readereither

Readereither is a construct in functional programming that combines the ideas of the Reader monad with the Either monad. It represents computations that both depend on a shared environment and may fail with an error, rather than producing a successful value directly.

Formally, a readereither type describes a function from an environment R to an either value E or A. In type form, readereither can be viewed as R -> Either<E, A>. If the function yields a Left, it signals an error of type E; if it yields a Right, it produces a value of type A. This combination enables dependency injection through the environment while propagating failures in a structured way.

The readereither pattern is common in languages with strong functional programming libraries. In Haskell, it corresponds to a monad transformer stack such as ReaderT r (Either e) a, or, when viewed through a type alias, a function r -> Either e a. In TypeScript and other ecosystems, libraries often expose a Readereither type as a simple alias for a function that takes an environment and returns an Either-like result.

Usage and benefits include composing computations that need access to a shared configuration or services (the environment) and that may fail due to validation, I/O issues, or domain errors. Operations typically available include mapping over successful results and chaining dependent steps, all while threading the environment transparently.

Common use cases involve dependency injection, configurable computations, and testable code paths where errors must be reported cleanly. Readereither provides a disciplined pattern for handling environment-aware logic with robust error handling, without resorting to exceptions. See also: Reader monad, Either monad, and monad transformers.