Module Format

structure Format = struct ... end

Pretty printing.
This module implements a pretty-printing facility to format text within ``pretty-printing boxes''. The pretty-printer breaks lines at specified break hints, and indents lines according to the box structure.
Warning: the material output by the following functions is delayed in the pretty-printer queue in order to compute the proper line breaking. Hence, you should not mix calls to the printing functions of the basic I/O system with calls to the functions of this module: this could result in some strange output seemingly unrelated with the evaluation order of printing commands.
You may consider this module as providing an extension to the printf facility to provide automatic line breaking. The addition of pretty-printing annotations to your regular printf formats gives you fancy indentation and line breaks. Pretty-printing annotations are described below in the documentation of the function Format.fprintf.
You may also use the explicit box management and printing functions provided by this module. This style is more basic but more verbose than the fprintf concise formats.
For instance, the sequence open_box (); print_string "x ="; print_space (); print_int 1; close_box () that prints x = 1 within a pretty-printing box, can be abbreviated as printf "@[@ 0@]" "x =" 1, or even shorter printf "@[x =@ 0@]" 1.
Rule of thumb for casual users of this library:

use simple boxes (as obtained by open_box 0);
use simple break hints (as obtained by print_cut () that outputs a simple break hint, or by print_space () that outputs a space indicating a break hint);
once a box is opened, display its material with basic printing functions (e. g. print_int and print_string);
when the material for a box has been printed, call close_box () to close the box;
at the end of your routine, evaluate print_newline () to close all remaining boxes and flush the pretty-printer.

The behaviour of pretty-printing commands is unspecified if there is no opened pretty-printing box. Each box opened via one of the open_ functions below must be closed using close_box for proper formatting. Otherwise, some of the material printed in the boxes may not be output, or may be formatted incorrectly.
In case of interactive use, the system closes all opened boxes and flushes all pending text (as with the print_newline function) after each phrase. Each phrase is therefore executed in the initial state of the pretty-printer.

Boxes

val open_box : int -> unit

open_box d opens a new pretty-printing box with offset d. This box is the general purpose pretty-printing box. Material in this box is displayed ``horizontal or vertical'': break hints inside the box may lead to a new line, if there is no more room on the line to print the remainder of the box, or if a new line may lead to a new indentation (demonstrating the indentation of the box). When a new line is printed in the box, d is added to the current indentation.

val close_box : unit -> unit

Close the most recently opened pretty-printing box.

Formatting functions

val print_string : string -> unit

print_string str prints str in the current box.

val print_as : int -> string -> unit

print_as len str prints str in the current box. The pretty-printer formats str as if it were of length len.

val print_int : int -> unit

Print an integer in the current box.

val print_real (print_float) : real -> unit

Print a floating point number in the current box.

val print_char : char -> unit

Print a character in the current box.

val print_bool : bool -> unit

Print a boolean in the current box.

Break hints

val print_space : unit -> unit

print_space () is used to separate items (typically to print a space between two words). It indicates that the line may be split at this point. It either prints one space or splits the line. It is equivalent to print_break 1 0.

val print_cut : unit -> unit

print_cut () is used to mark a good break position. It indicates that the line may be split at this point. It either prints nothing or splits the line. This allows line splitting at the current point, without printing spaces or adding indentation. It is equivalent to print_break 0 0.

val print_break : int -> int -> unit

Insert a break hint in a pretty-printing box. print_break nspaces offset indicates that the line may be split (a newline character is printed) at this point, if the contents of the current box does not fit on the current line. If the line is split at that point, offset is added to the current indentation. If the line is not split, nspaces spaces are printed.

val print_flush : unit -> unit

Flush the pretty printer: all opened boxes are closed, and all pending text is displayed.

val print_newline : unit -> unit

Equivalent to print_flush followed by a new line.

val force_newline : unit -> unit

Force a newline in the current box. Not the normal way of pretty-printing, you should prefer break hints.

val print_if_newline : unit -> unit

Execute the next formatting command if the preceding line has just been split. Otherwise, ignore the next formatting command.

