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|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).
?- atom_chars(hello, X). X = [h, e, l, l, o]
If CharList is parsed, it is parsed using the Prolog
syntax for numbers. Following the ISO standard, it allows for leading
white space (including newlines) and does not allow for trailing
white space.104ISO 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
also be achieved using format/3
syntax_error exception is raised if CharList
does not represent a valid Prolog number.
name(N, "300"), 400 is N + 100succeeds). 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, depending on the syntax, a number or atom. New code should consider the ISO predicates atom_codes/2, number_codes/2 or the SWI-Prolog predicate atom_number/2.
syntax_errorexception 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.
variable_namesand 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_errorexception is raised. New code should use read_term_from_atom/3.
?- atomic_concat(name, 42, X). X = name42.
atomic_list_concat(List, '', Atom).
?- 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]
?- sub_atom(Atom, 0, _, _, Prefix). Deprecated.
?- 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.