SWI-Prolog -- type

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(error): Error generating support
- Packages

Availability::- use_module(library(error)).(can be autoloaded)

type_error(+Type, +Term)

Tell the user that Term is not of the expected Type. This error is closely related to domain_error/2 because the notion of types is not really set in stone in Prolog. We introduce the difference using a simple example.

Suppose an argument must be a non-negative integer. If the actual argument is not an integer, this is a type_error. If it is a negative integer, it is a domain_error.

Typical borderline cases are predicates accepting a compound term, e.g., point(X,Y). One could argue that the basic type is a compound-term and any other compound term is a domain error. Most Prolog programmers consider each compound as a type and would consider a compoint that is not point(_,_) a type_error.