package containers

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Helpers for Format

  • since 0.8
type 'a sequence = ('a -> unit) -> unit
include module type of struct include Format end

Introduction

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 splitting. The addition of pretty-printing annotations to your regular printf format strings 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 pretty-printing box management and printing functions provided by this module. This style is more basic but more verbose than the concise fprintf format strings.

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 pretty-printing 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 pretty-printing box is open, display its material with basic printing functions (e. g. print_int and print_string);
  • when the material for a pretty-printing box has been printed, call close_box () to close the box;
  • at the end of pretty-printing, flush the pretty-printer to display all the remaining material, e.g. evaluate print_newline ().

The behavior of pretty-printing commands is unspecified if there is no open pretty-printing box. Each box opened by 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, each phrase is executed in the initial state of the standard pretty-printer: after each phrase execution, the interactive system closes all open pretty-printing boxes, flushes all pending text, and resets the standard pretty-printer.

Warning: mixing calls to pretty-printing functions of this module with calls to Pervasives low level output functions is error prone.

The pretty-printing functions output material that is delayed in the pretty-printer queue and stacks in order to compute proper line splitting. In contrast, basic I/O output functions write directly in their output device. As a consequence, the output of a basic I/O function may appear before the output of a pretty-printing function that has been called before. For instance, Pervasives.print_string "<"; Format.print_string "PRETTY"; Pervasives.print_string ">"; Format.print_string "TEXT"; leads to output <>PRETTYTEXT.

type formatter = Format.formatter

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

Pretty-printing boxes

The pretty-printing engine uses the concepts of pretty-printing box and break hint to drive indentation and line splitting behavior of the pretty-printer.

Each different pretty-printing box kind introduces a specific line splitting policy:

  • within an horizontal box, break hints never split the line (but the line may be split in a box nested deeper),
  • within a vertical box, break hints always split the line,
  • within an horizontal/vertical box, if the box fits on the current line then break hints never split the line, otherwise break hint always split the line,
  • within a compacting box, a break hint never splits the line, unless there is no more room on the current line.

Note that line splitting policy is box specific: the policy of a box does not rule the policy of inner boxes. For instance, if a vertical box is nested in an horizontal box, all break hints within the vertical box will split the line.

val pp_open_box : formatter -> int -> unit
val open_box : int -> unit

pp_open_box ppf d opens a new compacting pretty-printing box with offset d in the formatter ppf.

Within this box, the pretty-printer prints as much as possible material on every line.

A break hint splits the line if there is no more room on the line to print the remainder of the box.

Within this box, the pretty-printer emphasizes the box structure: a break hint also splits the line if the splitting ``moves to the left'' (i.e. the new line gets an indentation smaller than the one of the current line).

This box is the general purpose pretty-printing box.

If the pretty-printer splits the line in the box, offset d is added to the current indentation.

val pp_close_box : formatter -> unit -> unit
val close_box : unit -> unit

Closes the most recently open pretty-printing box.

val pp_open_hbox : formatter -> unit -> unit
val open_hbox : unit -> unit

pp_open_hbox ppf () opens a new 'horizontal' pretty-printing box.

This box prints material on a single line.

Break hints in a horizontal box never split the line. (Line splitting may still occur inside boxes nested deeper).

val pp_open_vbox : formatter -> int -> unit
val open_vbox : int -> unit

pp_open_vbox ppf d opens a new 'vertical' pretty-printing box with offset d.

This box prints material on as many lines as break hints in the box.

Every break hint in a vertical box splits the line.

If the pretty-printer splits the line in the box, d is added to the current indentation.

val pp_open_hvbox : formatter -> int -> unit
val open_hvbox : int -> unit

pp_open_hvbox ppf d opens a new 'horizontal/vertical' pretty-printing box with offset d.

This box behaves as an horizontal box if it fits on a single line, otherwise it behaves as a vertical box.

If the pretty-printer splits the line in the box, d is added to the current indentation.

val pp_open_hovbox : formatter -> int -> unit
val open_hovbox : int -> unit

pp_open_hovbox ppf d opens a new 'horizontal-or-vertical' pretty-printing box with offset d.

This box prints material as much as possible on every line.

A break hint splits the line if there is no more room on the line to print the remainder of the box.

If the pretty-printer splits the line in the box, d is added to the current indentation.

Formatting functions

val pp_print_string : formatter -> string -> unit
val print_string : string -> unit

pp_print_string ppf s prints s in the current pretty-printing box.

