maperr

Maperr is a functional programming concept and method used to transform the error value of a result-like type while preserving a successful value. It is commonly applied to data types such as Result<T, E> in Rust or Either<L, R> in other languages, and to similar error-handling abstractions in various libraries. The operation takes a function that maps the error type E to another error type F and returns a new result with the same success type T but with the mapped error F.

Semantics: Given a value r: Result<T, E> and a function f: E -> F, maperr(r, f) yields Result<T, F> where Ok(t) remains Ok(t) and Err(e) becomes Err(f(e)).

Examples: In Rust, a closely related method is map_err. For instance, let r: Result<i32, &str> = Err("bad"); let r2 = r.map_err(|e| format!("error: {}", e)); This produces Err(String) with a enriched message, while Ok values pass through unchanged.

Purpose and usage: Maperr is used to convert error types for uniform handling, attach contextual information, or adapt errors to a common error enumeration across interfaces. It is often used in chains of combinators to keep the success path separate from error handling, enabling consistent error reporting and recovery strategies.

Limitations: Maperr only affects the error branch of the result. If there is no error to map, the value remains unchanged. It also requires a defined transformation from the original error type to the target type.

Origin: The concept arises from monadic error handling in functional programming. In practice, languages implement it under names such as map_err (Rust) or mapError in other ecosystems, sometimes with slight variations in behavior. Maperr serves as a lightweight, composable tool for error normalization and enrichment.