As of SWI-Prolog version 7, text enclosed in double quotes
(e.g.,
"Hello world") is read as objects of the type string.
Strings are distinct from lists, which makes it possible to recognize
them at runtime and print them using the string syntax:
?- write("Hello world!").
Hello world!
?- writeq("Hello world!").
"Hello world!"
A string is a compact representation of a character sequence that
lives on the global (term) stack. Strings are represented by sequences
of Unicode character codes including the character code 0 (zero). The
length of strings is limited by the available space on the global (term)
stack (see
set_prolog_stack/2). Section
5.2.3 motivates the introduction of strings and mapping double
quoted text to this type.
Whereas in version 7, double-quoted text is mapped to strings,
back-quoted text (as in `text`) is mapped to a
list of
character codes, i.e. integers that are Unicode code points. In
a traditional setting, back-quoted would be mapped to a list of
characters (also known as chars), which are atoms of
length 1.
The settings for the flags that control how double- and back-quoted
text is read is summarised in table
8. Programs that aim for compatibility should realise that the ISO
standard defines back-quoted text, but does not define the back_quotes
Prolog flag and does not define the term that is produced by back-quoted
text.
Table 8 : Mapping of double and back quoted
text in the two modes.
With the introduction of strings as a Prolog data type, there are
three main ways to represent text: using strings, using atoms and using
lists of character codes. As a fourth way, one may also use lists of
chars. This section explains what to choose for what purpose. Both
strings and atoms are atomic objects: you can only look inside
them using dedicated predicates, while lists of character codes or chars
are compound data structures forming an extended structure that must
follow a convention.
- Lists of character codes
- is what you need if you want to parse text using Prolog grammar
rules (DCGs, see phrase/3).
Most of the text reading predicates (e.g.,
read_line_to_codes/2)
return a list of character codes because most applications need to parse
these lines before the data can be processed. As said above, the back-quoted
text notation (
`hello`) can be used to easily specify
a list of character codes. The 0'c notation can be used to
specify a single character code.
- Atoms
- are identifiers. They are typically used in cases where
identity comparison is the main operation and that are typically not
composed nor taken apart. Examples are RDF resources (URIs that identify
something), system identifiers (e.g.,
'Boeing 747'), but
also individual words in a natural language processing system. They are
also used where other languages would use enumerated types,
such as the names of days in the week. Unlike enumerated types, Prolog
atoms do not form a fixed set and the same atom can represent different
things in different contexts.
- Strings
- typically represents text that is processed as a unit most of the time,
but which is not an identifier for something. Format specifications for
format/3
is a good example. Another example is a descriptive text provided in an
application. Strings may be composed and decomposed using e.g., string_concat/3
and sub_string/5
or converted for parsing using string_codes/2
or created from codes generated by a generative grammar rule, also using string_codes/2.
Strings are manipulated using a set of predicates that mirrors the
set of predicates used for manipulating atoms. In addition to the list
below, string/1
performs the type check for this type and is described in section
4.5.
SWI-Prolog's string primitives are being synchronized with
ECLiPSe.
We expect the set of predicates documented in this section to be stable,
although it might be expanded. In general, SWI-Prolog's text
manipulation predicates accept any form of text as input argument - they
accept anytext input. anytext comprises:
- atoms
- strings
- lists of character codes
- list of characters
- number types: integers, floating point numbers and non-integer
rationals. Under the hood, these must first be formatted into a text
representation according to some inner convention before they can be
used.
The predicates produce the type indicated by the predicate name as
output. This policy simplifies migration and writing programs that can
run unmodified or with minor modifications on systems that do not
support strings. Code should avoid relying on this feature as much as
possible for clarity as well as to facilitate a more strict mode and/or
type checking in future releases.
- atom_string(?Atom,
?String)
- Bi-directional conversion between an atom and a string. At least one of
the two arguments must be instantiated. An initially uninstantiated
variable on the “string side'' is always instantiated to a string.
An initially uninstantiated variable on the “atom side'' is always
instantiated to an atom. If both arguments are instantiated, their
list-of-character representations must match, but the types are not
enforced. The following all succeed:
atom_string("x",'x').
atom_string('x',"x").
atom_string(3.1415,3.1415).
atom_string('3r2',3r2).
atom_string(3r2,'3r2').
atom_string(6r4,3r2).