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

pp_print_as ppf len s prints s in the current pretty-printing box. The pretty-printer formats s as if it were of length len.

val pp_print_int : formatter -> int -> unit
val print_int : int -> unit

Print an integer in the current pretty-printing box.

val pp_print_float : formatter -> float -> unit
val print_float : float -> unit

Print a floating point number in the current pretty-printing box.

val pp_print_char : formatter -> char -> unit
val print_char : char -> unit

Print a character in the current pretty-printing box.

val pp_print_bool : formatter -> bool -> unit
val print_bool : bool -> unit

Print a boolean in the current pretty-printing box.

Break hints

A 'break hint' tells the pretty-printer to output some space or split the line whichever way is more appropriate to the current pretty-printing box splitting rules.

Break hints are used to separate printing items and are mandatory to let the pretty-printer correctly split lines and indent items.

Simple break hints are:

  • the 'space': output a space or split the line if appropriate,
  • the 'cut': split the line if appropriate.

Note: the notions of space and line splitting are abstract for the pretty-printing engine, since those notions can be completely redefined by the programmer. However, in the pretty-printer default setting, ``output a space'' simply means printing a space character (ASCII code 32) and ``split the line'' means printing a newline character (ASCII code 10).

val pp_print_space : formatter -> unit -> unit
val print_space : unit -> unit

pp_print_space ppf () emits a 'space' break hint: the pretty-printer may split the line at this point, otherwise it prints one space.

pp_print_space ppf () is equivalent to pp_print_break ppf 1 0.

val pp_print_cut : formatter -> unit -> unit
val print_cut : unit -> unit

pp_print_cut ppf () emits a 'cut' break hint: the pretty-printer may split the line at this point, otherwise it prints nothing.

pp_print_cut ppf () is equivalent to pp_print_break ppf 0 0.

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

pp_print_break ppf nspaces offset emits a 'full' break hint: the pretty-printer may split the line at this point, otherwise it prints nspaces spaces.

If the pretty-printer splits the line, offset is added to the current indentation.

val pp_force_newline : formatter -> unit -> unit
val force_newline : unit -> unit

Force a new line in the current pretty-printing box.

The pretty-printer must split the line at this point,

Not the normal way of pretty-printing, since imperative line splitting may interfere with current line counters and box size calculation. Using break hints within an enclosing vertical box is a better alternative.

val pp_print_if_newline : formatter -> unit -> unit
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.

Pretty-printing termination

val pp_print_flush : formatter -> unit -> unit
val print_flush : unit -> unit

End of pretty-printing: resets the pretty-printer to initial state.

All open pretty-printing boxes are closed, all pending text is printed. In addition, the pretty-printer low level output device is flushed to ensure that all pending text is really displayed.

Note: never use print_flush in the normal course of a pretty-printing routine, since the pretty-printer uses a complex buffering machinery to properly indent the output; manually flushing those buffers at random would conflict with the pretty-printer strategy and result to poor rendering.

Only consider using print_flush when displaying all pending material is mandatory (for instance in case of interactive use when you want the user to read some text) and when resetting the pretty-printer state will not disturb further pretty-printing.

Warning: If the output device of the pretty-printer is an output channel, repeated calls to print_flush means repeated calls to Pervasives.flush to flush the out channel; these explicit flush calls could foil the buffering strategy of output channels and could dramatically impact efficiency.

val pp_print_newline : formatter -> unit -> unit
val print_newline : unit -> unit

End of pretty-printing: resets the pretty-printer to initial state.

All open pretty-printing boxes are closed, all pending text is printed.

Equivalent to print_flush followed by a new line. See corresponding words of caution for print_flush.

Note: this is not the normal way to output a new line; the preferred method is using break hints within a vertical pretty-printing box.

Margin

val pp_set_margin : formatter -> int -> unit
val set_margin : int -> unit

pp_set_margin ppf d sets the right margin to d (in characters): the pretty-printer splits lines that overflow the right margin according to the break hints given. 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). If d is less than the current maximum indentation limit, the maximum indentation limit is decreased while trying to preserve a minimal ratio max_indent/margin>=50% and if possible the current difference margin - max_indent.

val pp_get_margin : formatter -> unit -> int
val get_margin : unit -> int

Returns the position of the right margin.

Maximum indentation limit

val pp_set_max_indent : formatter -> int -> unit
val set_max_indent : int -> unit

pp_set_max_indent ppf d sets the maximum indentation limit of lines to d (in characters): once this limit is reached, new pretty-printing 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).

