# Module Stdlib.Format

    module Format: Format

## Introduction

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 `Stdlib` 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, `
Stdlib.print_string "<";
Format.print_string "PRETTY";
Stdlib.print_string ">";
Format.print_string "TEXT";
` leads to output `<>PRETTYTEXT`.

## Formatters

    type 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.

Moreover, opening a box after the maximum indentation limit splits the line whether or not the box would end up fitting on 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: if a structural box does not fit fully on a simple line, 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_substring : pos:int -> len:int -> formatter -> string -> unit

    val print_substring : pos:int -> len:int -> string -> unit

`pp_print_substring ~pos ~len ppf s` prints the substring of `s` that starts at position `pos` and stops at position `pos+len` in the current pretty-printing box.

- Since 5.3

    val pp_print_bytes : formatter -> bytes -> unit

    val print_bytes : bytes -> unit

`pp_print_bytes ppf b` prints `b` in the current pretty-printing box.

- Since 4.13

    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_substring_as : pos:int -> len:int -> formatter -> int -> string -> unit

    val print_substring_as : pos:int -> len:int -> int -> string -> unit

`pp_print_substring_as ~first ~len ppf len_as s` prints the substring of `s` that starts at position `pos` and stop at position `pos+len` in the current pretty-printing box as if it were of length `len_as`.

- Since 5.1

    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.

    val pp_print_nothing : formatter -> unit -> unit

Print nothing.

- Since 5.2

## 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_print_custom_break : formatter ->
           fits:string * int * string -> breaks:string * int * string -> unit

`pp_print_custom_break ppf ~fits:(s1, n, s2) ~breaks:(s3, m, s4)` emits a custom break hint: the pretty-printer may split the line at this point.

If it does not split the line, then the `s1` is emitted, then `n` spaces, then `s2`.

If it splits the line, then it emits the `s3` string, then an indent (according to the box rules), then an offset of `m` spaces, then the `s4` string.

While `n` and `m` are handled by `formatter_out_functions.out_indent`, the strings will be handled by `formatter_out_functions.out_string`. This allows for a custom formatter that handles indentation distinctly, for example, outputs `<br/>` tags or `&nbsp;` entities.

The custom break is useful if you want to change which visible (non-whitespace) characters are printed in case of break or no break. For example, when printing a list `[a; b; c]` , you might want to add a trailing semicolon when it is printed vertically:

    [
      a;
      b;
      c;
    ]

You can do this as follows:

    printf "@[<v 0>[@;<0 2>@[<v 0>a;@,b;@,c@]%t]@]@\n"
      (pp_print_custom_break ~fits:("", 0, "") ~breaks:(";", 0, ""))

- Since 4.08

    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 `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 `Format.print_flush` with a new line emitted on the pretty-printer low-level output device immediately before the device is flushed. See corresponding words of caution for `Format.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_infinity : int

`pp_infinity` is the maximal size of the margin. Its exact value is implementation dependent but is guaranteed to be greater than 109.

- Since 5.2

    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. Setting the margin to `d` means that the formatting engine aims at printing at most `d-1` characters per line. Nothing happens if `d` is smaller than 2. If `d >=` `Format.pp_infinity`, the right margin is set to `Format.pp_infinity``- 1`. 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`.

See also `Format.pp_set_geometry`.

    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, unless the enclosing box fully fits on the current line. As an illustration,

     set_margin 10; set_max_indent 5; printf "@[123456@[7@]89A@]@."

yields

        123456
        789A

because the nested box `"@[7@]"` is opened after the maximum indentation limit (`7>5`) and its parent box does not fit on the current line. Either decreasing the length of the parent box to make it fit on a line:

     printf "@[123456@[7@]89@]@."

or opening an intermediary box before the maximum indentation limit which fits on the current line

     printf "@[123@[456@[7@]89@]A@]@."

avoids the rejection to the left of the inner boxes and print respectively `"123456789"` and `"123456789A"` . Note also that vertical boxes never fit on a line whereas horizontal boxes always fully fit on the current line. Opening a box may split a line whereas the contents may have fit. If this behavior is problematic, it can be curtailed by setting the maximum indentation limit to `margin - 1`. Note that setting the maximum indentation limit to `margin` is invalid.

Nothing happens if `d` is smaller than 2.

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

See also `Format.pp_set_geometry`.

    val pp_get_max_indent : formatter -> unit -> int

    val get_max_indent : unit -> int

Return the maximum indentation limit (in characters).

