SWI-Prolog supports the data type string. Strings are a
time and space efficient mechanism to handle text in Prolog. Atoms are
under some circumstances not suitable because garbage collection on them
is next to impossible (Although it is possible: BIM_prolog does it).
Representing text as a list of ASCII values is, from the logical point
of view, the cleanest solution. It however has two drawbacks: 1) they
cannot be distinguished from a list of (small) integers; and 2) they
consume (in SWI-Prolog) 12 bytes for each character stored.
Within strings each character only requires 1 byte storage. Strings
live on the global stack and their storage is thus reclaimed on
backtracking. Garbage collection can easily deal with strings.
The ISO standard proposes " ... " is transformed into a
string object by read/1
and derivatives. This poses problems as in the old convention "
... " is transformed into a list of ASCII
characters. For this reason the style check option `string'
is available (see style_check/1).
The set of predicates associated with strings is incomplete and
tentative. Names and definitions might change in the future to confirm
to the emerging standard.
- string_to_atom(?String,
?Atom)
-
Logical conversion between a string and an atom. At least one of the two
arguments must be instantiated. Atom can also be an integer
or floating point number.
- string_to_list(?String,
?List)
-
Logical conversion between a string and a list of ASCII characters. At
least one of the two arguments must be instantiated.
- string_length(+String,
-Length)
-
Unify Length with the number of characters in String.
This predicate is functionally equivalent to atom_length/2
and also accepts atoms, integers and floats as its first argument.
- string_concat(?String1,
?String2, ?String3)
-
Similar to concat/3,
but the unbound argument will be unified with a string object rather
than an atom. Also, if both String1 and
String2 are unbound and String3 is bound to text,
it breaks String3, unifying the start with String1
and the end with String2 as append does with lists. Note that
this is not particularly fast on long strings as for each redo the
system has to create two entirely new strings, while the list equivalent
only creates a single new list-cell and moves some pointers around.
- substring(+String,
+Start, +Length, -Sub)
-
Create a substring of String that starts at character Start
(1 base) and has Length characters. Unify this substring with
Sub. (13)