If d is greater or equal than the current margin, it is ignored, and the current maximum indentation limit is kept.

val pp_get_max_indent : formatter -> unit -> int
val get_max_indent : unit -> int

Return the maximum indentation limit (in characters).

Maximum formatting depth

The maximum formatting depth is the maximum number of pretty-printing boxes simultaneously open.

Material inside boxes nested deeper is printed as an ellipsis (more precisely as the text returned by get_ellipsis_text ()).

val pp_set_max_boxes : formatter -> int -> unit
val set_max_boxes : int -> unit

pp_set_max_boxes ppf max sets the maximum number of pretty-printing boxes simultaneously open.

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 pp_get_max_boxes : formatter -> unit -> int
val get_max_boxes : unit -> int

Returns the maximum number of pretty-printing boxes allowed before ellipsis.

val pp_over_max_boxes : formatter -> unit -> bool
val over_max_boxes : unit -> bool

Tests if the maximum number of pretty-printing boxes allowed have already been opened.

Tabulation boxes

A tabulation box prints material on lines divided into cells of fixed length. A tabulation box provides a simple way to display vertical columns of left adjusted text.

This box features command set_tab to define cell boundaries, and command print_tab to move from cell to cell and split the line when there is no more cells to print on the line.

Note: printing within tabulation box is line directed, so arbitrary line splitting inside a tabulation box leads to poor rendering. Yet, controlled use of tabulation boxes allows simple printing of columns within module Format.

val pp_open_tbox : formatter -> unit -> unit
val open_tbox : unit -> unit

open_tbox () opens a new tabulation box.

This box prints lines separated into cells of fixed width.

Inside a tabulation box, special tabulation markers defines points of interest on the line (for instance to delimit cell boundaries). Function Format.set_tab sets a tabulation marker at insertion point.

A tabulation box features specific tabulation breaks to move to next tabulation marker or split the line. Function Format.print_tbreak prints a tabulation break.

val pp_close_tbox : formatter -> unit -> unit
val close_tbox : unit -> unit

Closes the most recently opened tabulation box.

val pp_set_tab : formatter -> unit -> unit
val set_tab : unit -> unit

Sets a tabulation marker at current insertion point.

val pp_print_tab : formatter -> unit -> unit
val print_tab : unit -> unit

print_tab () emits a 'next' tabulation break hint: if not already set on a tabulation marker, the insertion point moves to the first tabulation marker on the right, or the pretty-printer splits the line and insertion point moves to the leftmost tabulation marker.

It is equivalent to print_tbreak 0 0.

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

print_tbreak nspaces offset emits a 'full' tabulation break hint.

If not already set on a tabulation marker, the insertion point moves to the first tabulation marker on the right and the pretty-printer prints nspaces spaces.

If there is no next tabulation marker on the right, the pretty-printer splits the line at this point, then insertion point moves to the leftmost tabulation marker of the box.

If the pretty-printer splits the line, offset is added to the current indentation.

Ellipsis

val pp_set_ellipsis_text : formatter -> string -> unit
val set_ellipsis_text : string -> unit

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

val pp_get_ellipsis_text : formatter -> unit -> string
val get_ellipsis_text : unit -> string

Return the text of the ellipsis.

Semantic tags

type tag = string

Semantic tags (or simply tags) are user's defined delimiters to associate user's specific operations to printed entities.

Common usage of semantic tags is text decoration to get specific font or text size rendering for a display device, or marking delimitation of entities (e.g. HTML or TeX elements or terminal escape sequences). More sophisticated usage of semantic tags could handle dynamic modification of the pretty-printer behavior to properly print the material within some specific tags.

In order to properly delimit printed entities, a semantic tag must be opened before and closed after the entity. Semantic tags must be properly nested like parentheses.

Tag specific operations occur any time a tag is opened or closed, At each occurrence, two kinds of operations are performed tag-marking and tag-printing:

  • The tag-marking operation is the simpler tag specific operation: it simply writes a tag specific string into the output device of the formatter. Tag-marking does not interfere with line-splitting computation.
  • The tag-printing operation is the more involved tag specific operation: it can print arbitrary material to the formatter. Tag-printing is tightly linked to the current pretty-printer operations.

Roughly speaking, tag-marking is commonly used to get a better rendering of texts in the rendering device, while tag-printing allows fine tuning of printing routines to print the same entity differently according to the semantic tags (i.e. print additional material or even omit parts of the output).

