basics.pl -- Various general DCG utilities

This library provides various commonly used DCG primitives acting on list of character codes. Character classification is based on code_type/2.

This module started its life as library(http/dcg_basics) to support the HTTP protocol. Since then, it was increasingly used in code that has no relation to HTTP and therefore this library was moved to the core library.

To be done: - This is just a starting point. We need a comprehensive set of generally useful DCG primitives.

string_without(+EndCodes, -Codes)// is det

Take as many codes from the input until the next character code appears in the list EndCodes. The terminating code itself is left on the input. Typical use is to read upto a defined delimiter such as a newline or other reserved character. For example:

    ...,
    string_without("\n", RestOfLine)

Arguments:

EndCodes

- is a list of character codes.

See also

- string//1.

string(-Codes)// is nondet

Take as few as possible tokens from the input, taking one more each time on backtracking. This code is normally followed by a test for a delimiter. For example:

upto_colon(Atom) -->
        string(Codes), ":", !,
        { atom_codes(Atom, Codes) }.

See also: - string_without//2.

blanks// is det

Skip zero or more white-space characters.

blank// is semidet

Take next space character from input. Space characters include newline.

See also: - white//0

nonblanks(-Codes)// is det

Take all graph characters

nonblank(-Code)// is semidet

Code is the next non-blank (graph) character.

blanks_to_nl// is semidet

Take a sequence of blank//0 codes if blanks are followed by a newline or end of the input.

whites// is det

Skip white space inside a line.

See also: - blanks//0 also skips newlines.

white// is semidet

Take next white character from input. White characters do not include newline.

alpha_to_lower(?C)// is semidet

Read a letter (class alpha) and return it as a lowercase letter. If C is instantiated and the DCG list is already bound, C must be lower and matches both a lower and uppercase letter. If the output list is unbound, its first element is bound to C. For example:

?- alpha_to_lower(0'a, `AB`, R).
R = [66].
?- alpha_to_lower(C, `AB`, R).
C = 97, R = [66].
?- alpha_to_lower(0'a, L, R).
L = [97|R].

digits(?Chars)// is det

digit(?Char)// is det

integer(?Integer)// is det

Number processing. The predicate digits//1 matches a possibly empty set of digits, digit//1 processes a single digit and integer processes an optional sign followed by a non-empty sequence of digits into an integer.

float(?Float)// is det

Process a floating point number. The actual conversion is controlled by number_codes/2.

number(+Number)// is det

number(-Number)// is semidet

Generate extract a number. Handles both integers and floating point numbers.

xinteger(+Integer)// is det

xinteger(-Integer)// is semidet

Generate or extract an integer from a sequence of hexadecimal digits. Hexadecimal characters include both uppercase (A-F) and lowercase (a-f) letters. The value may be preceded by a sign (+/-)

xdigit(-Weight)// is semidet

True if the next code is a hexdecimal digit with Weight. Weight is between 0 and 15. Hexadecimal characters include both uppercase (A-F) and lowercase (a-f) letters.

xdigits(-WeightList)// is det

List of weights of a sequence of hexadecimal codes. WeightList may be empty. Hexadecimal characters include both uppercase (A-F) and lowercase (a-f) letters.

eos//

Matches end-of-input. The implementation behaves as the following portable implementation:

eos --> call(eos_).
eos_([], []).

To be done: - This is a difficult concept and violates the context free property of DCGs. Explain the exact problems.

remainder(-List)//

Unify List with the remainder of the input.

prolog_var_name(-Name:atom)// is semidet

Matches a Prolog variable name. Primarily intended to deal with quasi quotations that embed Prolog variables.

atom(++Atom)// is det

Generate codes of Atom. Current implementation uses write/1, dealing with any Prolog term. Atom must be ground though.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

digit(Arg1, Arg2, Arg3)
integer(Arg1, Arg2, Arg3)