## Geometry

Geometric functions can be used to manipulate simultaneously the coupled variables, margin and maximum indentation limit.

    type geometry = {

`` | `max_indent : `int`;`
`` | `margin : `int`;`

`}`

- Since 4.08

    val check_geometry : geometry -> bool

Check if the formatter geometry is valid: `1 < max_indent < margin <` `Format.pp_infinity`

- Since 4.08

    val pp_set_geometry : formatter -> max_indent:int -> margin:int -> unit

    val set_geometry : max_indent:int -> margin:int -> unit

    val pp_safe_set_geometry : formatter -> max_indent:int -> margin:int -> unit

    val safe_set_geometry : max_indent:int -> margin:int -> unit

`pp_set_geometry ppf ~max_indent ~margin` sets both the margin and maximum indentation limit for `ppf`.

When `1 < max_indent < margin <` `Format.pp_infinity`, `pp_set_geometry ppf ~max_indent ~margin` is equivalent to `pp_set_margin ppf margin; pp_set_max_indent ppf max_indent`; and avoids the subtly incorrect `pp_set_max_indent ppf max_indent; pp_set_margin ppf margin`;

Outside of this domain, `pp_set_geometry` raises an invalid argument exception whereas `pp_safe_set_geometry` does nothing.

- Since 4.08

    val pp_update_geometry : formatter -> (geometry -> geometry) -> unit

`pp_update_geometry ppf (fun geo -> { geo with ... })` lets you update a formatter's geometry in a way that is robust to extension of the `geometry` record with new fields.

Raises an invalid argument exception if the returned geometry does not satisfy `Format.check_geometry`.

- Since 4.11

    val update_geometry : (geometry -> geometry) -> unit

    val pp_get_geometry : formatter -> unit -> geometry

    val get_geometry : unit -> geometry

Return the current geometry of the formatter

- Since 4.08

## 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 `Format.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 `Format.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 stag = ..

Semantic tags (or simply tags) are user's defined annotations 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. For instance, we can define an RGB tag like so:

    type stag += RGB of {r:int;g:int;b:int}

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 using `Format.pp_open_stag` and `Format.pp_close_stag`.

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_stag` (resp. `print_close_stag`) 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_stag` (resp. `mark_close_stag`) 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: string tags are enclosed in "<" and ">" while other tags are ignored; hence, opening marker for tag string `"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 `Format.set_formatter_stag_functions`.

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

- Since 4.08

    type tag = string

    type Format.stag +=

`|` | `String_tag of `tag`` | `(*` | `String_tag s` is a string tag `s`. String tags can be inserted either by explicitly using the constructor `String_tag` or by using the dedicated format syntax `"@{<s> ... @}"`. Since 4.08 | `*)`

    val pp_open_stag : formatter -> stag -> unit

    val open_stag : stag -> unit

`pp_open_stag ppf t` opens the semantic tag named `t`.

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

- Since 4.08

    val pp_close_stag : formatter -> unit -> unit

    val close_stag : unit -> unit

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

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

- Since 4.08

    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 -> out_channel -> unit

## Redirecting the standard formatter output

    val set_formatter_out_channel : 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 `Format.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 = {

`` | `out_string : `string -> int -> int -> unit`;`
`` | `out_width : `string -> pos:int -> len:int -> int`;` | `(*` | Since 5.4 | `*)`
`` | `out_flush : `unit -> unit`;`
`` | `out_newline : `unit -> unit`;`
`` | `out_spaces : `int -> unit`;`
`` | `out_indent : `int -> unit`;` | `(*` | Since 4.06 | `*)`

`}`

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_width` function informs the formatting engine of the width of the substring as rendered on the output device. Explicit width information as provided by `@<n>`, `Format.pp_print_as` or `Format.pp_print_substring_as` takes priority over this function. Moreover, the formatting engine evaluates the width of the string arguments of `Format.pp_print_string` and substring arguments of `Format.pp_print_substring` as a whole. Consequently, `out_width` can be used to compute an approximative width for unicode substrings.
- 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:

- fields `out_string` and `out_flush` are output device specific; (e.g. `output_string` and `flush` for a `out_channel` device, or `Buffer.add_substring` and `ignore` for a `Buffer.t` output device),
- field `out_width` is the number of unicode scalar values (see `Format.utf_8_scalar_width`) in the substring.
- field `out_newline` is equivalent to `out_string "\n" 0 1`;
- fields `out_spaces` and `out_indent` are equivalent to `out_string (String.make n ' ') 0 n`.