More precisely: when a semantic tag is opened or closed then both and successive 'tag-printing' and 'tag-marking' operations occur:

  • Tag-printing a semantic tag means calling the formatter specific function print_open_tag (resp. print_close_tag) 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 splitting computation).
  • Tag-marking a semantic tag means calling the formatter specific function mark_open_tag (resp. mark_close_tag) with the name of the tag as argument: that tag-marking function can then return the 'tag-opening marker' (resp. `tag-closing marker') for direct output into the output device of the formatter.

Being written directly into the output device of the formatter, semantic tag marker strings are not considered as part of the printing material that drives line splitting (in other words, the length of the strings corresponding to tag markers is considered as zero for line splitting).

Thus, semantic 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 honors tags and decorates the output accordingly.

Default tag-marking functions behave the HTML way: tags are enclosed in "<" and ">"; hence, opening marker for tag t is "<t>" and closing marker is "</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.

Semantic tag operations may be set on or off with set_tags. Tag-marking operations may be set on or off with set_mark_tags. Tag-printing operations may be set on or off with set_print_tags.

val pp_open_tag : formatter -> string -> unit
val open_tag : tag -> unit

pp_open_tag ppf t opens the semantic tag named t.

The print_open_tag tag-printing function of the formatter is called with t as argument; then the opening tag marker for t, as given by mark_open_tag t, is written into the output device of the formatter.

val pp_close_tag : formatter -> unit -> unit
val close_tag : unit -> unit

pp_close_tag ppf () closes the most recently opened semantic tag t.

The closing tag marker, as given by mark_close_tag t, is written into the output device of the formatter; then the print_close_tag tag-printing function of the formatter is called with t as argument.

val pp_set_tags : formatter -> bool -> unit
val set_tags : bool -> unit

pp_set_tags ppf b turns on or off the treatment of semantic tags (default is off).

val pp_set_print_tags : formatter -> bool -> unit
val set_print_tags : bool -> unit

pp_set_print_tags ppf b turns on or off the tag-printing operations.

val pp_set_mark_tags : formatter -> bool -> unit
val set_mark_tags : bool -> unit

pp_set_mark_tags ppf b turns on or off the tag-marking operations.

val pp_get_print_tags : formatter -> unit -> bool
val get_print_tags : unit -> bool

Return the current status of tag-printing operations.

val pp_get_mark_tags : formatter -> unit -> bool
val get_mark_tags : unit -> bool

Return the current status of tag-marking operations.

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

Redirecting the standard formatter output

val set_formatter_out_channel : Pervasives.out_channel -> unit

Redirect the standard 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.)

set_formatter_out_channel is equivalent to pp_set_formatter_out_channel std_formatter.

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

pp_set_formatter_output_functions ppf out flush redirects the standard 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 pp_get_formatter_output_functions : formatter -> unit -> (string -> int -> int -> unit) * (unit -> unit)
val get_formatter_output_functions : unit -> (string -> int -> int -> unit) * (unit -> unit)

Return the current output functions of the standard pretty-printer.

Redefining formatter output

The Format module is versatile enough to let you completely redefine the meaning of pretty-printing output: you may provide your own functions to define how to handle indentation, line splitting, and even printing of all the characters that have to be printed!

Redefining output functions

type formatter_out_functions = Format.formatter_out_functions = {
  1. out_string : string -> int -> int -> unit;
  2. out_flush : unit -> unit;
  3. out_newline : unit -> unit;
  4. out_spaces : int -> unit;
  5. out_indent : int -> unit;
}

The set of output functions specific to a formatter:

  • the out_string 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 out_flush function flushes the pretty-printer output device.
  • out_newline is called to open a new line when the pretty-printer splits the line.
  • the out_spaces function outputs spaces when a break hint leads to spaces instead of a line split. It is called with the number of spaces to output.
  • the out_indent function performs new line indentation when the pretty-printer splits the line. It is called with the indentation value of the new line.

By default:

  • since 4.01.0
val pp_set_formatter_out_functions : formatter -> formatter_out_functions -> unit
val set_formatter_out_functions : formatter_out_functions -> unit

pp_set_formatter_out_functions ppf out_funs Set all the pretty-printer output functions of ppf to those of argument out_funs,

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).

Reasonable defaults for functions out_spaces and out_newline are respectively out_funs.out_string (String.make n ' ') 0 n and out_funs.out_string "\n" 0 1.