Margin

val set_margin : int -> unit

set_margin d sets the value of the right margin to d (in characters): this value is used to detect line overflows that leads to split lines. Nothing happens if d is smaller than 2 or bigger than 999999999.

val get_margin : unit -> int

Return the position of the right margin.

Maximum indentation limit

val set_max_indent : int -> unit

set_max_indent d sets the value of the maximum indentation limit to d (in characters): once this limit is reached, boxes are rejected to the left, if they do not fit on the current line. Nothing happens if d is smaller than 2 or bigger than 999999999.

val get_max_indent : unit -> int

Return the value of the maximum indentation limit (in characters).

Formatting depth: maximum number of boxes allowed before ellipsis

val set_max_boxes : int -> unit

set_max_boxes max sets the maximum number of boxes simultaneously opened. Material inside boxes nested deeper is printed as an ellipsis (more precisely as the text returned by get_ellipsis_text ()). Nothing happens if max is not greater than 1.

val get_max_boxes : unit -> int

Return the maximum number of boxes allowed before ellipsis.

val over_max_boxes : unit -> bool

Test if the maximum number of boxes allowed have already been opened.

Advanced formatting

val open_hbox : unit -> unit

open_hbox () opens a new pretty-printing box. This box is ``horizontal'': the line is not split in this box (new lines may still occur inside boxes nested deeper).

val open_vbox : int -> unit

open_vbox d opens a new pretty-printing box with offset d. This box is ``vertical'': every break hint inside this box leads to a new line. When a new line is printed in the box, d is added to the current indentation.

val open_hvbox : int -> unit

open_hvbox d opens a new pretty-printing box with offset d. This box is ``horizontal-vertical'': it behaves as an ``horizontal'' box if it fits on a single line, otherwise it behaves as a ``vertical'' box. When a new line is printed in the box, d is added to the current indentation.

val open_hovbox : int -> unit

open_hovbox d opens a new pretty-printing box with offset d. This box is ``horizontal or vertical'': break hints inside this box may lead to a new line, if there is no more room on the line to print the remainder of the box. When a new line is printed in the box, d is added to the current indentation.

Tabulations

val open_tbox : unit -> unit

Open a tabulation box.

val close_tbox : unit -> unit

Close the most recently opened tabulation box.

val print_tbreak : int -> int -> unit

Break hint in a tabulation box. print_tbreak spaces offset moves the insertion point to the next tabulation (spaces being added to this position). Nothing occurs if insertion point is already on a tabulation mark. If there is no next tabulation on the line, then a newline is printed and the insertion point moves to the first tabulation of the box. If a new line is printed, offset is added to the current indentation.

val set_tab : unit -> unit

Set a tabulation mark at the current insertion point.

val print_tab : unit -> unit

print_tab () is equivalent to print_tbreak (0,0).

Ellipsis

val set_ellipsis_text : string -> unit

Set the text of the ellipsis printed when too many boxes are opened (a single dot, ., by default).

val get_ellipsis_text : unit -> string

Return the text of the ellipsis.

Redirecting formatter output

val set_formatter_out_channel : Pervasives.out_channel -> unit

Redirect the pretty-printer output to the given channel.

Changing the meaning of printing material

val set_formatter_output_functions : (string -> int -> int -> unit) -> (unit -> unit) -> unit

set_formatter_output_functions out flush redirects the pretty-printer output to the functions out and flush. The out function performs the pretty-printer output. It is called with a string s, a start position p, and a number of characters n; it is supposed to output characters p to p + n - 1 of s. The flush function is called whenever the pretty-printer is flushed using print_flush or print_newline.

val get_formatter_output_functions : unit -> (string -> int -> int -> unit) * (unit -> unit)

Return the current output functions of the pretty-printer.

