1:- module(md_line, [
    2    merge_lines/2,               % +Lines, -Codes
    3    indent//0,
    4    non_empty_line//1,           % -Codes
    5    discard_to_line_end//0,
    6    empty_lines//0,
    7    empty_line//0,
    8    inline_string//1,            % -Codes
    9    ln_or_eos//0,
   10    ln//0,
   11    string_limit//2,             % -Codes, +Limit
   12    lookahead//1,                % ?Code
   13    lookahead_ln//0,
   14    lookahead_ln_or_eos//0
   15]).

   22:- use_module(library(dcg/basics)).

 merge_lines(+Lines, -Codes) is det

Merges list of lines into a flat code list.

   29merge_lines([], []).
   30
   31merge_lines([Line], Line):- !.
   32
   33merge_lines([Line|Lines], Codes):-
   34    merge_lines(Lines, Merged),
   35    append(Line, [0'\n|Merged], Codes).

 indent// is semidet

Recognizes normal indent which is a tab or 4 spaces.

   42indent --> "\t".
   43indent --> "    ".

 non_empty_line(-Codes)// is semidet

Single non-empty line ending with newline or end-of-stream.

   50non_empty_line([Code|Codes]) -->
   51    [Code], { Code \= 0'\n },
   52    non_empty_line_rest(Codes).
   53
   54non_empty_line_rest([Code|Codes]) -->
   55    [Code], { Code \= 0'\n }, !,
   56    non_empty_line_rest(Codes).
   57
   58non_empty_line_rest([]) -->
   59    "\n", !.
   60
   61non_empty_line_rest([]) -->
   62    "".

 discard_to_line_end// is det

Discards zero or more symbol codes untill the first line end or eos is reached.

   70discard_to_line_end -->
   71    ln_or_eos, !.
   72
   73discard_to_line_end -->
   74    [_], discard_to_line_end.

 empty_lines// is det

List of consequtive empty lines. Consumes as many empty lines as possible.

   82empty_lines -->
   83    eos, !.
   84
   85empty_lines -->
   86    empty_line, !,
   87    empty_lines.
   88
   89empty_lines --> "".

 empty_line// is semidet

Recognizes a single empty line.

   95empty_line -->
   96    whites, ln_or_eos.

 lookahead(?Code)// is semidet

Looks ahead a single symbol code.

  102lookahead(Code), [Code] -->
  103    [Code].

 string_limit(-Codes, +Limit)// is multi

Same as string//1 but with a length limit.

  110string_limit([], Limit) -->
  111    { Limit =< 0 }, !.
  112
  113string_limit([], Limit) -->
  114    { Limit >= 0 }.
  115
  116string_limit([Code|Codes], Limit) -->
  117    [Code],
  118    { Next is Limit - 1 },
  119    string_limit(Codes, Next).

 inline_string(-Codes)// is multi

Takes as few symbol codes as possible up to line end.

  126inline_string([]) --> "".
  127
  128inline_string([]) -->
  129    lookahead_ln, !.
  130
  131inline_string([Code|Codes]) -->
  132    [Code],
  133    inline_string(Codes).

 lookahead_ln_or_eos// is semidet

Looks ahead a line end or end-of-stream. Puts back \n when a line end is recognized.

  141lookahead_ln_or_eos -->
  142    lookahead_ln, !.
  143
  144lookahead_ln_or_eos -->
  145    eos.

 lookahead_ln// is semidet

Looks ahead a line end. Puts back \n when it is recognized.

  152lookahead_ln, "\n" --> ln.

 ln_or_eos// is semidet

Recognizes either a line end or eos.

  159ln_or_eos -->
  160    "\n", !.
  161
  162ln_or_eos -->
  163    eos.

 ln// is semidet

Recognizes line ending.

  169ln --> "\n"