illtyped

Illtyped refers to code that cannot be assigned a type in a given type system. In software development and programming language research, ill-typed programs are used to study the limits of a type system and to test compilers or type-checkers for correct error reporting.

Illtyped is also the name of a small, widely used test-suite in the Haskell ecosystem. The illtyped collection comprises a curated set of Haskell modules and expressions that intentionally provoke type errors, serving as regression tests and teaching examples. The goal is to verify that the compiler or type-checker rejects these programs and to provide concrete counter-examples for teaching advanced type-system features (such as parametricity, type families, higher-rank types, and GADTs). Each example is typically accompanied by a short explanation of the intended type error.

Usage typically involves including the suite in a project where the user runs the tests to ensure that the type checker emits the expected errors, thus guarding against regressions in type inference or error messaging. It is commonly discussed in Haskell type-level programming circles as a resource for illustrating subtle points about type systems, constraints, and type-level programming.

In related terms, a well-typed program is one that type-checks successfully; ill-typed contrasts with well-typed, and the study of ill-typed programs informs both language design and tool development.

See also: type system, type error, type checking, Haskell, type-level programming, regression testing.