error:must

swipl
- library

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.

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`
callable	Atom or compound term
char	Atom of length 1
chars	Proper list of 1-character atoms
code	Representation Unicode code point (0..0x10ffff)
codes	Proper list of Unicode character codes
compound	compound term
`compound(Term)`	Compound with same name/arity as term; checks arguments
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
`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
negative_integer	Integer < 0
nonneg	Integer >= 0
`oneof(L)`	Ground term that is member of L
pair	Key-Value pair. Same as `compound(any-any)`
positive_integer	Integer > 0
proper_list	Same as list
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 specification

In addition, types may be composed using TypeA,TypeB, TypeA;TypeB and negated using \Type.

Errors: - instantiation_error if Term is insufficiently instantiated and type_error(Type, Term) if Term is not of Type.