  • since 4.01.0
val pp_get_formatter_out_functions : formatter -> unit -> formatter_out_functions
val get_formatter_out_functions : unit -> formatter_out_functions

Return the current output functions of the pretty-printer, including line splitting and indentation functions. Useful to record the current setting and restore it afterwards.

  • since 4.01.0

Redefining semantic tag operations

type formatter_tag_functions = Format.formatter_tag_functions = {
  1. mark_open_tag : tag -> string;
  2. mark_close_tag : tag -> string;
  3. print_open_tag : tag -> unit;
  4. print_close_tag : tag -> unit;
}

The semantic 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 write 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 pp_set_formatter_tag_functions : formatter -> formatter_tag_functions -> unit
val set_formatter_tag_functions : formatter_tag_functions -> unit

pp_set_formatter_tag_functions ppf tag_funs changes the meaning of opening and closing semantic tag operations to use the functions in tag_funs when printing on ppf.

When opening a semantic tag with 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 semantic 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 tag-printing functions that are called at tag opening and tag closing time, to output regular material in the pretty-printer queue.

val pp_get_formatter_tag_functions : formatter -> unit -> formatter_tag_functions
val get_formatter_tag_functions : unit -> formatter_tag_functions

Return the current semantic tag operation functions of the standard pretty-printer.

Defining formatters

Defining new formatters permits unrelated output of material in parallel on several output devices. All the parameters of a formatter are local to the formatter: right margin, maximum indentation limit, maximum number of pretty-printing boxes simultaneously open, ellipsis, and so on, are specific to each formatter and may be fixed independently.

For instance, given a Buffer.t buffer b, formatter_of_buffer b returns a new formatter using buffer b as its output device. Similarly, given a Pervasives.out_channel output channel oc, formatter_of_out_channel oc returns a new formatter using channel oc as its output device.

Alternatively, given out_funs, a complete set of output functions for a formatter, then formatter_of_out_functions out_funs computes a new formatter using those functions for output.

val formatter_of_out_channel : out_channel -> formatter

formatter_of_out_channel oc returns a new formatter writing to the corresponding output channel oc.

val std_formatter : formatter

The standard formatter to write to standard output.

It is defined as formatter_of_out_channel Pervasives.stdout.

val err_formatter : formatter

A formatter to write to standard error.

It is defined as formatter_of_out_channel Pervasives.stderr.

val formatter_of_buffer : Buffer.t -> formatter

formatter_of_buffer b returns a new formatter writing to buffer b. At the end of pretty-printing, the formatter must be flushed using pp_print_flush or pp_print_newline, to print all the pending material into the buffer.

val stdbuf : Buffer.t

The string buffer in which str_formatter writes.

val str_formatter : formatter

A formatter to 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 outputs with function out, and flushes with function flush.

For instance, 

make_formatter
  (Pervasives.output oc)
  (fun () -> Pervasives.flush oc)

returns a formatter to the Pervasives.out_channel oc.

val formatter_of_out_functions : formatter_out_functions -> formatter

formatter_of_out_functions out_funs returns a new formatter that writes with the set of output functions out_funs.

See definition of type formatter_out_functions for the meaning of argument out_funs.

  • since 4.06.0

Symbolic pretty-printing

Symbolic pretty-printing is pretty-printing using a symbolic formatter, i.e. a formatter that outputs symbolic pretty-printing items.

When using a symbolic formatter, all regular pretty-printing activities occur but output material is symbolic and stored in a buffer of output items. At the end of pretty-printing, flushing the output buffer allows post-processing of symbolic output before performing low level output operations.

In practice, first define a symbolic output buffer b using:

  • let sob = make_symbolic_output_buffer (). Then define a symbolic formatter with:
  • let ppf = formatter_of_symbolic_output_buffer sob

Use symbolic formatter ppf as usual, and retrieve symbolic items at end of pretty-printing by flushing symbolic output buffer sob with:

  • flush_symbolic_output_buffer sob.
type symbolic_output_item = Format.symbolic_output_item =
  1. | Output_flush
    (*

    symbolic flush command

    *)
  2. | Output_newline
    (*

    symbolic newline command

    *)
  3. | Output_string of string
    (*

    Output_string s: symbolic output for string s

    *)
  4. | Output_spaces of int
    (*

    Output_spaces n: symbolic command to output n spaces

    *)
  5. | Output_indent of int
    (*

    Output_indent i: symbolic indentation of size i

    *)

Items produced by symbolic pretty-printers

  • since 4.06.0
type symbolic_output_buffer = Format.symbolic_output_buffer

The output buffer of a symbolic pretty-printer.

