strings.pl -- String utilities

This module provides string handling utilities, currently notably for dealing with multi-line strings and interpolation. The library provides a couple of primitives as well definitions for the string quasi quotation syntax. The latter allows for constructing both single line and multi-line long strings based on template interpolation. Below is a simple example using the quasi quotation syntax.

test(To) :-
    write({|string(To)||
           | Dear {To},
           |
           | I'm happy to announce a string interpolation quasi quoter.
           |}.

Warning

The general purpose string interpolation implemented by this library should not be used to create strings for a formal language such as HTML, JavaScript, SQL, etc. because the result will be subject to injection attacks, providing a serious security risc. The core idea of quasi quotation is to know about the target language and interpolate Prolog data into the template while respecting the syntax of the target language, notable to escape certain characters where needed. See also library(http/html_write) and library(http/js_write) which define quasi quotation rules for HTML and JavaScript.

See also

- format/3 can format to a string as well. The library(lynx/format) provides primitive to wrap long strings.

- The core system provides many additional string processing predicates.

To be done

- There are probably many other high level string predicates that belong in this library. For example, predicates similar to the functions in https://docs.python.org/3/library/textwrap.html

 string(+Content, +Args, +Binding, -DOM)

Implements the quasi quotation syntax string. If the first character of the content is a newline (i.e., there is a newline immediately after the || token) this first uses dedent_lines/3 to the remove common white space prefix from the lines. This is called with the option chars("\s\t|"), i.e., also removing | characters and tab(8).

If the quasi quotation syntax carries arguments (e.g., string(To)), the string is compiled into a function that produces the result of interpolating the arguments into the template. See user functions on dict objects. If there are no arguments, the result is simply the final string.

See also

- interpolate_string/4 for the interpolation syntax.

- Section for examples and discussion.

To be done

- Specify tab width and allow for {@Goal} templates.

 interpolate_string(:In, -Out, +Map, +Options)

Establish a string from a template by replacing patterns. Supported patterns are:

{Name}

If Map contains Name=Value, insert Value using write/1. If Name does not appear in Map, raise an existence error. Name must satisfy the rules for a Prolog variable.

{Name,Default}

As above, but if Name does not appear in Map, use Value

{@Goal}

Insert the output (to current_output) of Goal here. For safety reasons only accepted if Options contains goals(true)

 string_lines(?String, ?Lines) is det

True when String represents Lines. This follows the normal text convention that a line is defined as a possible empty string followed by a newline character ("\n"). E.g.

?- string_lines("a\nb\n", L).
L = ["a", "b"].
?- string_lines(S, ["a", "b"]).
S = "a\nb\n".

This predicate is a true relation if both arguments are in canonical form, i.e. all text is represented as strings and the first argument ends with a newline. The implementation tolerates non-canonical input: other types than strings are accepted and String does not need to end with a newline.

See also

- split_string/4. Using split_string(String, "\n", "", Lines) on a string that ends in a newline adds an additional empty string compared to string_lines/2.

 dedent_lines(+In, -Out, +Options)

Remove shared indentation for all lines in a string. Lines are separated by "\n" -- conversion to and from external forms (such as "\r\n") are typically done by the I/O predicates. A final "\n" is preserved.

Options:

tab(N)

Assume tabs at columns of with N. When omitted, tabs are taken literally and only exact matches are removed.

chars(CodesOrString)

Characters to remove. This can notably be used to remove additional characters such as * or `|`. Default is " \t".

 line_indent(+Line, -Indent, +Sep) is det[private]

Determine the indentation as a list of character codes. If the line only holds white space Indent is left unbound.

 line_indent_width(+Line, -Indent, +Tab, +Sep) is det[private]

Determine the indentation as a column, compensating for the Tab width. This is used if the tab(Width) option is provided.

 dedent_line(+Tab, +Indent, +String, -Dedented)[private]

Dedent a single line according to Tab and Indent. Indent is either an integer, deleting the first Indent characters or a string, deleting the string literally.

 indent_lines(+Prefix, +In, -Out) is det

Add Prefix to the beginning of lines in In. Lines are separated by "\n" -- conversion to and from external forms (such as "\r\n") are typically done by the I/O predicates. Lines that consist entirely of whitespace are left as-is.

 indent_lines(:Filter, +Prefix, +In, -Out) is det

Similar to indent_lines/3, but only adds Prefix to lines for which call(Filter, Line) succeeds.