A.12 library(error): Error generating support

author

- Jan Wielemaker
- Richard O'Keefe
- Ulrich Neumerkel

See also

- library(debug) and library(prolog_stack).
- print_message/2 is used to print (uncaught) error terms.

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(+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.

domain_error(+Type, +Term)

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(+Type, +Term)

Term is of the correct type and correct domain, but there is no existing (external) resource that is represented by it.

permission_error(+Action, +Type, +Term)

It is not allowed to perform Action on the object Term that is of the given Type.

instantiation_error(+Term)

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.

Term	is the term that needs (further) instantiation. Unfortunately, the ISO error does not allow for passing this term along with the error, but we pass it to this predicate for documentation purposes and to allow for future enhancement.

uninstantiation_error(+Term)

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 input.

representation_error(+Reason)

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.

syntax_error(+Culprit)

A text has invalid syntax. The error is described by Culprit.

To be done

Deal with proper description of the location of the error. For short texts, we allow for Type(Text), meaning Text is not a valid Type. E.g. syntax_error(number('1a')) means that 1a is not a valid number.

resource_error(+Culprit)

A goal cannot be completed due to lack of resources.

[det]must_be(+Type, @Term)

True if Term satisfies the type constraints for Type. Defined types are atom, atomic, between, boolean, callable, chars, codes, text, compound, constant, float, integer, nonneg, positive_integer, negative_integer, nonvar, number, oneof, list, list_or_partial_list, symbol, var, rational, encoding, dict and string.

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.

boolean	one of true or false
char	Atom of length 1
code	Representation Unicode code point
chars	Proper list of 1-character atoms
codes	Proper list of Unicode character codes
text	One of atom, string, chars or codes
between(IntL,IntU)	Integer [IntL..IntU]
between(FloatL,FloatU)	Number [FloatL..FloatU]
nonneg	Integer >= 0
positive_integer	Integer > 0
negative_integer	Integer < 0
oneof(L)	Ground term that is member of L
encoding	Valid name for a character encoding
cyclic	Cyclic term (rational tree)
acyclic	Acyclic term (tree)
list(Type)	Proper list with elements of Type
list_or_partial_list	A list or an open list (ending in a variable

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.

throws

instantiation_error if Term is insufficiently instantiated and type_error(Type, Term) if Term is not of Type.

[semidet]is_of_type(+Type, @Term)

True if Term satisfies Type.

[semidet,multifile]has_type(+Type, @Term)

True if Term satisfies Type.

[nondet]current_type(?Type, @Var, -Body)

True when Type is a currently defined type and Var satisfies Type of the body term Body succeeds.