  • since 4.06.0
val make_symbolic_output_buffer : unit -> symbolic_output_buffer

make_symbolic_output_buffer () returns a fresh buffer for symbolic output.

  • since 4.06.0
val clear_symbolic_output_buffer : symbolic_output_buffer -> unit

clear_symbolic_output_buffer sob resets buffer sob.

  • since 4.06.0
val get_symbolic_output_buffer : symbolic_output_buffer -> symbolic_output_item list

get_symbolic_output_buffer sob returns the contents of buffer sob.

  • since 4.06.0
val flush_symbolic_output_buffer : symbolic_output_buffer -> symbolic_output_item list

flush_symbolic_output_buffer sob returns the contents of buffer sob and resets buffer sob. flush_symbolic_output_buffer sob is equivalent to let items = get_symbolic_output_buffer sob in clear_symbolic_output_buffer sob; items

  • since 4.06.0
val add_symbolic_output_item : symbolic_output_buffer -> symbolic_output_item -> unit

add_symbolic_output_item sob itm adds item itm to buffer sob.

  • since 4.06.0
val formatter_of_symbolic_output_buffer : symbolic_output_buffer -> formatter

formatter_of_symbolic_output_buffer sob returns a symbolic formatter that outputs to symbolic_output_buffer sob.

  • since 4.06.0

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 items of list l, using pp_v to print each item, and calling pp_sep between items (pp_sep defaults to 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 using pp_print_space and pp_force_newline.

  • since 4.02.0

Formatted pretty-printing

Module Format provides a complete set of printf like functions for pretty-printing using format string specifications.

Specific annotations may be added in the format strings to give pretty-printing commands to the pretty-printing engine.

Those annotations are introduced in the format strings using the @ character. For instance, @ means a space break, @, means a cut, @[ opens a new box, and @] closes the last open box.

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 string 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. Pretty-printing box type is one of h, v, hv, b, or hov. 'h' stands for an 'horizontal' pretty-printing box, 'v' stands for a 'vertical' pretty-printing box, 'hv' stands for an 'horizontal/vertical' pretty-printing box, 'b' stands for an 'horizontal-or-vertical' pretty-printing box demonstrating indentation, 'hov' stands a simple 'horizontal-or-vertical' pretty-printing box. For instance, @[<hov 2> opens an 'horizontal-or-vertical' pretty-printing box with indentation 2 as obtained with open_hovbox 2. For more details about pretty-printing boxes, see the various box opening functions open_*box.
  • @]: close the most recently opened pretty-printing box.
  • @,: output a 'cut' break hint, as with print_cut ().
  • @ : output a 'space' break hint, as with print_space ().
  • @;: output a 'full' break hint as with print_break. The nspaces and offset parameters of the break hint 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 'space' break hint.
  • @.: flush the pretty-printer and split the 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 semantic 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 semantic tags, see the functions open_tag and close_tag.
  • @\}: close the most recently opened semantic tag.
  • @?: flush the pretty-printer as with print_flush (). This is equivalent to the conversion %!.
  • @\n: force a newline, as with force_newline (), not the normal way of pretty-printing, you should prefer using break hints inside a vertical pretty-printing box.

Note: To prevent the interpretation of a @ character as a pretty-printing indication, escape it with a % character. Old quotation mode @@ is deprecated since it is not compatible with formatted input interpretation of 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 'horizontal-or-vertical' 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 asprintf : ('a, formatter, unit, string) 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) format -> 'a

Same as fprintf above, but does not print anything. Useful to ignore some material when conditionally printing.

  • since 3.10.0

Formatted Pretty-Printing with continuations.

val kfprintf : (formatter -> 'a) -> formatter -> ('b, formatter, unit, 'a) 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) format4 -> 'b

Same as kfprintf above, but does not print anything. Useful to ignore some material when conditionally printing.

  • since 3.12.0
val kasprintf : (string -> 'a) -> ('b, formatter, unit, 'a) format4 -> 'b

Same as asprintf above, but instead of returning the string, passes it to the first argument.

  • since 4.03

Deprecated

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

    This function is error prone. Do not use it. This function is neither compositional nor incremental, since it flushes the pretty-printer queue at each call.

