error.pl -- Error generating support

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.

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.

 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 point(_,_) a type_error.

 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)

Compatibility

- This error is outside the ISO Standard.

 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).

 instantiation_error(+FormalSubTerm)

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.

Arguments:

FormalSubTerm

- 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(+Culprit)

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(+Flag)

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. According to the ISO Standard, Culprit should be an implementation-dependent atom.

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(+Resource)

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 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.

acyclic	Acyclic term (tree); see acyclic_term/1
any	any term
between(FloatL,FloatU)	Number [FloatL..FloatU]
between(IntL,IntU)	Integer [IntL..IntU]
boolean	One of true or false
callable	Atom or compound term
char	Atom of length 1
chars	Proper list of 1-character atoms
code	Representation Unicode code point (0..0x10ffff)
codes	Proper list of Unicode character codes
compound	compound term
compound(Term)	Compound with same name/arity as term; checks arguments
constant	Same as atomic
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
list(Type)	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
negative_integer	Integer < 0
nonneg	Integer >= 0
oneof(L)	Ground term that is member of L
pair	Key-Value pair. Same as compound(any-any)
positive_integer	Integer > 0
proper_list	Same as list
stream	A stream name or valid stream handle; see is_stream/1
symbol	Same as atom
text	One of atom, string, chars or codes
type	Term is a valid type specification

In addition, types may be composed using TypeA,TypeB, TypeA;TypeB and negated using \Type.

Errors

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

 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_encoding(?Name) is nondet

True if Name is the name of a supported encoding. See encoding option of e.g., open/4.

 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.