OptionalMaybeT

OptionalMaybeT is a monad transformer used in functional programming to compose an outer effect with the possibility of absence. It represents computations in a base monad m that may yield no value, encoded as an inner Optional type. This allows sequencing effectful computations that can also opt out at any step without introducing a separate error type.

The type is typically written as OptionalMaybeT m a, with the core operation runOptionalMaybeT :: OptionalMaybeT m a -> m (Optional a). Conceptually it is a wrapper around m (Optional a). If the language provides a distinct Optional type, OptionalMaybeT specializes the generic MaybeT pattern to reflect that semantics.

Instances are defined whenever the base monad m is a Monad. Functor maps a function over the present value inside the Optional. Applicative combines effects while propagating the absence. Monad defines binding that threads the outer effect and the inner optional; when the inner Optional is Nothing, the computation short-circuits to Nothing; when it is Just a, the function is applied and its result lifted back into OptionalMaybeT. In more descriptive terms, (OptionalMaybeT ma) >>= f applies ma, then, if the inner value exists, applies f and unwraps the result; otherwise, it yields Nothing in the outer monad.

Common helpers include lift to insert a base m a value, runOptionalMaybeT to access the underlying computation, and fromOptional or toOptional to convert between Optional and OptionalMaybeT contexts. These enable modular composition with IO, error handling, or asynchronous effects.

Use cases arise in pipelines where steps may fail to produce a value yet still perform side effects. OptionalMaybeT thus provides a uniform interface for combining optional results with other effectful computations, without resorting to separate error types for absence.

See also: MaybeT, Optional, Monad transformer.