4.22 Analysing and Constructing Atoms

These predicates convert between certain Prolog atomic values on one hand and lists of character codes (or, for atom_chars/2, characters) on the other. The Prolog atomic values can be atoms, characters (which are atoms of length 1), SWI-Prolog strings, as well as numbers (integers, floats and non-integer rationals).

The character codes, also known as code values, are integers. In SWI-Prolog, these integers are Unicode code points.^{bugOn Windows the range is limited to UCS-2, 0..65535.}

To ease the pain of all text representation variations in the Prolog community, all SWI-Prolog predicates behave as flexible as possible. This implies the‘list-side’accepts both a character-code-list and a character-list and the‘atom-side’accepts all atomic types (atom, number and string). For example, the predicates atom_codes/2, number_codes/2 and name/2 behave the same in mode (+,-), i.e.,‘listwards’, 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 throw an exception;
• name/2 will generate a number if possible and an atom otherwise.

[ISO]atom_codes(?Atom, ?CodeList)

Convert between an atom and a list of character codes (integers denoting characters).

• If Atom is instantiated, it will be translated into a list of character codes, which are unified with CodeList.
• If Atom is uninstantiated and CodeList is a list of character codes, then Atom will be unified with an atom constructed from this list.

?- atom_codes(hello, X).
X = [104, 101, 108, 108, 111].

The‘listwards’call to atom_codes/2 can also be written (functionally) using backquotes instead:

?- Cs = `hello`.
Cs = [104, 101, 108, 108, 111].

Backquoted strings can be mostly found in the body of DCG rules that process lists of character codes.

Note that this is the default interpretation for backquotes. It can be changed on a per-module basis by setting the value of the Prolog flag back_quotes.

[ISO]atom_chars(?Atom, ?CharList)

Similar to atom_codes/2, but CharList is a list of characters (atoms of length 1) rather than a list of character codes (integers denoting characters).

?- atom_chars(hello, X).
X = [h, e, l, l, o]

[ISO]char_code(?Atom, ?Code)

Convert between a single character (an atom of length 1), and its character code (an integer denoting the corresponding character). The predicate alternatively accepts an SWI-Prolog string of length 1 at Atom place.

[ISO]number_chars(?Number, ?CharList)

Similar to atom_chars/2, but converts between a number and its representation as a list of characters (atoms of length 1).

• If CharList is a proper list, i.e., not unbound or a partial list, CharList is parsed according to the Prolog syntax for numbers and the resulting number is unified with Number. A syntax_error exception is raised if CharList is instantiated to a ground, proper list but does not represent a valid Prolog number.
• Otherwise, if Number is indeed a number, Number is serialized and the result is unified with CharList.

Following the ISO standard, the Prolog syntax for number allows for leading white space (including newlines) and does not allow for trailing white space.^{118ISO also allows for Prolog comments in leading white space. We--and most other implementations--believe this is incorrect. We also believe 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 also 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 characters. 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, float or non-integer rational) in one predicate, avoiding the intermediate list. Unlike the ISO standard number_codes/2 predicates, atom_number/2 fails silently in mode (+,-) if Atom does not represent a number.

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.
• If Atomic is an atom or number, the unquoted print representation of it as a character code list is unified with CodeList.

This predicate is part of the Edinburgh tradition. It should be considered deprecated although, given its long tradition, it is unlikely to be removed from the system. It still has some value for converting input to a number or an atom (depending on the syntax). New code should consider the ISO predicates atom_codes/2, number_codes/2 or the SWI-Prolog predicate 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, floating point numbers or non-integer rationals. 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‘atomwards` transformation is usually called a string join operation in other programming languages.

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, ?Length, ?After, ?SubAtom)

ISO predicate for breaking atoms. It maintains the following relation: SubAtom is a sub-atom of Atom that starts at (0-based index) Before, has Length characters, and Atom contains After characters after the match. The implementation minimises non-determinism and creation of atoms. This is a flexible predicate that can do search, prefix- and suffix-matching, etc. Scenarios that use this predicate often generate atoms that with a short lifetime; in such cases sub_string/5 may be a better alternative. Examples:

Pick out a sub-atom of length 3 starting a 0-based index 2:

?- sub_atom(aaxyzbbb, 2, 3, After, SubAtom).
After = 3,
SubAtom = xyz.

The following example splits a string of the form <name>=<value> into the name part (an atom) and the value (a string).

name_value(String, Name, Value) :-
    sub_atom(String, Before, _, After, "="),
    !,
    sub_atom(String, 0, Before, _, Name),
    sub_atom(String, _, After, 0, Value).

This example defines a predicate that inserts a value at a position. Note that sub_string/5 is used here instead of sub_atom/5 to avoid the overhead of creating atoms for the intermediate results.

atom_insert(Str, Val, At, NewStr) :-
    sub_string(Str, 0, At, A1, S1),
    sub_string(Str, At, A1, _, S2),
    atomic_list_concat([S1,Val,S2], NewStr).

On backtracking, matches are delivered in order left-to-right (i.e. Before increases monotonically):

?- sub_atom('xATGATGAxATGAxATGAx', Before, Length, After, 'ATGA').
Before = 1, Length = 4, After = 14 ;
Before = Length, Length = 4, After = 11 ;
Before = 9, Length = 4, After = 6 ;
Before = 14, Length = 4, After = 1 ;
false.

See also sub_string/5, the corresponding predicate for SWI-Prolog strings.

[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.^{119This 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.}