    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 with formatter to_b.

val kprintf : (string -> 'a) -> ('b, unit, string, 'a) 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.

type 'a printer = t -> 'a -> unit

Combinators

val silent : 'a printer

Prints nothing.

val unit : unit printer

Prints "()".

val int : int printer
val string : string printer
val bool : bool printer
val float3 : float printer
val float : float printer
val newline : unit printer

Force newline (see Format.pp_force_newline).

  • since 1.2
val substring : (string * int * int) printer

Print the substring (s,i,len), where i is the offset in s and len the number of bytes in the substring.

  • raises Invalid_argument

    if the triple (s,i,len) does not describe a proper substring.

  • since 1.2
val text : string printer

Print string, but replacing spaces with breaks and newlines with newline. See pp_print_text on recent versions of OCaml.

  • since 1.2
val char : char printer
  • since 0.14
val int32 : int32 printer
  • since 0.14
val int64 : int64 printer
  • since 0.14
val nativeint : nativeint printer
  • since 0.14
val flush : unit printer

Alias to Format.pp_print_flush.

  • since 1.2
val string_quoted : string printer

Similar to CCString.print.

  • since 0.14
val list : ?sep:unit printer -> 'a printer -> 'a list printer
val array : ?sep:unit printer -> 'a printer -> 'a array printer
val arrayi : ?sep:unit printer -> (int * 'a) printer -> 'a array printer
val seq : ?sep:unit printer -> 'a printer -> 'a sequence printer
val opt : 'a printer -> 'a option printer

opt pp prints options as follows:

  • Some x will become "some foo" if pp x ---> "foo".
  • None will become "none".

In the tuple printers, the sep argument is only available.

  • since 0.17
val pair : ?sep:unit printer -> 'a printer -> 'b printer -> ('a * 'b) printer
val triple : ?sep:unit printer -> 'a printer -> 'b printer -> 'c printer -> ('a * 'b * 'c) printer
val quad : ?sep:unit printer -> 'a printer -> 'b printer -> 'c printer -> 'd printer -> ('a * 'b * 'c * 'd) printer
val within : string -> string -> 'a printer -> 'a printer

within a b p wraps p inside the strings a and b. Convenient, for instances, for brackets, parenthesis, quotes, etc.

  • since 0.17
val map : ('a -> 'b) -> 'b printer -> 'a printer
val vbox : ?i:int -> 'a printer -> 'a printer

Wrap the printer in a vertical box.

  • parameter i

    level of indentation within the box (default 0).

  • since 0.16
val hvbox : ?i:int -> 'a printer -> 'a printer

Wrap the printer in a horizontal/vertical box.

  • parameter i

    level of indentation within the box (default 0).

  • since 0.16
val hovbox : ?i:int -> 'a printer -> 'a printer

Wrap the printer in a horizontal or vertical box.

  • parameter i

    level of indentation within the box (default 0).

  • since 0.16
val hbox : 'a printer -> 'a printer

Wrap the printer in an horizontal box.

  • since 0.16
val return : ('a, _, _, 'a) format4 -> unit printer

return "some_format_string" takes a argument-less format string and returns a printer actionable by (). Examples:

  • return ",@ "
  • return "@{<Red>and then@}@,"
  • return "@[<v>a@ b@]"
  • since 1.0
val of_to_string : ('a -> string) -> 'a printer

of_to_string f converts its input to a string using f, then prints the string.

  • since 1.0
val const : 'a printer -> 'a -> unit printer

const pp x is a unit printer that uses pp on x.

  • since 1.0
val some : 'a printer -> 'a option printer

some pp will print options as follows:

  • Some x is printed using pp on x
  • None is not printed at all
  • since 1.0
val lazy_force : 'a printer -> 'a lazy_t printer

lazy_force pp out x forces x and prints the result with pp.

  • since 2.0
val lazy_or : ?default:unit printer -> 'a printer -> 'a lazy_t printer

lazy_or ?default pp out x prints default if x is not evaluated yet, or uses pp otherwise.

  • since 2.0

ANSI codes

Use ANSI escape codes https://en.wikipedia.org/wiki/ANSI_escape_code to put some colors on the terminal.

