louthy/language-ext · error · NotSupportedException

Type is not a valid monoid

Error message

Type {typeof(F).Name} is not a valid monoid

What it means

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.

Solutions

1. Register a Monoid instance for type F
2. Use a monoidal type (Unit, Lst<A>, string, etc.) as the ValidationT slot type
3. Avoid BindSecond; use Map/Bind on the inner monad instead

Example fix

// before
ValidationT<M, MyType, B>.BindSecond(...) // NSE
// after
// ensure MyType has: public record MyTypeMonoid : Monoid<MyType> { ... }

if (MonoidInstance<F>.Instance == null) throw new InvalidOperationException($"Register a Monoid for {typeof(F).Name} before BindSecond");

bool SupportsBindSecond<F>() => MonoidInstance<F>.Instance is not null;

try { return BindSecond(ma, f); } catch (NSE ex) { /* restructure to plain Map/Bind */ }

When it happens

Trigger: Calling Bimonad<ValidationT<M>>.BindSecond<F, A, B> where MonoidInstance<F>.Instance is null/unregistered for the F used as the validation slot type.

Common situations: Attempting second-slot binding (mapping the bound value while keeping the validation type) on ValidationT parameterised by a success type lacking a Monoid instance.

Understand the failure class

Background: "not installed", "pip install", "required for": how missing-dependency errors surface across open-source libraries — this error's family across 34 libraries.

Related errors

• Type is not a valid monoid
• must be a Monoid
• Ord attribute should have a struct type that derives from…
• Hashable attribute should have a struct type that derives…
• Don't use Equals - use either RecordType

AI-assisted analysis of louthy/language-ext@2f0e362824 (2026-09-15). Data as JSON: /api/errors/74b686a8af430988. Report an issue: GitHub.

Appendix: source

Thrown at LanguageExt.Core/Monads/Alternative Monads/ValidationT/Trait/ValidationT.TraitImpl.2.cs:93

        K<ValidationT<M>, F, A> ma, 
        Func<A, K<ValidationT<M>, F, B>> f) =>
        new ValidationT<F, M, B>(ty => MonoidInstance<F>.Instance switch
                                       {
                                           { IsSome: true, Value: { } tx } =>
                                               ma.As2()
                                                 .Run(tx)
                                                 .Bind(v => v switch
                                                            {
                                                                Validation<F, A>.Fail (var e) =>
                                                                    M.Pure(Validation.FailI<F, B>(e)),

                                                                Validation<F, A>.Success (var x) =>
                                                                    f(x).As2().Run(ty),

                                                                _ => throw new NSE()
                                                            }),

                                           _ => throw new NSE($"Type {typeof(F).Name} is not a valid monoid")
                                       });
}

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