{"record":{"id":"74b686a8af430988","repo":"louthy/language-ext","slug":"type-typeof-f-name-is-not-a-valid-monoid","errorCode":null,"errorMessage":"Type {typeof(F).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":93,"sourceCode":"        K<ValidationT<M>, F, A> ma, \n        Func<A, K<ValidationT<M>, F, B>> f) =>\n        new ValidationT<F, M, B>(ty => MonoidInstance<F>.Instance switch\n                                       {\n                                           { IsSome: true, Value: { } tx } =>\n                                               ma.As2()\n                                                 .Run(tx)\n                                                 .Bind(v => v switch\n                                                            {\n                                                                Validation<F, A>.Fail (var e) =>\n                                                                    M.Pure(Validation.FailI<F, B>(e)),\n\n                                                                Validation<F, A>.Success (var x) =>\n                                                                    f(x).As2().Run(ty),\n\n                                                                _ => throw new NSE()\n                                                            }),\n\n                                           _ => throw new NSE($\"Type {typeof(F).Name} is not a valid monoid\")\n                                       });\n}\n","sourceCodeStart":75,"sourceCodeEnd":96,"githubUrl":"https://github.com/louthy/language-ext/blob/2f0e3628242889774d4141960a35671a0280051f/LanguageExt.Core/Monads/Alternative Monads/ValidationT/Trait/ValidationT.TraitImpl.2.cs#L75-L96","documentation":"BindSecond on ValidationT switches on MonoidInstance<F>.Instance for the validation-slot type F; without a registered monoid the switch has no matching arm and throws NSE. The slot type must be monoidal so a pure value can be lifted into the validation.","triggerScenarios":"Calling Bimonad<ValidationT<M>>.BindSecond<F, A, B> where MonoidInstance<F>.Instance is null/unregistered for the F used as the validation slot type.","commonSituations":"Attempting second-slot binding (mapping the bound value while keeping the validation type) on ValidationT parameterised by a success type lacking a Monoid instance.","solutions":["Register a Monoid instance for type F","Use a monoidal type (Unit, Lst<A>, string, etc.) as the ValidationT slot type","Avoid BindSecond; use Map/Bind on the inner monad instead"],"exampleFix":"// before\nValidationT<M, MyType, B>.BindSecond(...) // NSE\n// after\n// ensure MyType has: public record MyTypeMonoid : Monoid<MyType> { ... }","handlingStrategy":"validation","validationCode":"if (MonoidInstance<F>.Instance == null) throw new InvalidOperationException($\"Register a Monoid for {typeof(F).Name} before BindSecond\");","typeGuard":"bool SupportsBindSecond<F>() => MonoidInstance<F>.Instance is not null;","tryCatchPattern":"try { return BindSecond(ma, f); } catch (NSE ex) { /* restructure to plain Map/Bind */ }","preventionTips":["Keep a Monoid instance for the ValidationT slot type alongside the type definition","Favour Map over BindSecond when no monoid exists","Centralise monoid registrations in one bootstrap file"],"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"}