Changing the meaning of pretty printing (indentation, line breaking, and printing material)

val set_all_formatter_output_functions : out:(string -> int -> int -> unit) ->
       flush:(unit -> unit) ->
       newline:(unit -> unit) -> spaces:(int -> unit) -> unit

set_all_formatter_output_functions out flush outnewline outspace redirects the pretty-printer output to the functions out and flush as described in set_formatter_output_functions. In addition, the pretty-printer function that outputs a newline is set to the function outnewline and the function that outputs indentation spaces is set to the function outspace. This way, you can change the meaning of indentation (which can be something else than just printing a space character) and the meaning of new lines opening (which can be connected to any other action needed by the application at hand). The two functions outspace and outnewline are normally connected to out and flush: respective default values for outspace and outnewline are out (String.make n ' ') 0 n and out "\n" 0 1.

val get_all_formatter_output_functions : unit ->
       (string -> int -> int -> unit) * (unit -> unit) * (unit -> unit) *
       (int -> unit)

Return the current output functions of the pretty-printer, including line breaking and indentation functions.

Multiple formatted output

type formatter

Abstract data type corresponding to a pretty-printer and all its machinery. Defining new pretty-printers permits the output of material in parallel on several channels. Parameters of a pretty-printer are local to this pretty-printer: margin, maximum indentation limit, maximum number of boxes simultaneously opened, ellipsis, and so on, are specific to each pretty-printer and may be fixed independently. Given an output channel oc, a new formatter writing to that channel is obtained by calling formatter_of_out_channel oc. Alternatively, the make_formatter function allocates a new formatter with explicit output and flushing functions (convenient to output material to strings for instance).

val formatter_of_out_channel : Pervasives.out_channel -> formatter

formatter_of_out_channel oc returns a new formatter that writes to the corresponding channel oc.

val std_formatter : formatter

The standard formatter used by the formatting functions above. It is defined as formatter_of_out_channel stdout.

val err_formatter : formatter

A formatter to use with formatting functions below for output to standard error. It is defined as formatter_of_out_channel stderr.

val formatter_of_buffer : Buffer.t -> formatter

formatter_of_buffer b returns a new formatter writing to buffer b. As usual, the formatter has to be flushed at the end of pretty printing, using pp_print_flush or pp_print_newline, to display all the pending material.

val stdbuf : Buffer.t

The string buffer in which str_formatter writes.

val str_formatter : formatter

A formatter to use with formatting functions below for output to the stdbuf string buffer. str_formatter is defined as formatter_of_buffer stdbuf.

val flush_str_formatter : unit -> string

Returns the material printed with str_formatter, flushes the formatter and reset the corresponding buffer.

val make_formatter : (string -> int -> int -> unit) -> (unit -> unit) -> formatter

make_formatter out flush returns a new formatter that writes according to the output function out, and the flushing function flush. Hence, a formatter to the out channel oc is returned by make_formatter (output oc) (fun () -> flush oc).

Basic functions to use with formatters

val pp_open_hbox : formatter -> unit -> unit

val pp_open_vbox : formatter -> int -> unit

val pp_open_hvbox : formatter -> int -> unit

val pp_open_hovbox : formatter -> int -> unit

val pp_open_box : formatter -> int -> unit

val pp_close_box : formatter -> unit -> unit

val pp_print_string : formatter -> string -> unit

val pp_print_as : formatter -> int -> string -> unit

val pp_print_int : formatter -> int -> unit

val pp_print_real (pp_print_float) : formatter -> real -> unit

val pp_print_char : formatter -> char -> unit

val pp_print_bool : formatter -> bool -> unit

val pp_print_break : formatter -> int -> int -> unit

val pp_print_cut : formatter -> unit -> unit

val pp_print_space : formatter -> unit -> unit

val pp_force_newline : formatter -> unit -> unit

val pp_print_flush : formatter -> unit -> unit

val pp_print_newline : formatter -> unit -> unit

val pp_print_if_newline : formatter -> unit -> unit

val pp_open_tbox : formatter -> unit -> unit

val pp_close_tbox : formatter -> unit -> unit

val pp_print_tbreak : formatter -> int -> int -> unit

val pp_set_tab : formatter -> unit -> unit

val pp_print_tab : formatter -> unit -> unit

val pp_set_margin : formatter -> int -> unit

val pp_get_margin : formatter -> unit -> int

val pp_set_max_indent : formatter -> int -> unit

val pp_get_max_indent : formatter -> unit -> int

val pp_set_max_boxes : formatter -> int -> unit

val pp_get_max_boxes : formatter -> unit -> int

val pp_over_max_boxes : formatter -> unit -> bool

val pp_set_ellipsis_text : formatter -> string -> unit

val pp_get_ellipsis_text : formatter -> unit -> string

val pp_set_formatter_out_channel : formatter -> Pervasives.out_channel -> unit

val pp_set_formatter_output_functions : formatter -> (string -> int -> int -> unit) -> (unit -> unit) -> unit

