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basics.pl -- Various general DCG utilities
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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.
Source 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)
EndCodes- is a list of character codes.
See also
- string//1.
Source 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.
Source blanks// is det
Skip zero or more white-space characters.
Source blank// is semidet
Take next space character from input. Space characters include newline.
See also
- white//0
Source nonblanks(-Codes)// is det
Take all graph characters
Source nonblank(-Code)// is semidet
Code is the next non-blank (graph) character.
Source blanks_to_nl// is semidet
Take a sequence of blank//0 codes if blanks are followed by a newline or end of the input.
Source whites// is det
Skip white space inside a line.
See also
- blanks//0 also skips newlines.
Source white// is semidet
Take next white character from input. White characters do not include newline.
Source 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].
Source digits(?Chars)// is det
Source digit(?Char)// is det
Source integer(?Integer)// is det
Number processing. The predicate digits//1 matches a posibly 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.
Source float(?Float)// is det
Process a floating point number. The actual conversion is controlled by number_codes/2.
Source number(+Number)// is det
number(-Number)// is semidet
Generate extract a number. Handles both integers and floating point numbers.
Source 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 preceeded by a sign (+/-)
Source 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.
Source 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.
Source 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.
Source remainder(-List)//
Unify List with the remainder of the input.
Source prolog_var_name(-Name:atom)// is semidet
Matches a Prolog variable name. Primarily intended to deal with quasi quotations that embed Prolog variables.
Source 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.

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