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error.pl -- Error generating support
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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.

- 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.
Source 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 compound that is not point(_,_) a type_error.

Source 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.
Source 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.
Source existence_error(+Type, +Term, +Set)
Term is of the correct type and correct domain, but there is no existing (external) resource that is represented by it in the provided set.
- This error is not in ISO.
Source permission_error(+Action, +Type, +Term)
It is not allowed to perform Action on the object Term that is of the given Type.
Source 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.
Source 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.
Source 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.
Source 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.
Source resource_error(+Culprit)
A goal cannot be completed due to lack of resources.
Source 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.

acyclicAcyclic term (tree); see acyclic_term/1
between(FloatL,FloatU)Number [FloatL..FloatU]
between(IntL,IntU)Integer [IntL..IntU]
booleanOne of true or false
charAtom of length 1
charsProper list of 1-character atoms
codeRepresentation Unicode code point
codesProper list of Unicode character codes
constantSame as atomic
cyclicCyclic term (rational tree); see cyclic_term/1
dictA dictionary term; see is_dict/1
encodingValid name for a character encoding; see current_encoding/1
listA (non-open) list; see is_list/1
negative_integerInteger < 0
nonnegInteger >= 0
oneof(L)Ground term that is member of L
positive_integerInteger > 0
proper_listSame as list
list(Type)Proper list with elements of Type
list_or_partial_listA list or an open list (ending in a variable); see is_list_or_partial_list/1
streamA stream name or valid stream handle; see is_stream/1
symbolSame as atom
textOne of atom, string, chars or codes
typeTerm 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.

- instantiation_error if Term is insufficiently instantiated and type_error(Type, Term) if Term is not of Type.
Source is_of_type(+Type, @Term) is semidet
True if Term satisfies Type.
Source has_type(+Type, @Term) is semidet[multifile]
True if Term satisfies Type.
Source 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.