This uses tags in format strings to specify the style. Current styles are the following:

  • "reset" resets style
  • "black"
  • "red"
  • "green"
  • "yellow"
  • "blue"
  • "magenta"
  • "cyan"
  • "white"
  • "bold" bold font
  • "Black" bold black
  • "Red" bold red
  • "Green" bold green
  • "Yellow" bold yellow
  • "Blue" bold blue
  • "Magenta" bold magenta
  • "Cyan" bold cyan
  • "White" bold white

Example:

set_color_default true;;

Format.printf
  "what is your @{<White>favorite color@}? @{<blue>blue@}! No, @{<red>red@}! Ahhhhhhh@.";;

status: unstable

  • since 0.15
val set_color_tag_handling : t -> unit

Add functions to support color tags to the given formatter.

  • since 0.15
val set_color_default : bool -> unit

set_color_default b enables color handling on the standard formatters (stdout, stderr) if b = true as well as on sprintf formatters; it disables the color handling if b = false.

val with_color : string -> 'a printer -> 'a printer

with_color "Blue" pp behaves like the printer pp, but with the given style.

status: unstable

  • since 0.16
val with_colorf : string -> t -> ('a, t, unit, unit) format4 -> 'a

with_colorf "Blue" out "%s %d" "yolo" 42 will behave like Format.fprintf, but wrapping the content with the given style.

status: unstable

  • since 0.16
val with_color_sf : string -> ('a, t, unit, string) format4 -> 'a

with_color_sf "Blue" out "%s %d" "yolo" 42 will behave like sprintf, but wrapping the content with the given style.

Example:

CCFormat.with_color_sf "red" "%a" CCFormat.Dump.(list int) [1;2;3] |> print_endline;;

status: unstable

  • since 0.21
val with_color_ksf : f:(string -> 'b) -> string -> ('a, t, unit, 'b) format4 -> 'a

with_color_ksf "Blue" ~f "%s %d" "yolo" 42 will behave like ksprintf, but wrapping the content with the given style.

Example: the following with raise Failure with a colored message

CCFormat.with_color_ksf "red" ~f:failwith "%a" CCFormat.Dump.(list int) [1;2;3];;
  • since 1.2

IO

val output : t -> 'a printer -> 'a -> unit
val to_string : 'a printer -> 'a -> string
val of_chan : out_channel -> t

Alias to Format.formatter_of_out_channel.

  • since 1.2
val with_out_chan : out_channel -> (t -> 'a) -> 'a

with_out_chan oc f turns oc into a formatter fmt, and call f fmt. Behaves like f fmt from then on, but whether the call to f fails or returns, fmt is flushed before the call terminates.

  • since 1.2
val stdout : t
val stderr : t
val tee : t -> t -> t

tee a b makes a new formatter that writes in both a and b.

  • since 1.0
val sprintf : ('a, t, unit, string) format4 -> 'a

Print into a string any format string that would usually be compatible with fprintf. Like Format.asprintf.

val sprintf_no_color : ('a, t, unit, string) format4 -> 'a

Like sprintf but never prints colors.

  • since 0.16
val sprintf_dyn_color : colors:bool -> ('a, t, unit, string) format4 -> 'a

Like sprintf but enable/disable colors depending on colors.

Example:

(* with colors *)
CCFormat.sprintf_dyn_color ~colors:true "@{<Red>%a@}"
  CCFormat.Dump.(list int) [1;2;3] |> print_endline;;

(* without colors *)
CCFormat.sprintf_dyn_color ~colors:false "@{<Red>%a@}"
  CCFormat.Dump.(list int) [1;2;3] |> print_endline;;
  • since 0.21
val fprintf : t -> ('a, t, unit) format -> 'a

Alias to Format.fprintf.

  • since 0.14
val fprintf_dyn_color : colors:bool -> t -> ('a, t, unit) format -> 'a

Like fprintf but enable/disable colors depending on colors.

  • since 0.21
val ksprintf : ?margin:int -> f:(string -> 'b) -> ('a, Format.formatter, unit, 'b) format4 -> 'a

ksprintf fmt ~f formats using fmt, in a way similar to sprintf, and then calls f on the resulting string.

  • parameter margin

    set margin (since 2.1)

  • since 0.14
val to_file : string -> ('a, t, unit, unit) format4 -> 'a

Print to the given file.

Dump

Print structures as OCaml values, so that they can be parsed back by OCaml (typically, in the toplevel, for debugging).

Example:

Format.printf "%a@." CCFormat.Dump.(list int) CCList.(1 -- 200);;

Format.printf "%a@." CCFormat.Dump.(array (list (pair int bool)))
  [| [1, true; 2, false]; []; [42, false] |];;
  • since 0.21
module Dump : sig ... end
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