SWI-Prolog 7.7.1 Reference Manual: Section 4.22

## 4.22 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.99Up 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.100This 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.101ISO 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.102Versions 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/2.103Unfortunately, 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 parsed and the result 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, with_output_to/2 and term_string/2.
[deprecated]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. New code should use read_term_from_atom/3.
[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 strings, atoms, integers or floating point numbers. Succeeds if Atom can be unified with the concatenated elements of List. Equivalent to `atomic_list_concat(List, '', Atom)`.
[commons]atomic_list_concat(+List, +Separator, -Atom)
  Creates an atom just like atomic_list_concat/2, but inserts Separator between each pair of inputs. 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. When used in mode (-,+,+), Separator must be a non-empty atom. See also split_string/4.

      ?- 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 flexible predicate that can do search, prefix- and suffix-matching, etc.

[semidet]sub_atom_icasechk(+Haystack, ?Start, +Needle)
  True when Needle is a sub atom of Haystack starting at Start. The match is `half case insensitive', i.e., uppercase letters in Needle only match themselves, while lowercase letters in Needle match case insensitively. Start is the first 0-based offset inside Haystack where Needle matches.104This predicate replaces $apropos_match/2, used by the help system, while extending it with locating the (first) match and performing case insensitive prefix matching. We are still not happy with the name and interface.