ResultT

ResultT is a parameterized type used in functional programming to model computations that may fail, returning either a successful value of type A or an error of type E, within an underlying context or monad M. It is commonly implemented as a monad transformer that augments an existing monad with error handling, typically represented as a wrapper around M (Either E A). The design allows error propagation to be composed with other effects such as I/O, state, or asynchrony, while preserving the ability to sequence computations.

In many languages the concept appears under related names such as ExceptT or EitherT, with ResultT serving a similar role when a project prefers the explicit terminology of a result type. A typical representation is newtype ResultT e m a = ResultT { runResultT :: m (Either e a) }, where the inner monad m carries the effectful context and Either encodes success (Right a) or error (Left e). Functor, Applicative, and Monad (or their equivalents) instances enable mapping, applying, and chaining computations while preserving the error semantics. There are also utility operations for lifting pure values into the transformer, throwing or returning errors, and running the computation to obtain the underlying monadic result.

ResultT is used to compose error-prone computations in a principled way, allowing richer effect stacks without abandoning structured error handling. It is closely related to, and often interchangeable with, constructs like Rust’s Result, Haskell’s Either, and various monad transformers that provide similar error semantics.