- number_string(?Number,
?String)
- Bi-directional conversion between a number and a string. At least one of
the two arguments must be instantiated. Besides the type used to
represent the text, this predicate differs in several ways from its ISO
cousin:170Note that SWI-Prolog's
syntax for numbers is not ISO compatible either.
- If String does not represent a number, the predicate fails
rather than throwing a syntax error exception.
- Leading white space and Prolog comments are not allowed.
- Numbers may start with
+-
- It is not allowed to have white space between a leading
+-
- Floating point numbers in exponential notation do not require a dot
before exponent, i.e.,
"1e10" is a valid number.
Unlike other predicates of this family, if instantiated, String
cannot be an atom.
The corresponding‘atom-handling' predicate is atom_number/2,
with reversed argument order.
- term_string(?Term,
?String)
- Bi-directional conversion between a term and a string. If String
is instantiated, it is parsed and the result is unified with Term.
Otherwise Term is‘written' using the option
quoted(true)
and the result is converted to String.
- term_string(?Term,
?String, +Options)
- As term_string/2,
passing Options to either read_term/2
or write_term/2.
For example:
?- term_string(Term, 'a(A)', [variable_names(VNames)]).
Term = a(_9674),
VNames = ['A'=_9674].
- string_chars(?String,
?Chars)
- Bi-directional conversion between a string and a list of characters. At
least one of the two arguments must be instantiated.
See also: atom_chars/2.
- string_codes(?String,
?Codes)
- Bi-directional conversion between a string and a list of character
codes. At least one of the two arguments must be instantiated.
- string_bytes(?String,
?Bytes, +Encoding)
- True when the (Unicode) String is represented by Bytes
in
Encoding. If String is instantiated it may
represent text as an atom, string, list of character codes or list or
characters.
Bytes is always a list of integers in the range 0 ...
255. At least one of String or Bytes must be
instantiated. This predicate is notably intended as an intermediate step
to perform byte oriented operations on text. Examples are (base64)
encoding, encryption, computing a (secure) hash, etc. Encoding
is typically
utf8. All valid stream encodings except for wchar_t
are supported. See section
2.19.1. Note that this translation is only provided for strings.
Creating an atom from bytes requires
atom_string/2.171Strings
are an efficient intermediate and this conversion is needed only in some
uncommon scenarios.
- [det]text_to_string(+Text,
-String)
- Converts Text to a string. Text is anytext
excluding the number types. When running in
--traditional mode,
'[]' is ambiguous and
interpreted as an empty string.
- 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
anytext as its first argument. Number types must first be
formatted into strings before the length of their string representation
can be determined.
- string_code(?Index,
+String, ?Code)
- True when Code represents the character at the 1-based Index
position in String. If Index is unbound the string
is scanned from index 1. Raises a domain error if Index is
negative. Fails silently if Index is zero or greater than the
length of
String. The mode
string_code(-,+,+) is
deterministic if the searched-for Code appears only once in String.
See also
sub_string/5.
- get_string_code(+Index,
+String, -Code)
- Semi-deterministic version of string_code/3.
In addition, this version provides strict range checking, throwing a
domain error if Index is less than 1 or greater than the
length of String. ECLiPSe provides this to support
String[Index]
notation.
- string_concat(?String1,
?String2, ?String3)
- Similar to atom_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.
- [det]split_string(+String,
+SepChars, +PadChars, -SubStrings)
- Break String into SubStrings. The SepChars
argument provides the characters that act as separators and thus the
length of
SubStrings is one more than the number of separators found if
SepChars and PadChars do not have common
characters. If
SepChars and PadChars are equal, sequences of
adjacent separators act as a single separator. Leading and trailing
characters for each substring that appear in PadChars are
removed from the substring. The input arguments can be either atoms,
strings or char/code lists. Compatible with ECLiPSe. Below are some
examples:
A simple split wherever there is a‘.':
?- split_string("a.b.c.d", ".", "", L).
L = ["a", "b", "c", "d"].
Consider sequences of separators as a single one:
?- split_string("/home//jan///nice/path", "/", "/", L).
L = ["home", "jan", "nice", "path"].
Split and remove white space:
?- split_string("SWI-Prolog, 7.0", ",", " ", L).
