These built-in predicates enable atomic terms to be processed as a sequence of characters and character codes. Facilities exist to split and join atoms, to convert a single character to and from the corresponding character code, and to convert a number to and from a list of characters and character codes.
Templates
Description
atom_length(Atom, Length) succeeds if Length unifies with the number of characters of the name of Atom.
Errors
| Atom is a variable | instantiation_error | |
| Atom is neither a variable nor an atom | type_error(atom, Atom) | |
| Length is neither a variable nor an integer | type_error(integer, Length) | |
| Length is an integer < 0 | domain_error(not_less_than_zero, Length) | |
Portability
ISO predicate.
Templates
Description
atom_concat(Atom1, Atom2, Atom12) succeeds if the name of Atom12 is the concatenation of the name of Atom1 with the name of Atom1. This predicate is re-executable on backtracking (e.g. if Atom12 is instantiated and both Atom1 and Atom2 are variables).
Errors
| Atom1 and Atom12 are variables | instantiation_error | |
| Atom2 and Atom12 are variables | instantiation_error | |
| Atom1 is neither a variable nor an atom | type_error(atom, Atom1) | |
| Atom2 is neither a variable nor an atom | type_error(atom, Atom2) | |
| Atom12 is neither a variable nor an atom | type_error(atom, Atom12) | |
Portability
ISO predicate.
Templates
Description
sub_atom(Atom, Before, Length, After, SubAtom) succeeds if atom Atom can be split into three atoms, AtomL, SubAtom and AtomR such that Before is the number of characters of the name of AtomL, Length is the number of characters of the name of SubAtom and After is the number of characters of the name of AtomR. This predicate is re-executable on backtracking.
Errors
| Atom is a variable | instantiation_error | |
| Atom is neither a variable nor an atom | type_error(atom, Atom) | |
| SubAtom is neither a variable nor an atom | type_error(atom, SubAtom) | |
| Before is neither a variable nor an integer | type_error(integer, Before) | |
| Length is neither a variable nor an integer | type_error(integer, Length) | |
| After is neither a variable nor an integer | type_error(integer, After) | |
| Before is an integer < 0 | domain_error(not_less_than_zero, Before) | |
| Length is an integer < 0 | domain_error(not_less_than_zero, Length) | |
| After is an integer < 0 | domain_error(not_less_than_zero, After) | |
Portability
ISO predicate.
Templates
Description
char_code(Char, Code) succeeds if the character code for the one-char atom Char is Code.
Errors
| Char and Code are variables | instantiation_error | |
| Char is neither a variable nor a one-char atom | type_error(character, Char) | |
| Code is neither a variable nor an integer | type_error(integer, Code) | |
| Code is an integer but not a character code | representation_error(character_code) | |
Portability
ISO predicate.
Templates
Description
lower_upper(Char1, Char2) succeeds if Char1 and Char2 are one-char atoms and if Char2 is the upper conversion of Char1. If Char1 (resp. Char2) is a character that is not a lower (resp. upper) letter then Char2 is equal to Char1.
Errors
| Char1 and Char2 are variables | instantiation_error | |
| Char1 is neither a variable nor a one-char atom | type_error(character, Char1) | |
| Char2 is neither a variable nor a one-char atom | type_error(character, Char2) | |
Portability
GNU Prolog predicate.
Templates
Description
atom_chars(Atom, Chars) succeeds if Chars is the list of one-char atoms whose names are the successive characters of the name of Atom.
atom_codes(Atom, Codes) is similar to atom_chars/2 but deals with a list of character codes.
Errors
| Atom is a variable and Chars (or Codes) is a partial list or a list with an element which is a variable | instantiation_error | |
| Atom is neither a variable nor an atom | type_error(atom, Atom) | |
| Chars is neither a list nor a partial list | type_error(list, Chars) | |
| Codes is neither a list nor a partial list | type_error(list, Codes) | |
| An element E of the list Chars is neither a variable nor a one-char atom | type_error(character, E) | |
| An element E of the list Codes is neither a variable nor an integer | type_error(integer, E) | |
| An element E of the list Codes is an integer but not a character code | representation_error(character_code) | |
Portability
ISO predicates. The ISO reference only causes a type_error(list, Chars) if Atom is a variable and Chars is neither a list nor a partial list. GNU Prolog always checks if Chars is a list. Similarly for Codes. The type_error(integer, E) when an element E of the Codes is not an integer is a GNU Prolog extension. This seems to be an omission in the ISO reference since this error is detected for many other built-in predicates accepting a character code (e.g. char_code/2, put_code/2).
