- Documentation
- Reference manual
- The SWI-Prolog library
- library(aggregate): Aggregation operators on backtrackable predicates
- library(apply): Apply predicates on a list
- library(assoc): Association lists
- library(broadcast): Broadcast and receive event notifications
- library(charsio): I/O on Lists of Character Codes
- library(check): Consistency checking
- library(clpb): CLP(B): Constraint Logic Programming over Boolean Variables
- library(clpfd): CLP(FD): Constraint Logic Programming over Finite Domains
- library(clpqr): Constraint Logic Programming over Rationals and Reals
- library(csv): Process CSV (Comma-Separated Values) data
- library(debug): Print debug messages and test assertions
- library(error): Error generating support
- library(gensym): Generate unique identifiers
- library(iostream): Utilities to deal with streams
- library(lists): List Manipulation
- library(main): Provide entry point for scripts
- library(nb_set): Non-backtrackable set
- library(www_browser): Activating your Web-browser
- library(option): Option list processing
- library(optparse): command line parsing
- library(ordsets): Ordered set manipulation
- library(pairs): Operations on key-value lists
- library(persistency): Provide persistent dynamic predicates
- library(pio): Pure I/O
- library(predicate_options): Declare option-processing of predicates
- library(prolog_pack): A package manager for Prolog
- library(prolog_xref): Cross-reference data collection library
- library(quasi_quotations): Define Quasi Quotation syntax
- library(random): Random numbers
- library(readutil): Reading lines, streams and files
- library(record): Access named fields in a term
- library(registry): Manipulating the Windows registry
- library(simplex): Solve linear programming problems
- library(solution_sequences): Modify solution sequences
- library(tabling): Tabled execution (SLG)
- library(thread_pool): Resource bounded thread management
- library(ugraphs): Unweighted Graphs
- library(url): Analysing and constructing URL
- library(varnumbers): Utilities for numbered terms
- library(yall): Lambda expressions

- The SWI-Prolog library
- Packages

- Reference manual

## 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 `>=`

0positive_integer Integer `>`0negative_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.