stateeither

Stateeither is a conceptual type used in functional programming to combine stateful computations with early error reporting. It is typically realized as a state monad transformer applied to an Either error functor, commonly written as StateT s (Either e) a in languages like Haskell. A value of type StateEither s e a represents a computation that, given an initial state of type s, may either produce a result of type a along with a new state s, or fail with an error of type e, in which case no final state or result is produced.

Semantics: runStateEither returns an Either e (a, s). If the computation succeeds, you obtain a pair of the result and the updated state; if it fails, you obtain a Left e and the computation stops.

Operations and composition: Stateeither inherits the capabilities of both the State monad and the Either monad through the StateT structure. You can sequence actions that read or modify the state, while propagating errors without throwing exceptions. Lifting lets you embed ordinary Either values into the StateEither context.

Typical use cases: to model parsers or interpreters that carry around a symbol table or environment and may report the first error encountered; to implement configuration pipelines where the state represents cached results and errors should halt processing.

See also: State monad, Either monad, StateT transformer.