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)
| 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. |
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/1 any any term between(FloatL,FloatU)Number [FloatL..FloatU] between(IntL,IntU)Integer [IntL..IntU] boolean One of trueorfalsechar 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 atomiccyclic 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 >=0oneof(L)Ground term that is member of L pair Key-Value pair positive_integer Integer > 0 proper_list Same as list 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 stream A stream name or valid stream handle; see is_stream/1 symbol Same as atomtext One of atom,string,charsorcodestype 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.
type_error(Type, Term) if Term is not of Type.