IsUnion returns false for typeof<Option<int>> #2110

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1. alfonsogarciacaro commented on Jul 1, 2020

   

   alfonsogarciacaro

   on Jul 1, 2020

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   Yes, in Fable option is a special type and is not like other unions. Although I guess we could fix it in the reflection info.

   Fable/src/fable-library/Reflection.ts

   Lines 111 to 113 in 7d02d3e

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

   Do you have an example of a scenario that's being blocked by this?
2. kerams commented on Jul 1, 2020

   

   kerams

   on Jul 1, 2020

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   https://github.com/Zaid-Ajaj/Fable.Remoting/pull/158/files#diff-5bdc4da2122faa430fe9e2f88bdb24e6R425

   Not really a blocker since one can always specifically typecheck for Option.

   👍React with 👍1Reacted by Alfonso Garcia-Caro
3. alfonsogarciacaro

   added

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   on Nov 1, 2020
4. github-actions commented on Apr 17, 2026

   

   github-actionsBot

   on Apr 17, 2026 – with GitHub Actions

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   🤖 This is an automated response from Repo Assist.

   This issue is addressed by draft PR #4529: Fix FSharpOption not recognized as union type in F# reflection.

   The root cause is that option_type() in fable-library-ts/Reflection.ts and fable-library-py/fable_library/reflection.py creates a TypeInfo without a cases property, so:

   • FSharpType.IsUnion(typeof<option<int>>) returns false
   • FSharpType.GetUnionCases(typeof<option<int>>) throws
   • FSharpValue.GetUnionFields and MakeUnion also fail for option types

   PR #4529 rewrites option_type() to expose None (tag 0) and Some (tag 1) as union cases, and adds option-aware handling in getUnionFields/makeUnion to deal with Fable's erased runtime representation (None = undefined/null, Some(x) = x directly).

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5. MangelMaxime commented on May 28, 2026

   

   MangelMaxime

   on May 28, 2026

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   Fixed in #4529
6. MangelMaxime

   closed this as completedon May 28, 2026