L = ["SWI-Prolog", "7.0"].
Only remove leading and trailing white space (trim the
string):
?- split_string(" SWI-Prolog ", "", "\s\t\n", L).
L = ["SWI-Prolog"].
In the typical use cases, SepChars either does not overlap
PadChars or is equivalent to handle multiple adjacent
separators as a single (often white space). The behaviour with partially
overlapping sets of padding and separators should be considered
undefined. See also read_string/5.
- sub_string(+String,
?Before, ?Length, ?After, ?SubString)
- This predicate is functionally equivalent to sub_atom/5,
but operates on strings. Note that this implies the string input
arguments can be either strings or atoms. If SubString is
unbound (output) it is unified with a string. 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_string(String, Before, _, After, "="),
!,
sub_atom(String, 0, Before, _, Name),
sub_string(String, _, After, 0, Value).
The next example defines a predicate that inserts a value at a
position. See sub_atom/5
for more examples.
string_insert(Str, Val, At, NewStr) :-
sub_string(Str, 0, At, A1, S1),
sub_string(Str, At, A1, _, S2),
atomics_to_string([S1,Val,S2], NewStr).
- atomics_to_string(+List,
-String)
- List is a list of strings, atoms, or number types. Succeeds
if String can be unified with the concatenated elements of List.
Equivalent to
atomics_to_string(List,’’, String).
- atomics_to_string(+List,
+Separator, -String)
- Creates a string just like atomics_to_string/2,
but inserts
Separator between each pair of inputs. For example:
?- atomics_to_string([gnu, "gnat", 1], ', ', A).
A = "gnu, gnat, 1"
- string_upper(+String,
-UpperCase)
- Convert String to upper case and unify the result with
UpperCase.
- string_lower(+String,
LowerCase)
- Convert String to lower case and unify the result with
LowerCase.
- read_string(+Stream,
?Length, -String)
- Read at most Length characters from Stream and
return them in the string String. If Length is
unbound, Stream is read to the end and Length is
unified with the number of characters read. The number of bytes
read depends on the encoding of Stream (see section
2.19.1). This predicate may be used to read a sequence of bytes when
the stream is in
octet encoding. See open/4
and set_stream/2
for controlling the encoding.
- read_string(+Stream,
+SepChars, +PadChars, -Sep, -String)
- Read a string from Stream, providing functionality similar to
split_string/4.
The predicate performs the following steps:
- Skip all characters that match PadChars
- Read up to a character that matches SepChars or end of
file
- Discard trailing characters that match PadChars from the
collected input
- Unify String with a string created from the input and
Sep with the code of the separator character read. If input
was terminated by the end of the input, Sep is unified with
-1.
The predicate read_string/5
called repeatedly on an input until
Sep is -1 (end of file) is equivalent to reading the entire
file into a string and calling split_string/4,
provided that SepChars and PadChars are not partially
overlapping.172Behaviour that
is fully compatible would require unlimited look-ahead.
Below are some examples:
Read a line:
read_string(Input, "\n", "\r", Sep, String)
Read a line, stripping leading and trailing white space:
read_string(Input, "\n", "\r\t ", Sep, String)
Read up to‘,’or‘)’,
unifying Sep with 0', i.e. Unicode 44, or 0'),
i.e. Unicode 41:
read_string(Input, ",)", "\t ", Sep, String)
- open_string(+String,
-Stream)
- True when Stream is an input stream that accesses the content
of
String. String can be any text representation,
i.e., string, atom, list of codes or list of characters. The created Stream
has the
reposition property (see stream_property/2).
Note that the internal encoding of the data is either ISO Latin 1 or
UTF-8.
Prolog defines two forms of quoted text. Traditionally, single quoted
text is mapped to atoms while double quoted text is mapped to a list of
character codes (integers) or characters (atoms of length 1).
Representing text using atoms is often considered inadequate for several
reasons:
- It hides the conceptual difference between text and program symbols.
Where content of text often matters because it is used in I/O, program
symbols are merely identifiers that match with the same symbol
elsewhere. Program symbols can often be consistently replaced, for
example to obfuscate or compact a program.
- Atoms are globally unique identifiers. They are stored in a shared
table. Volatile strings represented as atoms come at a significant price
due to the required cooperation between threads for creating atoms.
