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Below is the API for the OCaml standard library. It's directly copied over from the OCaml Manual, formatted to the Reason syntax and styled accordingly. The API docs are work-in-progress; we'll be polishing these gradually!

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Module Format

module Format: sig .. 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.

For a gentle introduction to the basics of pretty-printing using Format, read http://caml.inria.fr/resources/doc/guides/format.en.html.

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 0; print_string "x ="; print_space (); print_int 1; close_box (); print_newline () that prints x = 1 within a pretty-printing box, can be abbreviated as printf "@[%s@ %i@]@." "x =" 1, or even shorter printf "@[x =@ %i@]@." 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, flush the pretty-printer to display all the remaining material, e.g. evaluate print_newline ().
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.

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.


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
Closes 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
Prints an integer in the current box.
val print_float : float -> unit
Prints a floating point number in the current box.
val print_char : char -> unit
Prints a character in the current box.
val print_bool : bool -> unit
Prints 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
Inserts 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
Flushes 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
Forces a newline in the current box. Not the normal way of pretty-printing, you should prefer break hints.
val print_if_newline : unit -> unit
Executes 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. If d is too large, the right margin is set to the maximum admissible value (which is greater than 10^9).
val get_margin : unit -> int
Returns 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. If d is too large, the limit is set to the maximum admissible value (which is greater than 10^9).
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 smaller than 2.
val get_max_boxes : unit -> int
Returns the maximum number of boxes allowed before ellipsis.
val over_max_boxes : unit -> bool
Tests 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
Opens a tabulation box.
val close_tbox : unit -> unit
Closes 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
Sets 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.

Semantics Tags

type tag = string

Semantics tags (or simply tags) are used to decorate printed entities for user's defined purposes, e.g. setting font and giving size indications for a display device, or marking delimitation of semantics entities (e.g. HTML or TeX elements or terminal escape sequences).

By default, those tags do not influence line breaking calculation: the tag 'markers' are not considered as part of the printing material that drives line breaking (in other words, the length of those strings is considered as zero for line breaking).

Thus, tag handling is in some sense transparent to pretty-printing and does not interfere with usual indentation. Hence, a single pretty printing routine can output both simple 'verbatim' material or richer decorated output depending on the treatment of tags. By default, tags are not active, hence the output is not decorated with tag information. Once set_tags is set to true, the pretty printer engine honours tags and decorates the output accordingly.

When a tag has been opened (or closed), it is both and successively 'printed' and 'marked'. Printing a tag means calling a formatter specific function with the name of the tag as argument: that 'tag printing' function can then print any regular material to the formatter (so that this material is enqueued as usual in the formatter queue for further line-breaking computation). Marking a tag means to output an arbitrary string (the 'tag marker'), directly into the output device of the formatter. Hence, the formatter specific 'tag marking' function must return the tag marker string associated to its tag argument. Being flushed directly into the output device of the formatter, tag marker strings are not considered as part of the printing material that drives line breaking (in other words, the length of the strings corresponding to tag markers is considered as zero for line breaking). In addition, advanced users may take advantage of the specificity of tag markers to be precisely output when the pretty printer has already decided where to break the lines, and precisely when the queue is flushed into the output device.

In the spirit of HTML tags, the default tag marking functions output tags enclosed in "<" and ">": hence, the opening marker of tag t is "<t>" and the closing marker "</t>".

Default tag printing functions just do nothing.

Tag marking and tag printing functions are user definable and can be set by calling set_formatter_tag_functions.

val open_tag : tag -> unit
open_tag t opens the tag named t; the print_open_tag function of the formatter is called with t as argument; the tag marker mark_open_tag t will be flushed into the output device of the formatter.
val close_tag : unit -> unit
close_tag () closes the most recently opened tag t. In addition, the print_close_tag function of the formatter is called with t as argument. The marker mark_close_tag t will be flushed into the output device of the formatter.
val set_tags : bool -> unit
set_tags b turns on or off the treatment of tags (default is off).
val set_print_tags : bool -> unit
set_print_tags b turns on or off the printing of tags.
val set_mark_tags : bool -> unit
set_mark_tags b turns on or off the output of tag markers.
val get_print_tags : unit -> bool
Return the current status of tags printing.
val get_mark_tags : unit -> bool
Return the current status of tags marking.

Redirecting the standard formatter output

val set_formatter_out_channel : Pervasives.out_channel -> unit
Redirect the pretty-printer output to the given channel. (All the output functions of the standard formatter are set to the default output functions printing to the given channel.)
val set_formatter_output_functions : (string -> int -> int -> unit) -> (unit -> unit) -> unit
set_formatter_output_functions out flush redirects the pretty-printer output functions to the functions out and flush.