Templates
Description
number_atom(Number, Atom) succeeds if Atom is an atom whose name corresponds to the characters of Number.
number_chars(Number, Chars) is similar to number_atom/2 but deals with a list of characters.
number_codes(Number, Codes) is similar to number_atom/2 but deals with a list of character codes.
Errors
| Number and Atom are variables | instantiation_error | |
| Number is a variable and Chars (or Codes) is a partial list or a list with an element which is a variable | instantiation_error | |
| Number is neither a variable nor an number | type_error(number, Number) | |
| Atom is neither a variable nor an atom | type_error(atom, Atom) | |
| Chars is neither a list nor a partial list | type_error(list, Chars) | |
| Codes is neither a list nor a partial list | type_error(list, Codes) | |
| An element E of the list Chars is neither a variable nor a one-char atom | type_error(character, E) | |
| An element E of the list Codes is neither a variable nor an integer | type_error(integer, E) | |
| An element E of the list Codes is an integer but not a character code | representation_error(character_code) | |
| Number is a variable, Atom (or Chars or Codes) cannot be parsed as a number and the value of the syntax_error Prolog flag is error (section 8.22.1) | syntax_error(atom explaining the error) | |
Portability
number_atom/2 is a GNU Prolog predicate. number_chars/2 and number_codes/2 are ISO predicates.
GNU Prolog only raises an error about an element E of the Chars (or Codes) list when Number is a variable while the ISO reference always check this. This seems an error since the list itself is only checked if Number is a variable.
The type_error(integer, E) when an element E of the Codes is not an integer is a GNU Prolog extension. This seems to be an omission in the ISO reference since this error is detected for many other built-in predicates accepting a character code (e.g. char_code/2, put_code/2).
Templates
Description
name(Constant, Codes) succeeds if Codes is a list whose elements are the character codes corresponding to the successive characters of Constant (a number or an atom). However, there atoms are for which name(Constant, Codes) is true, but which will not be constructed if name/2 is called with Constant uninstantiated, e.g. the atom ’1024’. For this reason the use of name/2 is discouraged and should be limited to compatibility purposes. It is preferable to use atom_codes/2 (section 8.19.6) or number_chars/2 (section 8.19.7).
Errors
| Constant is a variable and Codes is a partial list or a list with an element which is a variable | instantiation_error | |
| Constant is neither a variable nor an atomic term | type_error(atomic, Constant) | |
| Constant is a variable and Codes is neither a list nor a partial list | type_error(list, Codes) | |
| Constant is a variable and an element E of the list Codes is neither a variable nor an integer | type_error(integer, E) | |
| Constant is a variable and an element E of the list Codes is an integer but not a character code | representation_error(character_code) | |
Portability
GNU Prolog predicate.
Templates
Description
new_atom(Prefix, Atom) unifies Atom with a new atom whose name begins with the characters of the name of Prefix. This predicate is then a symbol generator. It is guaranteed that Atom does not exist before the invocation of new_atom/3. The characters appended to Prefix to form Atom are in: A-Z (capital letter), a-z (small letter) and 0-9 (digit).
new_atom/1 is similar to new_atom(term_, Atom), i.e. the generated atom begins with term_.
Errors
| Prefix is a variable | instantiation_error | |
| Prefix is neither a variable nor an atom | type_error(atom, Prefix) | |
| Atom is not a variable | uninstantiation_error(Atom) | |
Portability
GNU Prolog predicate.
Templates
Description
current_atom(Atom) succeeds if there exists an atom that unifies with Atom. All atoms are found except those beginning with a ’$’ (system atoms). This predicate is re-executable on backtracking.
Errors
| Atom is neither a variable nor an atom | type_error(atom, Atom) | |
Portability
GNU Prolog predicate.
Templates
Description
atom_property(Atom, Property) succeeds if current_atom(Atom) succeeds (section 8.19.10) and if Property unifies with one of the properties of the atom. This predicate is re-executable on backtracking.
Atom properties:
Errors
| Atom is neither a variable nor an atom | type_error(atom, Atom) | |
| Property is neither a variable nor a n atom property term | domain_error(atom_property, Property) | |
| Property = length(E) or hash(E) and E is neither a variable nor an integer | type_error(integer, E) | |
Portability
GNU Prolog predicate.