src/Pure/Syntax/pretty.ML

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(*  Title:      Pure/Syntax/pretty.ML
    ID:         $Id$
    Author:     Lawrence C Paulson
    Copyright   1991  University of Cambridge

Generic pretty printing module.

Loosely based on
  D. C. Oppen, "Pretty Printing",
  ACM Transactions on Programming Languages and Systems (1980), 465-483.

The object to be printed is given as a tree with indentation and line
breaking information.  A "break" inserts a newline if the text until
the next break is too long to fit on the current line.  After the newline,
text is indented to the level of the enclosing block.  Normally, if a block
is broken then all enclosing blocks will also be broken.  Only "inconsistent
breaks" are provided.

The stored length of a block is used in breakdist (to treat each inner block as
a unit for breaking).
*)

type pprint_args = (string -> unit) * (int -> unit) * (int -> unit) *
  (unit -> unit) * (unit -> unit);

signature PRETTY =
  sig
  type T
  val str: string -> T
  val strlen: string -> int -> T
  val brk: int -> T
  val fbrk: T
  val blk: int * T list -> T
  val lst: string * string -> T list -> T
  val quote: T -> T
  val commas: T list -> T list
  val breaks: T list -> T list
  val fbreaks: T list -> T list
  val block: T list -> T
  val strs: string list -> T
  val enclose: string -> string -> T list -> T
  val list: string -> string -> T list -> T
  val str_list: string -> string -> string list -> T
  val big_list: string -> T list -> T
  val string_of: T -> string
  val writeln: T -> unit
  val str_of: T -> string
  val pprint: T -> pprint_args -> unit
  val setdepth: int -> unit
  val setmargin: int -> unit
  end;

structure Pretty : PRETTY =
struct

(*printing items: compound phrases, strings, and breaks*)
datatype T =
  Block of T list * int * int |         (*body, indentation, length*)
  String of string * int |              (*text, length*)
  Break of bool * int;                  (*mandatory flag, width if not taken*);

(*Add the lengths of the expressions until the next Break; if no Break then
  include "after", to account for text following this block. *)
fun breakdist (Block(_,_,len)::es, after) = len + breakdist(es, after)
  | breakdist (String (s, len) :: es, after) = len + breakdist (es, after)
  | breakdist (Break _ :: es, after) = 0
  | breakdist ([], after) = after;

fun repstring a 0 = ""
  | repstring a 1 = a
  | repstring a k =
      if k mod 2 = 0 then repstring(a^a) (k div 2)
                     else repstring(a^a) (k div 2) ^ a;

(*** Type for lines of text: string, # of lines, position on line ***)

type text = {tx: string, nl: int, pos: int};

val emptytext = {tx="", nl=0, pos=0};

fun blanks wd {tx,nl,pos} =
    {tx  = tx ^ repstring " " wd,
     nl  = nl,
     pos = pos+wd};

fun newline {tx,nl,pos} =
    {tx  = tx ^ "\n",
     nl  = nl+1,
     pos = 0};

fun string s len {tx,nl,pos:int} =
    {tx  = tx ^ s,
     nl  = nl,
     pos = pos + len};


(*** Formatting ***)

val margin      = ref 80        (*right margin, or page width*)
and breakgain   = ref 4         (*minimum added space required of a break*)
and emergencypos = ref 40;      (*position too far to right*)

fun setmargin m =
    (margin     := m;
     breakgain  := !margin div 20;
     emergencypos := !margin div 2);

(*Search for the next break (at this or higher levels) and force it to occur*)
fun forcenext [] = []
  | forcenext (Break(_,wd) :: es) = Break(true,0) :: es
  | forcenext (e :: es) = e :: forcenext es;