The out function performs all the pretty-printer string 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 (via conversion %!, or pretty-printing indications @? or @., or using low level functions 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 standard formatter pretty printing


Changing the meaning of standard formatter pretty printing


The Format module is versatile enough to let you completely redefine the meaning of pretty printing: you may provide your own functions to define how to handle indentation, line breaking, and even printing of all the characters that have to be printed!
type formatter_out_functions = {
   out_string : string -> int -> int -> unit;
   out_flush : unit -> unit;
   out_newline : unit -> unit;
   out_spaces : int -> unit;
} 
val set_formatter_out_functions : formatter_out_functions -> unit
set_formatter_out_functions f Redirect the pretty-printer output to the functions f.out_string and f.out_flush as described in set_formatter_output_functions. In addition, the pretty-printer function that outputs a newline is set to the function f.out_newline and the function that outputs indentation spaces is set to the function f.out_spaces.

This way, you can change the meaning of indentation (which can be something else than just printing space characters) and the meaning of new lines opening (which can be connected to any other action needed by the application at hand). The two functions f.out_spaces and f.out_newline are normally connected to f.out_string and f.out_flush: respective default values for f.out_space and f.out_newline are f.out_string (String.make n ' ') 0 n and f.out_string "\n" 0 1.

val get_formatter_out_functions : unit -> formatter_out_functions
Return the current output functions of the pretty-printer, including line breaking and indentation functions. Useful to record the current setting and restore it afterwards.

Changing the meaning of printing semantics tags

type formatter_tag_functions = {
   mark_open_tag : tag -> string;
   mark_close_tag : tag -> string;
   print_open_tag : tag -> unit;
   print_close_tag : tag -> unit;
}
The tag handling functions specific to a formatter: mark versions are the 'tag marking' functions that associate a string marker to a tag in order for the pretty-printing engine to flush those markers as 0 length tokens in the output device of the formatter. print versions are the 'tag printing' functions that can perform regular printing when a tag is closed or opened.
val set_formatter_tag_functions : formatter_tag_functions -> unit
set_formatter_tag_functions tag_funs changes the meaning of opening and closing tags to use the functions in tag_funs.

When opening a tag name t, the string t is passed to the opening tag marking function (the mark_open_tag field of the record tag_funs), that must return the opening tag marker for that name. When the next call to close_tag () happens, the tag name t is sent back to the closing tag marking function (the mark_close_tag field of record tag_funs), that must return a closing tag marker for that name.

The print_ field of the record contains the functions that are called at tag opening and tag closing time, to output regular material in the pretty-printer queue.

val get_formatter_tag_functions : unit -> formatter_tag_functions
Return the current tag functions of the pretty-printer.

Multiple formatted output

type formatter
Abstract data corresponding to a pretty-printer (also called a formatter) and all its machinery.

Defining new pretty-printers permits unrelated output of material in parallel on several output channels. All the 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 a Pervasives.out_channel output channel oc, a new formatter writing to that channel is simply 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 resets 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. For instance, a formatter to the Pervasives.out_channel oc is returned by make_formatter (Pervasives.output oc) (fun () -> Pervasives.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_open_tag : formatter -> string -> unit
val pp_close_tag : 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_float : formatter -> float -> 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_tags : formatter -> bool -> unit
val pp_set_print_tags : formatter -> bool -> unit
val pp_set_mark_tags : formatter -> bool -> unit
val pp_get_print_tags : formatter -> unit -> bool
val pp_get_mark_tags : formatter -> unit -> bool
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_formatter_tag_functions : formatter -> formatter_tag_functions -> unit
val pp_get_formatter_tag_functions : formatter -> unit -> formatter_tag_functions
val pp_set_formatter_out_functions : formatter -> formatter_out_functions -> unit
val pp_get_formatter_out_functions : formatter -> unit -> formatter_out_functions
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.

Convenience formatting functions.

val pp_print_list : ?pp_sep:(formatter -> unit -> unit) ->       (formatter -> 'a -> unit) -> formatter -> 'a list -> unit
pp_print_list ?pp_sep pp_v ppf l prints the list l. pp_v is used on the elements of l and each element is separated by a call to pp_sep (defaults to Format.pp_print_cut). Does nothing on empty lists.
Since 4.02.0
val pp_print_text : formatter -> string -> unit
pp_print_text ppf s prints s with spaces and newlines respectively printed with Format.pp_print_space and Format.pp_force_newline.
Since 4.02.0

printf like functions for pretty-printing.

