exactatype

Exactatype is a hypothetical typing concept proposed in programming language theory to enforce exact type equality between values and their declared types. It aims to remove implicit coercions and subtyping that currently blur type boundaries, requiring programmers to be explicit when a value should be treated as a different type. The approach emphasizes precise reasoning about code.

Key principles include exact correspondence between a value and its type, prohibition of subtyping for ordinary types, elimination of automatic implicit conversions, and a preference for explicit casts or conversion operators. Type inference, if present, must produce the most specific type allowed by annotations, with no widening or narrowing through implicit rules. Optional dynamic checks may be retained only when a program opts into runtime validation of type boundaries.

Adoption considerations include potential benefits for safety, debugging, and optimization, since exact types reduce ambiguity. Trade-offs involve increased verbosity, more explicit conversions, and potential incompatibilities with existing libraries and frameworks relying on subtyping or coercions.

Use cases commonly cited are safety-critical systems, formal verification workflows, numeric and scientific computing requiring exactness, and API design where precise type contracts are essential.

Example: In a language with exactatype, a function add expects two values of the exact type Int and returns an Int. Passing an argument with the exact type Float would be a type error, even if both are numeric. A separate cast or constructor would be required to convert between them.

See also: type systems, strong typing, subtyping, type inference, coercion.