3.21 Analysing and constructing atoms

These predicates convert between Prolog constants and lists of ASCII values. The predicates atom_codes/2, number_codes/2 and name/2 behave the same when converting from a constant to a list of ASCII values. 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. Upto version 3.2.x, SWI-Prolog's atom_chars/2 behaved, compatible to Quintus and SICStus Prolog, like atom_codes. 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' accept all atomic types (atom, number and string).

atom_codes(?Atom, ?String)

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

atom_chars(?Atom, ?CharList)

As atom_codes/2, but CharList is a list of one-character atoms rather than a list of ASCII values (18).

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

char_code(?Atom, ?ASCII)

Convert between character and ASCII value for a single character. (19)

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 representation_error if Number is unbound and CharList does not describe a number.

number_codes(?Number, ?CodeList)

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

name(?AtomOrInt, ?String)

String is a list of ASCII values describing Atom. Each of the arguments may be a variable, but not both. When String is bound to an ASCII value list describing an integer and Atom is a variable Atom will be unified with the integer value described by String (e.g. `name(N, "300"), 400 is N + 100' succeeds).

int_to_atom(+Int, +Base, -Atom)

Convert Int to an ASCII representation using base Base and unify the result with Atom. If Base <> 10 the base will be prepended to Atom. Base = 0 will try to interpret Int as an ASCII value and return 0'<c>. Otherwise 2 =<Base =< 36. Some examples are given below.

int_to_atom(45, 2, A)	-->	A = 2'101101
int_to_atom(97, 0, A)	-->	A = 0'a
int_to_atom(56, 10, A)	-->	A = 56

int_to_atom(+Int, -Atom)

Equivalent to int_to_atom(Int, 10, Atom).

term_to_atom(?Term, ?Atom)

Succeeds 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/1.

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.

atom_concat(?Atom1, ?Atom2, ?Atom3)

Atom3 forms the concatenation of Atom1 and Atom2. At least two of the arguments must be instantiated to atoms, integers or floating point numbers. For ISO compliance, the instantiation-pattern -, -, + is allowed too, non-deterministically splitting the 3-th argument into two parts (as append/3 does for lists). See also string_concat/3.

concat_atom(+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. If List has exactly 2 elements it is equivalent to atom_concat/3, allowing for variables in the list.

concat_atom(?List, +Separator, ?Atom)

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

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

This predicate can also be used to split atoms by instantiating Separator and Atom:

?- concat_atom(L, -, 'gnu-gnat'). L = [gnu, gnat]

atom_length(+Atom, -Length)

Succeeds if Atom is an atom of Length characters long. This predicate also works for integers and floats, expressing the number of characters output when given to write/1.

atom_prefix(+Atom, +Prefix)

Succeeds if Atom starts with the characters from Prefix. Its behaviour is equivalent to ?- concat(Prefix, _, Atom), but avoids the construction of an atom for the `remainder'.

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