val pp_get_formatter_output_functions : formatter -> unit -> (string -> int -> int -> unit) * (unit -> unit)

val pp_set_all_formatter_output_functions : formatter ->
       out:(string -> int -> int -> unit) ->
       flush:(unit -> unit) ->
       newline:(unit -> unit) -> spaces:(int -> unit) -> unit

val pp_get_all_formatter_output_functions : formatter ->
       unit ->
       (string -> int -> int -> unit) * (unit -> unit) * (unit -> unit) *
       (int -> unit)

These functions are the basic ones: usual functions operating on the standard formatter are defined via partial evaluation of these primitives. For instance, print_string is equal to pp_print_string std_formatter.

printf like functions for pretty-printing.

val fprintf : formatter -> ('a, formatter, unit) format -> 'a

fprintf ff format arg1 ... argN formats the arguments arg1 to argN according to the format string format, and outputs the resulting string on the formatter ff. The format is a character string which contains three types of objects: plain characters and conversion specifications as specified in the printf module, and pretty-printing indications. The pretty-printing indication characters are introduced by a @ character, and their meanings are:

@[: open a pretty-printing box. The type and offset of the box may be optionally specified with the following syntax: the < character, followed by an optional box type indication, then an optional integer offset, and the closing > character. Box type is one of h, v, hv, b, or hov, which stand respectively for an horizontal box, a vertical box, an ``horizontal-vertical'' box, or an ``horizontal or vertical'' box (b standing for an ``horizontal or vertical'' box demonstrating indentation and hov standing for a regular``horizontal or vertical'' box). For instance, @[<hov 2> opens an ``horizontal or vertical'' box with indentation 2 as obtained with open_hovbox 2. For more details about boxes, see the various box opening functions open_*box.
@]: close the most recently opened pretty-printing box.
@,: output a good break as with print_cut ().
@ : output a space, as with print_space ().
@\n: force a newline, as with force_newline ().
@;: output a good break as with print_break. The nspaces and offset parameters of the break may be optionally specified with the following syntax: the < character, followed by an integer nspaces value, then an integer offset, and a closing > character.
@?: flush the pretty printer as with print_flush ().
@.: flush the pretty printer and output a new line, as with print_newline ().
@<n>: print the following item as if it were of length n. Hence, printf "@<0>" arg is equivalent to print_as 0 arg. If @<n> is not followed by a conversion specification, then the following character of the format is printed as if it were of length n.
@@: print a plain @ character.

Example: printf "@[@ 0@]" "x =" 1 is equivalent to open_box (); print_string "x ="; print_space (); print_int 1; close_box (). It prints x = 1 within a pretty-printing box.

val printf : ('a, formatter, unit) format -> 'a

Same as fprintf above, but output on std_formatter.

val eprintf : ('a, formatter, unit) format -> 'a

Same as fprintf above, but output on err_formatter.

val sprintf : ('a, unit, string) format -> 'a

Same as printf above, but instead of printing on a formatter, return a string containing the result of formatting the arguments. Note that the pretty-printer queue is flushed at the end of each call to sprintf. In case of multiple and related calls to sprintf to output material on a string, you should consider using fprintf with a formatter writing to a buffer: flushing the buffer at the end of pretty-printing returns the desired string. You can use the predefined formatter str_formatter and call

flush_str_formatter
   ()

to get the result.

val bprintf : Buffer.t -> ('a, formatter, unit) format -> 'a

Same as sprintf above, but instead of printing on a string, writes into the given extensible buffer. As for sprintf, the pretty-printer queue is flushed at the end of each call to bprintf. In case of multiple and related calls to bprintf to output material on the same buffer b, you should consider using fprintf with a formatter writing to the buffer b (as obtained by formatter_of_buffer b), otherwise the repeated flushes of the pretty-printer queue would result in badly formatted output.