Stronglyeither

Stronglyeither is a term used in discussions of type systems and functional programming to describe a strongly-typed sum type that represents a value with one of two distinct possibilities, with explicit mechanisms to track and enforce which side is present. The concept emphasizes compile-time guarantees around exhaustiveness and information preservation when transforming or composing values.

Definition and motivation

A stronglyeither encodes two disjoint alternatives, typically denoted A and B, and carries a phantom or explicit tag that prevents mixing or losing track of which alternative is active. The goal is to enable safer composition of operations, ensuring that code handling the value can reason about its state without resorting to runtime checks or unsafe casts.

Implementation notes

In languages with generalized algebraic data types (GADTs) or dependent types, stronglyeither can be modeled as a two-branch type with a phantom discriminant, allowing the compiler to enforce that certain functions preserve or reveal the active branch. In languages without such features, a discriminated union or tagged union can be used, with pattern matching required to exhaustively handle both cases. A common pattern is to provide fold, map, or bimap-style combinators and explicit constructors for each side, together with a way to reflect or transform the tag.

Typical usage

Stronglyeither is used to represent results that must be handled carefully, such as error-safety or state-dependent computations. Functions on a stronglyeither generally require handlers for both sides, promoting complete pattern matching and preventing silent loss of information as values flow through pipelines.

See also

Sum type, Either, discriminated union, GADT, type safety, exhaustiveness checking.