4.21 Analysing and Constructing Atoms

These predicates convert between Prolog constants and lists of character codes. The predicates atom_codes/2, number_codes/2 and name/2 behave the same when converting from a constant to a list of character codes. When converting the other way around, atom_codes/2 will generate an atom, number_codes/2 will generate a number or exception and name/2 will return a number if possible and an atom otherwise.

The ISO standard defines atom_chars/2 to describe the `broken-up' atom as a list of one-character atoms instead of a list of codes. Up to version 3.2.x, SWI-Prolog's atom_chars/2 behaved like atom_codes, compatible with Quintus and SICStus Prolog. As of 3.3.x, SWI-Prolog atom_codes/2 and atom_chars/2 are compliant to the ISO standard.

To ease the pain of all variations in the Prolog community, all SWI-Prolog predicates behave as flexible as possible. This implies the `list-side' accepts either a code-list or a char-list and the `atom-side' accepts all atomic types (atom, number and string).

[ISO]atom_codes(?Atom, ?String)

Convert between an atom and a list of character codes. If Atom is instantiated, it will be translated into a list of character codes and the result is unified with String. If Atom is unbound and String is a list of character codes, Atom will be unified with an atom constructed from this list.

[ISO]atom_chars(?Atom, ?CharList)

As atom_codes/2, but CharList is a list of one-character atoms rather than a list of character codes.^{68Up to version 3.2.x, atom_chars/2 behaved as the current atom_codes/2. The current definition is compliant with the ISO standard.}

?- atom_chars(hello, X).

X = [h, e, l, l, o]

[ISO]char_code(?Atom, ?Code)

Convert between character and character code for a single character.^{69This is also called atom_char/2 in older versions of SWI-Prolog as well as some other Prolog implementations. The atom_char/2 predicate is available from the library backcomp.pl}

[ISO]number_chars(?Number, ?CharList)

Similar to atom_chars/2, but converts between a number and its representation as a list of one-character atoms. Fails with a syntax_error if Number is unbound or CharList does not describe a number. Following the ISO standard, it allows for leading white space (including newlines) and does not allow for trailing white space.^{70ISO also allows for Prolog comments in leading white space. We--and most other implementations--believe this is incorrect. We also beleive it would have been better not to allow for white space, or to allow for both leading and trailing white space. Prolog syntax-based conversion can be achieved using format/3 and read_from_chars/2.}

[ISO]number_codes(?Number, ?CodeList)

As number_chars/2, but converts to a list of character codes rather than one-character atoms. In the mode (-, +), both predicates behave identically to improve handling of non-ISO source.

atom_number(?Atom, ?Number)

Realises the popular combination of atom_codes/2 and number_codes/2 to convert between atom and number (integer or float) in one predicate, avoiding the intermediate list. Unlike the ISO number_codes/2 predicates, atom_number/2 fails silently in mode (+,-) if Atom does not represent a number.^{71Versions prior to 6.1.7 raise a syntax error, compliant to number_codes/2} See also atomic_list_concat/2 for assembling an atom from atoms and numbers.

name(?Atomic, ?CodeList)

CodeList is a list of character codes representing the same text as Atomic. Each of the arguments may be a variable, but not both. When CodeList describes an integer or floating point number and Atomic is a variable, Atomic will be unified with the numeric value described by CodeList (e.g., name(N, "300"), 400 is N + 100 succeeds). If CodeList is not a representation of a number, Atomic will be unified with the atom with the name given by the character code list. When Atomic is an atom or number, the unquoted print representation of it as a character code list will be unified with CodeList.

Note that it is not possible to produce the atom '300' using name/2, and that name(300, CodeList), name('300', CodeList) succeeds. For these reasons, new code should consider using the ISO predicates atom_codes/2 or number_codes/2.^{72Unfortunately, the ISO predicates provide no neat way to check that a string can be interpreted as a number. The most sensible way is to use catch/3 to catch the exception from number_codes/2; however, this is both slow and cumbersome. We consider making, e.g., number_codes(N, "abc") fail silently in future versions.} See also atom_number/2.

term_to_atom(?Term, ?Atom)

True if Atom describes a term that unifies with Term. When Atom is instantiated, Atom is converted and then unified with Term. If Atom has no valid syntax, a syntax_error exception is raised. Otherwise Term is ``written'' on Atom using write_term/2 with the option quoted(true). See also format/3 and with_output_to/2.

atom_to_term(+Atom, -Term, -Bindings)

Use Atom as input to read_term/2 using the option variable_names and return the read term in Term and the variable bindings in Bindings. Bindings is a list of Name = Var couples, thus providing access to the actual variable names. See also read_term/2. If Atom has no valid syntax, a syntax_error exception is raised.

[ISO]atom_concat(?Atom1, ?Atom2, ?Atom3)

Atom3 forms the concatenation of Atom1 and Atom2. At least two of the arguments must be instantiated to atoms. This predicate also allows for the mode (-,-,+), non-deterministically splitting the 3rd argument into two parts (as append/3 does for lists). SWI-Prolog allows for atomic arguments. Portable code must use atomic_concat/3 if non-atom arguments are involved.

atomic_concat(+Atomic1, +Atomic2, -Atom)

Atom represents the text after converting Atomic1 and Atomic2 to text and concatenating the result:

?- atomic_concat(name, 42, X).
X = name42.

[commons]atomic_list_concat(+List, -Atom)

List is a list of atoms, integers or floating point numbers. Succeeds if Atom can be unified with the concatenated elements of List.

[commons]atomic_list_concat(+List, +Separator, ?Atom)

Creates an atom just like atomic_list_concat/2, but inserts Separator between each pair of atoms. For example:

?- atomic_list_concat([gnu, gnat], ', ', A).

A = 'gnu, gnat'

The SWI-Prolog version of this predicate can also be used to split atoms by instantiating Separator and Atom as shown below. We kept this functionality to simplify porting old SWI-Prolog code where this predicate was called concat_atom/3.

?- atomic_list_concat(L, -, 'gnu-gnat').

L = [gnu, gnat]

[ISO]atom_length(+Atom, -Length)

True if Atom is an atom of Length characters. The SWI-Prolog version accepts all atomic types, as well as code-lists and character-lists. New code should avoid this feature and use write_length/3 to get the number of characters that would be written if the argument was handed to write_term/3.

[deprecated]atom_prefix(+Atom, +Prefix)

True if Atom starts with the characters from Prefix. Its behaviour is equivalent to ?- sub_atom(Atom, 0, _, _, Prefix). Deprecated.

[ISO]sub_atom(+Atom, ?Before, ?Len, ?After, ?Sub)

ISO predicate for breaking atoms. It maintains the following relation: Sub is a sub-atom of Atom that starts at Before, has Len characters, and Atom contains After characters after the match.

?- sub_atom(abc, 1, 1, A, S).

A = 1, S = b

The implementation minimises non-determinism and creation of atoms. This is a very flexible predicate that can do search, prefix- and suffix-matching, etc.