Elsecannot

Elsecannot is a hypothetical keyword or operator proposed in programming language theory to express that the else-branch of a conditional statement is unreachable under the given preconditions or invariants. It is not part of mainstream programming languages, and its exact syntax and behavior have varied in theoretical discussions.

Semantics and usage of elsecannot are not standardized. In static models, elsecannot signals that, for all valid inputs, the else branch cannot be executed, allowing compilers to optimize away the else-block or to issue compile-time checks that guarantee exhaustiveness. In dynamic or runtime models, elsecannot may require a proof or contract that the else path cannot occur; if reached, it can trigger an explicit assertion or a runtime error. Some proposals envision elsecannot as a refinement of exhaustive pattern matching, where every possible case has a proven counterpart in prior branches.

Examples in pseudo-code can illustrate the idea. For instance: if (user_is_admin) { grant_access(); } else cannot { deny_access(); }. In this sense, the else path is treated as unreachable, and the language or toolchain may optimize or verify that the else block is never taken. Another variant appears in switch-like constructs: switch (state) { case A: handleA(); case B: handleB(); else cannot { handleError(); } }.

History and reception: elsecannot has appeared mainly in theoretical discussions and design drafts about dead code elimination and exhaustive case analysis. It remains uncommon in practical languages, with limited tool support and potential ambiguity around guarantees and error handling.

See also: unreachable code, exhaustive pattern matching, contract programming.