These predicates convert between Prolog constants and lists of
character codes. The predicates atom_codes/2, number_codes/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, compatible to Quintus and SICStus Prolog, like atom_codes. As
of 3.3.x SWI-Prolog
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).
Convert between an atom and a list of character codes. If
Atom is instantiated, if 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, it will
Atom will be unified with an atom constructed from this list.
but CharList is a list of one-character atoms rather than a
list of character codes49Up-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]
Convert between character and character code for a single character.50This
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
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 and CharList
does not describe a number.
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.
Realises the popular combination of atom_codes/2
to convert between atom and number (integer or float) in one predicate,
avoiding the intermediate list. Calling in mode +,- to convert numbers
represented as atoms is often good style. Converting numbers to atoms,
which in turn are assembled into larger units before communication them
to the outside world is bad style. Consider using streams or
to reduce the number of expensive intermediate atoms.
String is a list of character codes representing the same
text as Atom. Each of the arguments may be a variable, but
not both. When String is bound to an character code list
describing an integer and Atom is a variable Atom
will be unified with the integer value described by String
name(N, "300"), 400 is N + 100' succeeds).
True if Atom describes a term that unifies with Term.
Atom is instantiated Atom is converted and then
Term. If Atom has no valid syntax, a
exception is raised. Otherwise Term is ``written'' on Atom
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
exception is raised.
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
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.
Creates an atom just like concat_atom/2,
but inserts Separator between each pair of atoms. For
?- 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]
True if Atom is an atom of Length characters long.
This predicate also works for strings (see section
4.23). If the prolog flag iso
is not set, it also accepts integers and floats, expressing the
number of characters output when given to write/1
as well as code-lists and character-lists, expressing the length of the
is both an atom an empty code/character list. The predicate atom_length/2
returns 2 for this atom.
True if Atom starts with the characters from Prefix.
Its behaviour is equivalent to
?- sub_atom(Atom, 0, _, _, Prefix).
?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,
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