{"record":{"id":"3e40cbfd888b4654","repo":"louthy/language-ext","slug":"type-typeof-f1-name-is-not-a-valid-monoid","errorCode":null,"errorMessage":"Type {typeof(F1).Name} is not a valid monoid","messagePattern":"Type (.+?) is not a valid monoid","errorType":"exception","errorClass":"NotSupportedException","httpStatus":null,"severity":"error","filePath":"LanguageExt.Core/Monads/Alternative Monads/ValidationT/Trait/ValidationT.TraitImpl.2.cs","lineNumber":49,"sourceCode":"        Func<A, B> second,\n        K<ValidationT<M>, F1, A> fab) =>\n        new ValidationT<F2, M, B>(_ => MonoidInstance<F1>.Instance switch\n                                       {\n                                           { IsSome: true, Value: { } t } =>\n                                               fab.As2()\n                                                  .Run(t)\n                                                  .Map(v => v switch\n                                                            {\n                                                                Validation<F1, A>.Fail(var e) =>\n                                                                    Validation.FailI<F2, B>(first(e)),\n\n                                                                Validation<F1, A>.Success(var x) =>\n                                                                    Validation.SuccessI<F2, B>(second(x)),\n\n                                                                _ => throw new NSE()\n                                                            }),\n\n                                           _ => throw new NSE($\"Type {typeof(F1).Name} is not a valid monoid\")\n                                       });\n\n    static K<ValidationT<M>, F2, A> Bimonad<ValidationT<M>>.BindFirst<F1, F2, A>(\n        K<ValidationT<M>, F1, A> ma,\n        Func<F1, K<ValidationT<M>, F2, A>> f) =>\n        new ValidationT<F2, M, A>(ty => MonoidInstance<F1>.Instance switch\n                                       {\n                                           { IsSome: true, Value: { } tx } =>\n                                               ma.As2()\n                                                 .Run(tx)\n                                                 .Bind(v => v switch\n                                                            {\n                                                                Validation<F1, A>.Fail (var e) =>\n                                                                    f(e).As2().Run(ty),\n\n                                                                Validation<F1, A>.Success (var x) =>\n                                                                    M.Pure(Validation.SuccessI<F2, A>(x)),\n","sourceCodeStart":31,"sourceCodeEnd":67,"githubUrl":"https://github.com/louthy/language-ext/blob/2f0e3628242889774d4141960a35671a0280051f/LanguageExt.Core/Monads/Alternative Monads/ValidationT/Trait/ValidationT.TraitImpl.2.cs#L31-L67","documentation":"ValidationT's BiFunctor/Bimonad operations (BiMap) switch on the MonoidInstance registered for the success type F1; if no monoid instance exists for that type, the library cannot combine/discard success values, so it throws NSE. This is a typeclass-lookup failure: the success type of the ValidationT must have a registered Monoid for these operations to work.","triggerScenarios":"Calling BiMap on a K<ValidationT<M>, F1, A> where MonoidInstance<F1>.Instance is not registered (F1 is a type with no Monoid trait implementation), and the underlying value is in a Failed state (the Success branch throws NSE separately, otherwise the monoid fallthrough throws).","commonSituations":"Using ValidationT with a custom success type (e.g. a domain record or int) for which no Monoid instance has been defined, then attempting BiMap/BiTransform-style operations on the outer functor.","solutions":["Implement a Monoid trait instance for the success type F1 so MonoidInstance<F1>.Instance resolves","Use a success type that already has a Monoid registered (e.g. Unit, string, Lst)","Avoid BiMap on ValidationT values whose success type lacks a monoid; restructure so only the failure type is transformed"],"exampleFix":"// before\nvar mapped = ma.BiMap(succ => succ.Count, fail => fail.Message); // NSE: int has no Monoid\n// after\n[Monoid] // or implement Traits/Monoid<int>\npublic partial record SuccCount(int Value);","handlingStrategy":"validation","validationCode":"if (MonoidInstance<F1>.Instance == null) throw new InvalidOperationException($\"{typeof(F1).Name} needs a Monoid before BiMap on ValidationT\");","typeGuard":"bool HasMonoid<F>() => MonoidInstance<F>.Instance is not null;","tryCatchPattern":"try { return ma.BiMap(f, g); } catch (NSE ex) { /* fall back to non-bimonad transform or log missing monoid */ }","preventionTips":["Only use BiMap/BindFirst/BindSecond on ValidationT with known-monoidal slot types","Register Monoid instances for all custom types used as ValidationT success types","Unit-test monoid instance resolution early"],"tags":["csharp","languageext","monoid","validationt"],"backgroundTag":"missing-dependency","analyzedSha":"2f0e3628242889774d4141960a35671a0280051f","analyzedAt":"2026-09-15T03:31:55.716Z","contentChangedAt":"2026-09-15T03:31:55.716Z","schemaVersion":2},"datasetVersion":"2026-09-16T09:17:16.951Z"}