testValue |})
   Browser.Dom.console.log ({| TestValueType = testValue.GetType() |})
   Browser.Dom.console.log ({| IsUnionFS = Microsoft.FSharp.Reflection.FSharpType.IsUnion(testValue.GetType())|})
   Browser.Dom.console.log ({| IsUnionFable = Fable.Core.Reflection.isUnion testValue |})
   Browser.Dom.console.log ({| CaseNames = Fable.Core.Reflection.getCaseName testValue |})
   Browser.Dom.console.log ({| CaseFields = Fable.Core.Reflection.getCaseFields testValue |})

   Displays the following in a console:

   

   The union case seen by Fable.Core.Reflection is actually not the option, but the wrapped value! (which is itself a custom DU)

   So mixing reflection calls from both namespaces at the same time seems very error prone...

   EDIT: this has been fixed here elmish/browser#77

   I'm still curious about your expert opinion on this:

   • Is all the above expected?
   • Is there something we could improve? In documentation at least?
   • What's your recommendation regarding reflection in Fable? Prefer using Microsoft.FSharp.Reflection or Fable.Core.Reflection?
3. kerams commented on Mar 21, 2025

   

   kerams

   on Mar 21, 2025

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   Related #2110
4. github-actions

   added

   bug

   on Mar 17, 2026
5. github-actions

   added a commit that references this issue on Apr 12, 2026

   Fix option<'T> not recognized as union type in reflection (fixes #4082)

   ...

   This issue will close once commit 6924ec4 is merged into the 'main' branch.

   6924ec4
6. github-actions commented on Apr 12, 2026

   

   github-actionsBot

   on Apr 12, 2026 – with GitHub Actions

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   ---

   Note

   The pull request was not created — a fallback review issue was created instead due to protected file changes: #4530

   🤖 This is an automated response from Repo Assist.

   I've submitted a draft PR that addresses this issue: I investigated the root cause and found that option_type() in fable-library-ts/Reflection.ts (and the equivalent in fable-library-py/fable_library/reflection.py) was building a TypeInfo without a cases property — the exact field that FSharpType.IsUnion and GetUnionCases rely on.

   The fix rewrites option_type() to expose None (tag 0) and Some (tag 1) as union cases, and adds option-aware handling in getUnionFields/makeUnion to account for Fable's erased option representation at runtime.

   Please check the PR for details and test coverage. CI will validate the build.

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7. dbrattli

   closed this as completedin #4529on Apr 27, 2026