runStateEither

runStateEither is a function found in functional programming libraries, particularly those inspired by Haskell, that combines state management with error handling. It operates on a computation that can either succeed with a value or fail with an error, while also carrying a state that can be read and modified throughout the computation.

The core idea is to provide a way to execute computations that have side effects in terms of state modification but can also encounter failure conditions. The `Either` type is commonly used to represent these two outcomes: a `Left` value typically signifies an error, while a `Right` value indicates success and contains the computed result.

When `runStateEither` is applied, it takes an initial state and the computation to be executed. The computation progresses, potentially updating the state and producing intermediate results. If an error occurs at any point, the computation short-circuits, and the error value is returned along with the state as it was at the time of the failure. If the computation completes successfully, it returns the final computed value and the final state.

This pattern is useful in scenarios where operations need to maintain context or mutable data but also have the possibility of failing. For example, parsing a complex data structure might involve maintaining a position in the input string (the state) while also handling potential syntax errors (the `Either` failure). `runStateEither` provides a clean way to manage both aspects simultaneously.