library(debug) and library(prolog_stack). 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.
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.
existence_error(ObjectType, Culprit, Set)
open(File, read, input) cannot succeed because the system
will allocate a new unique stream handle that will never unify with
input.syntax_error(number('1a'))
means that 1a is not a valid number.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/1any any termbetween(FloatL,FloatU)Number [FloatL..FloatU]between(IntL,IntU)Integer [IntL..IntU]boolean One oftrueorfalsecallable Atom or compound termchar Atom of length 1chars Proper list of 1-character atomscode Representation Unicode code point (0..0x10ffff)codes Proper list of Unicode character codescompound compound termcompound(Term)Compound with same name/arity as term; checks argumentsconstant Same asatomiccyclic Cyclic term (rational tree); see cyclic_term/1dict A dictionary term; see is_dict/1encoding Valid name for a character encoding; see current_encoding/1list A (non-open) list; see is_list/1list(Type)Proper list with elements of Typelist_or_partial_list A list or an open list (ending in a variable); see is_list_or_partial_list/1negative_integer Integer < 0nonneg Integer>=0oneof(L)Ground term that is member of Lpair Key-Value pair. Same ascompound(any-any)positive_integer Integer > 0proper_list Same as liststream A stream name or valid stream handle; see is_stream/1symbol Same asatomtext One ofatom,string,charsorcodestype Term is a valid type specification
In addition, types may be composed using TypeA,TypeB,
TypeA;TypeB and negated using \Type.
type_error(Type, Term) if Term is not of Type.according to the description is_of_type/2 and has_type/2 are identical, but there's a difference in fact. e.g.:
?-error:has_type(T,'ąćęłńóśżźĄĆĘŁŃÓŚŻŹ').
returns:
T = any ; T = atom ; T = atomic ; T = callable ; T = text ; T = constant ; T = ground ; T = acyclic ; T = nonvar ; T = oneof(_11302) ; T = symbol ; T = dependency ; false.
while:
?- error:is_of_type(T,'ąćęłńóśżźĄĆĘŁŃÓŚŻŹ').
produces an error "Arguments are not sufficiently instantiated"