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

fprintf ff fmt arg1 ... argN formats the arguments arg1 to argN according to the format string fmt, and outputs the resulting string on the formatter ff.

The format fmt 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 specific to the Format module.

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 hint, as with print_cut ().
  • @ : output a good break space, as with print_space ().
  • @;: output a fully specified 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. If no parameters are provided, the good break defaults to a good break space.
  • @.: 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>%s" arg prints arg as a zero length string. 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.
  • @{: open a tag. The name of the tag may be optionally specified with the following syntax: the < character, followed by an optional string specification, and the closing > character. The string specification is any character string that does not contain the closing character '>'. If omitted, the tag name defaults to the empty string. For more details about tags, see the functions open_tag and close_tag.
  • @}: close the most recently opened tag.
  • @?: flush the pretty printer as with print_flush (). This is equivalent to the conversion %!.
  • @\n: force a newline, as with force_newline ().
  • @@: print a single @ character.
Example: printf "@[%s@ %d@]@." "x =" 1 is equivalent to open_box (); print_string "x ="; print_space (); print_int 1; close_box (); print_newline (). It prints x = 1 within a pretty-printing box.

Note: If you need to prevent the interpretation of a @ character as a pretty-printing indication, you can also escape it with a % character.

val printf : ('a, formatter, unit) Pervasives.format -> 'a
Same as fprintf above, but output on std_formatter.
val eprintf : ('a, formatter, unit) Pervasives.format -> 'a
Same as fprintf above, but output on err_formatter.
val sprintf : ('a, unit, string) Pervasives.format -> 'a
Same as printf above, but instead of printing on a formatter, returns 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 single string, you should consider using fprintf with the predefined formatter str_formatter and call flush_str_formatter () to get the final result.

Alternatively, you can use Format.fprintf with a formatter writing to a buffer of your own: flushing the formatter and the buffer at the end of pretty-printing returns the desired string.

val asprintf : ('a, formatter, unit, string) Pervasives.format4 -> 'a
Same as printf above, but instead of printing on a formatter, returns a string containing the result of formatting the arguments. The type of asprintf is general enough to interact nicely with %a conversions.
Since 4.01.0
val ifprintf : formatter -> ('a, formatter, unit) Pervasives.format -> 'a
Same as fprintf above, but does not print anything. Useful to ignore some material when conditionally printing.
Since 3.10.0

Formatted output functions with continuations.
val kfprintf : (formatter -> 'a) ->       formatter -> ('b, formatter, unit, 'a) Pervasives.format4 -> 'b
Same as fprintf above, but instead of returning immediately, passes the formatter to its first argument at the end of printing.
val ikfprintf : (formatter -> 'a) ->       formatter -> ('b, formatter, unit, 'a) Pervasives.format4 -> 'b
Same as kfprintf above, but does not print anything. Useful to ignore some material when conditionally printing.
Since 3.12.0
val ksprintf : (string -> 'a) -> ('b, unit, string, 'a) Pervasives.format4 -> 'b
Same as sprintf above, but instead of returning the string, passes it to the first argument.

Deprecated

val bprintf : Buffer.t -> ('a, formatter, unit) Pervasives.format -> 'a
Deprecated.This function is error prone. Do not use it.

If you need to print to some buffer b, you must first define a formatter writing to b, using let to_b = formatter_of_buffer b; then use regular calls to Format.fprintf on formatter to_b.

val kprintf : (string -> 'a) -> ('b, unit, string, 'a) Pervasives.format4 -> 'b
Deprecated.An alias for ksprintf.
val set_all_formatter_output_functions : out:(string -> int -> int -> unit) ->       flush:(unit -> unit) ->       newline:(unit -> unit) -> spaces:(int -> unit) -> unit
Deprecated.Subsumed by set_formatter_out_functions.
val get_all_formatter_output_functions : unit ->       (string -> int -> int -> unit) * (unit -> unit) * (unit -> unit) *       (int -> unit)
Deprecated.Subsumed by get_formatter_out_functions.
val pp_set_all_formatter_output_functions : formatter ->       out:(string -> int -> int -> unit) ->       flush:(unit -> unit) ->       newline:(unit -> unit) -> spaces:(int -> unit) -> unit
Deprecated.Subsumed by pp_set_formatter_out_functions.
val pp_get_all_formatter_output_functions : formatter ->       unit ->       (string -> int -> int -> unit) * (unit -> unit) * (unit -> unit) *       (int -> unit)
Deprecated.Subsumed by pp_get_formatter_out_functions.