5.2 The string type and its double quoted syntax

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.

Mode	double_quotes	back_quotes
Version 7 default	string	codes
--traditional	codes	symbol_char

5.2.1 Representing text: strings, atoms and code lists

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.

5.2.2 Predicates that operate on strings

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:^{155Note 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 + or -.
• It is not allowed to have white space between a leading + or - and the number.
• 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.

[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).

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.

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:

1. Skip all characters that match PadChars
2. Read up to a character that matches SepChars or end of file
3. Discard trailing characters that match PadChars from the collected input
4. 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.^{156Behaviour 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.

5.2.3 Why has the representation of double quoted text changed?

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.

5.2.4 Adapting code for double quoted strings

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.

5.2.5 Predicates to support adapting code for double quoted strings

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:

• Rewrite the code. For example, change [X] = "a" into X = 0'a.
• If a particular module relies heavily on representing strings as lists of character code, consider adding the following directive to the module. Note that this flag only applies to the module in which it appears.

            :- set_prolog_flag(double_quotes, codes).
            

• Use a back quoted string (e.g., `text`). Note that this will not make your code run regardless of the --traditional command line option and code exploiting this mapping is also not portable to ISO compliant systems.
• If the strings appear in facts and usage is safe, add a clause to the multifile predicate check:string_predicate/1 to silence list_strings/0 on all clauses of that predicate.
• If the strings appear as an argument to a predicate that can handle string objects, add a clause to the multifile predicate check:valid_string_goal/1 to silence list_strings/0.

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).