Gleam case exhaustiveness, as a fan

The Gleam programming language boasts a robust compiler that meticulously assesses the exhaustiveness of case expressions. This crucial task is handled by the compiler-core/src/exhaustiveness.rs module, which diligently verifies whether every conceivable value a pattern can assume is adequately covered. Conversely, it also identifies any unwarranted clauses that are unlikely to ever be executed, thanks to the presence of prior matching clauses.

Compiler developers have strived to make this process as user-friendly as possible. For instance, if a programmer inadvertently omits a necessary case, the compiler will promptly notify them of the missing pattern. This feature underscores Gleam's commitment to providing a seamless and intuitive programming experience.

The theory behind this functionality can be traced back to seminal works from the 1980s, notably Lennart Augustsson's paper "Compiling Pattern Matching" from 1985. Augustsson introduced a systematic approach to pattern matching, grouping clauses based on their leading constructors, introducing fresh variables for sub-patterns, and then recursively applying these rules. This methodology laid the groundwork for subsequent advancements in the field.

As programming languages evolved in the 1990s, the focus shifted towards enhancing the ergonomics of diagnostics and expanding the richness of matching patterns. Luc Maranget's 2007 paper, "Warnings for Pattern Matching," delved into the concept of witnesses, which are specific values that demonstrate the absence of exhaustiveness in a case expression. Witnesses serve as invaluable "counter-examples" that help programmers identify and rectify their mistakes.

The process of identifying witnesses involves constructing a tree representation of the case expression, where each node corresponds to a branch of the expression. By analyzing this tree, the compiler can pinpoint the exact location where a clause fails to match any possible value, effectively unveiling the missing case. The diagnostics generated by the compiler then guide the programmer towards the correct solution, transforming what could have been a frustrating "non-exhaustive match" warning into an enlightening learning experience.