- Documentation
- Reference manual
- The SWI-Prolog library
- library(aggregate): Aggregation operators on backtrackable predicates
- library(ansi_term): Print decorated text to ANSI consoles
- 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(dcg/basics): Various general DCG utilities
- library(dcg/high_order): High order grammar operations
- library(debug): Print debug messages and test assertions
- library(dicts): Dict utilities
- library(error): Error generating support
- library(gensym): Generate unique identifiers
- library(intercept): Intercept and signal interface
- 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(occurs): Finding and counting sub-terms
- 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_jiti): Just In Time Indexing (JITI) utilities
- library(prolog_pack): A package manager for Prolog
- library(prolog_xref): Prolog cross-referencer data collection
- library(quasi_quotations): Define Quasi Quotation syntax
- library(random): Random numbers
- library(readutil): Read utilities
- library(record): Access named fields in a term
- library(registry): Manipulating the Windows registry
- library(settings): Setting management
- library(simplex): Solve linear programming problems
- library(solution_sequences): Modify solution sequences
- library(tables): XSB interface to tables
- library(thread): High level thread primitives
- 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.16 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**(`+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.
`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**(`+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. - [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.

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`

char 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 `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 negative_integer Integer `<`0nonneg Integer `>=`

0`oneof(L)`

Ground term that is member of L positive_integer Integer `>`0proper_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 `atom`

text One of `atom`

,`string`

,`chars`

or`codes`

type `Term`is a valid type specificationNote: 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.