Reclaiming temporary atoms using Atom garbage collection is a
costly process that requires significant synchronisation.
- Many Prolog systems (not SWI-Prolog) put severe restrictions on the
length of atoms or the maximum number of atoms.
Representing text as lists, be it of character codes or characters,
also comes at a price:
- It is not possible to distinguish (at runtime) a list of integers or
atoms from a string. Sometimes this information can be derived from
(implicit) typing. In other cases the list must be embedded in a
compound term to distinguish the two types. For example,
s("hello world")
could be used to indicate that we are dealing with a string.
Lacking runtime information, debuggers and the toplevel can only use
heuristics to decide whether to print a list of integers as such or as a
string (see portray_text/1).
While experienced Prolog programmers have learned to cope with this,
we still consider this an unfortunate situation.
- Lists are expensive structures, taking 2 cells per character (3 for
SWI-Prolog in its current form). This stresses memory consumption on the
stacks while pushing them on the stack and dealing with them during
garbage collection is unnecessarily expensive.
We observe that in many programs, most strings are only handled as a
single unit during their lifetime. Examining real code tells us that
double quoted strings typically appear in one of the following roles:
- A DCG literal
- Although represented as a list of codes is the correct representation
for handling in DCGs, the DCG translator can recognise the literal and
convert it to the proper representation. Such code need not be modified.
- A format string
- This is a typical example of text that is conceptually not a program
identifier. Format is designed to deal with alternative representations
of the format string. Such code need not be modified.
- Getting a character code
- The construct
[X] = "a" is a commonly used template for
getting the character code of the letter’a'. ISO Prolog defines
the syntax 0'a for this purpose. Code using this must be
modified. The modified code will run on any ISO compliant Prolog
Processor.
- As argument to list predicates to operate on strings
- Here, we might see code similar to
append("name:", Rest, Codes).
Such code needs to be modified. In this particular example, the
following is a good portable alternative: phrase("name:", Codes, Rest)
- Checks for a character to be in a set
- Such tests are often performed with code such as this:
memberchk(C, "~!@#$"). This is a rather inefficient check
in a traditional Prolog system because it pushes a list of character
codes cell-by-cell onto the Prolog stack and then traverses this list
cell-by-cell to see whether one of the cells unifies with C.
If the test is successful, the string will eventually be subject to
garbage collection. The best code for this is to write a predicate as
below, which pushes nothing on the stack and performs an indexed lookup
to see whether the character code is in‘my_class'.
my_class(0'~).
my_class(0'!).
...
An alternative to reach the same effect is to use term expansion to
create the clauses:
term_expansion(my_class(_), Clauses) :-
findall(my_class(C),
string_code(_, "~!@#$", C),
Clauses).
my_class(_).
Finally, the predicate string_code/3
can be exploited directly as a replacement for the memberchk/2
on a list of codes. Although the string is still pushed onto the stack,
it is more compact and only a single entity.
The predicates in this section can help adapting your program to the
new convention for handling double quoted strings. We have adapted a
huge code base with which we were not familiar in about half a day.
- list_strings
- This predicate may be used to assess compatibility issues due to the
representation of double quoted text as string objects. See
section 5.2 and section
5.2.3. To use it, load your program into Prolog and run list_strings/0.
The predicate lists source locations of string objects encountered in
the program that are not considered safe. Such string need to be
examined manually, after which one of the actions below may be
appropriate:
- check:string_predicate(:PredicateIndicator)
- Declare that PredicateIndicator has clauses that contain
strings, but that this is safe. For example, if there is a predicate
help_info/2 , where the second argument contains a double quoted string
that is handled properly by the predicates of the applications' help
system, add the following declaration to stop
list_strings/0
from complaining:
:- multifile check:string_predicate/1.
check:string_predicate(user:help_info/2).
- check:valid_string_goal(:Goal)
- Declare that calls to Goal are safe. The module qualification
is the actual module in which Goal is defined. For example, a
call to format/3
is resolved by the predicate system:format/3.
and the code below specifies that the second argument may be a string
(system predicates that accept strings are defined in the library).
:- multifile check:valid_string_goal/1.
check:valid_string_goal(system:format(_,S,_)) :- string(S).