This module provides predicates to simplify error generation and checking. It's implementation is based on a discussion on the SWI-Prolog mailinglist on best practices in error handling. The utility predicate must_be/2 provides simple run-time type validation. The *_error predicates are simple wrappers around throw/1 to simplify throwing the most common ISO error terms.
- type_error(+ValidType, +Culprit)
- Tell the user that Culprit is not of the expected ValidType. 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 compound that is not
- domain_error(+ValidDomain, +Culprit)
- The argument is of the proper type, but has a value that is outside the supported values. See type_error/2 for a more elaborate discussion of the distinction between type- and domain-errors.
- existence_error(+ObjectType, +Culprit)
- Culprit is of the correct type and correct domain, but there is no existing (external) resource of type ObjectType that is represented by it.
- existence_error(+ObjectType, +Culprit, +Set)
- Culprit is of the correct type and correct domain, but there is no
existing (external) resource of type ObjectType that is represented
by it in the provided set. The thrown exception term carries a
formal term structured as follows:
existence_error(ObjectType, Culprit, Set)
- permission_error(+Operation, +PermissionType, +Culprit)
- It is not allowed to perform Operation on (whatever is represented by) Culprit that is of the given PermissionType (in fact, the ISO Standard is confusing and vague about these terms' meaning).
- An argument is under-instantiated. I.e. it is not acceptable as it is, but if some variables are bound to appropriate values it would be acceptable.
- An argument is over-instantiated. This error is used for output
arguments whose value cannot be known upfront. For example, the goal
open(File, read, input)cannot succeed because the system will allocate a new unique stream handle that will never unify with
- A representation error indicates a limitation of the implementation. SWI-Prolog has no such limits that are not covered by other errors, but an example of a representation error in another Prolog implementation could be an attempt to create a term with an arity higher than supported by the system.
- A text has invalid syntax. The error is described by Culprit. According to the ISO Standard, Culprit should be an implementation-dependent atom.
- A goal cannot be completed due to lack of resources. According to the ISO Standard, Resource should be an implementation-dependent atom.
- must_be(+Type, @Term) is det
- True if Term satisfies the type constraints for Type. Defined
Most of these types are defined by an arity-1 built-in predicate of the same name. Below is a brief definition of the other types.
acyclic Acyclic term (tree); see acyclic_term/1 any any term
Integer [IntL..IntU] boolean One of
char Atom of length 1 chars Proper list of 1-character atoms code Representation Unicode code point codes Proper list of Unicode character codes constant Same as
cyclic Cyclic term (rational tree); see cyclic_term/1 dict A dictionary term; see is_dict/1 encoding Valid name for a character encoding; see current_encoding/1 list A (non-open) list; see is_list/1 negative_integer Integer < 0 nonneg Integer >= 0
Ground term that is member of L pair Key-Value pair positive_integer Integer > 0 proper_list Same as list
Proper list with elements of Type list_or_partial_list A list or an open list (ending in a variable); see is_list_or_partial_list/1 stream A stream name or valid stream handle; see is_stream/1 symbol Same as
text One of
type Term is a valid type specification
Note: The Windows version can only represent Unicode code points up to 2^16-1. Higher values cause a representation error on most text handling predicates.
- is_of_type(+Type, @Term) is semidet
- True if Term satisfies Type.
- has_type(+Type, @Term) is semidet[multifile]
- True if Term satisfies Type.
- current_type(?Type, @Var, -Body) is nondet
- True when Type is a currently defined type and Var satisfies Type of the body term Body succeeds.