(*es is list of expressions to print;
  blockin is the indentation of the current block;
  after is the width of the following context until next break. *)
fun format ([], _, _) text = text
  | format (e::es, blockin, after) (text as {pos,nl,...}) =
    (case e of
       Block(bes,indent,wd) =>
         let val blockin' = (pos + indent) mod !emergencypos
             val btext = format(bes, blockin', breakdist(es,after)) text
             (*If this block was broken then force the next break.*)
             val es2 = if nl < #nl(btext) then forcenext es else es
         in  format (es2,blockin,after) btext  end
     | String (s, len) => format (es,blockin,after) (string s len text)
     | Break(force,wd) => (*no break if text to next break fits on this line
                            or if breaking would add only breakgain to space *)
           format (es,blockin,after)
               (if not force andalso
                   pos+wd <= Int.max(!margin - breakdist(es,after),
                                     blockin + !breakgain)
                then blanks wd text  (*just insert wd blanks*)
                else blanks blockin (newline text)));


(*** Exported functions to create formatting expressions ***)

fun length (Block (_, _, len)) = len
  | length (String (_, len)) = len
  | length (Break(_, wd)) = wd;

fun str s = String (s, size s);
fun strlen s len = String (s, len);

fun brk wd = Break (false, wd);
val fbrk = Break (true, 0);

fun blk (indent, es) =
  let
    fun sum([], k) = k
      | sum(e :: es, k) = sum (es, length e + k);
  in Block (es, indent, sum (es, 0)) end;

(*Join the elements of es as a comma-separated list, bracketted by lp and rp*)
fun lst(lp,rp) es =
  let fun add(e,es) = str"," :: brk 1 :: e :: es;
      fun list(e :: (es as _::_)) = str lp :: e :: foldr add (es,[str rp])
        | list(e::[]) = [str lp, e, str rp]
        | list([]) = []
  in blk(size lp, list es) end;


(* utils *)

fun quote prt =
  blk (1, [str "\"", prt, str "\""]);

fun commas prts =
  flat (separate [str ",", brk 1] (map (fn x => [x]) prts));

fun breaks prts = separate (brk 1) prts;

fun fbreaks prts = separate fbrk prts;

fun block prts = blk (2, prts);

val strs = block o breaks o (map str);

fun enclose lpar rpar prts =
  block (str lpar :: (prts @ [str rpar]));

fun list lpar rpar prts =
  enclose lpar rpar (commas prts);

fun str_list lpar rpar strs =
  list lpar rpar (map str strs);

fun big_list name prts =
  block (fbreaks (str name :: prts));



(*** Pretty printing with depth limitation ***)

val depth       = ref 0;        (*maximum depth; 0 means no limit*)

fun setdepth dp = (depth := dp);

(*Recursively prune blocks, discarding all text exceeding depth dp*)
fun pruning dp (Block(bes,indent,wd)) =
      if dp>0 then blk(indent, map (pruning(dp-1)) bes)
              else str "..."
  | pruning dp e = e;

fun prune dp e = if dp>0 then pruning dp e else e;


fun string_of e = #tx (format ([prune (!depth) e], 0, 0) emptytext);

val writeln = writeln o string_of;


(*Create a single flat string: no line breaking*)
fun str_of prt =
  let
    fun s_of (Block (prts, _, _)) = implode (map s_of prts)
      | s_of (String (s, _)) = s
      | s_of (Break (false, wd)) = repstring " " wd
      | s_of (Break (true, _)) = " ";
  in
    s_of (prune (! depth) prt)
  end;

(*Part of the interface to the Poly/ML and New Jersey ML pretty printers*)
fun pprint prt (put_str, begin_blk, put_brk, put_fbrk, end_blk) =
  let
    fun pp (Block (prts, ind, _)) = (begin_blk ind; pp_lst prts; end_blk ())
      | pp (String (s, _)) = put_str s
      | pp (Break (false, wd)) = put_brk wd
      | pp (Break (true, _)) = put_fbrk ()
    and pp_lst [] = ()
      | pp_lst (prt :: prts) = (pp prt; pp_lst prts);
  in
    pp (prune (! depth) prt)
  end;


(*** Initialization ***)

val () = setmargin 80;

end;