- Since 4.01

    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`, `out_newline`, and `out_width` are respectively `out_funs.out_string (String.make n ' ') 0 n`, `out_funs.out_string "\n" 0 1` and `Format.utf_8_scalar_width`.

- Since 4.01

    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

    val utf_8_scalar_width : string -> pos:int -> len:int -> int

`utf_8_scalar_width s ~pos ~len` is the number of unicode scalar values in the substring `String.sub s pos len`. Invalid byte sequences are implictly replaced by `U+FFFD` since this yields a better width approximation for other ascii-based encoding scheme like ISO-8859-15. This is the default `out_width` function since OCaml 5.4.

- Since 5.4

    val ascii_width : string -> pos:int -> len:int -> int

`ascii_width s ~pos ~len` is `len`.

- Since 5.4

## Redefining semantic tag operations

    type formatter_stag_functions = {

`` | `mark_open_stag : `stag -> string`;`
`` | `mark_close_stag : `stag -> string`;`
`` | `print_open_stag : `stag -> unit`;`
`` | `print_close_stag : `stag -> 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.

- Since 4.08

    val pp_set_formatter_stag_functions : formatter -> formatter_stag_functions -> unit

    val set_formatter_stag_functions : formatter_stag_functions -> unit

`pp_set_formatter_stag_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_stag` field of the record `tag_funs`), that must return the opening tag marker for that name. When the next call to `close_stag ()` happens, the semantic tag name `t` is sent back to the closing tag-marking function (the `mark_close_stag` 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.

- Since 4.08

    val pp_get_formatter_stag_functions : formatter -> unit -> formatter_stag_functions

    val get_formatter_stag_functions : unit -> formatter_stag_functions

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

- Since 4.08

## 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`, `Format.formatter_of_buffer` `b` returns a new formatter using buffer `b` as its output device. Similarly, given a `out_channel` output channel `oc`, `Format.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 `Format.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 synchronized_formatter_of_out_channel : out_channel -> formatter Domain.DLS.key

`synchronized_formatter_of_out_channel oc` returns the key to the domain-local state that holds the domain-local formatter for writing to the corresponding output channel `oc`.

When the formatter is used with multiple domains, the output from the domains will be interleaved with each other at points where the formatter is flushed, such as with `Format.print_flush`.

- Alert unstable.

    val std_formatter : formatter

The initial domain's standard formatter to write to standard output.

It is defined as `Format.formatter_of_out_channel` `stdout`.

    val get_std_formatter : unit -> formatter

`get_std_formatter ()` returns the current domain's standard formatter used to write to standard output.

- Since 5.0

    val err_formatter : formatter

The initial domain's formatter to write to standard error.

It is defined as `Format.formatter_of_out_channel` `stderr`.

    val get_err_formatter : unit -> formatter

`get_err_formatter ()` returns the current domain's formatter used to write to standard error.

- Since 5.0

    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 `Format.pp_print_flush` or `Format.pp_print_newline`, to print all the pending material into the buffer.

    val stdbuf : Buffer.t

The initial domain's string buffer in which `str_formatter` writes.

    val get_stdbuf : unit -> Buffer.t

`get_stdbuf ()` returns the current domain's string buffer in which the current domain's string formatter writes.

- Since 5.0

    val str_formatter : formatter

The initial domain's formatter to output to the `Format.stdbuf` string buffer.

`str_formatter` is defined as `Format.formatter_of_buffer` `Format.stdbuf`.

    val get_str_formatter : unit -> formatter

The current domain's formatter to output to the current domains string buffer.

- Since 5.0

    val flush_str_formatter : unit -> string

Returns the material printed with `str_formatter` of the current domain, 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
          (Stdlib.output_substring oc)
          (fun () -> Stdlib.flush oc)

returns a formatter to the `out_channel` `oc`.

    val make_synchronized_formatter : (string -> int -> int -> unit) ->
           (unit -> unit) -> formatter Domain.DLS.key

`make_synchronized_formatter out flush` returns the key to the domain-local state that holds the domain-local formatter that outputs with function `out`, and flushes with function `flush`.

When the formatter is used with multiple domains, the output from the domains will be interleaved with each other at points where the formatter is flushed, such as with `Format.print_flush`.

- Since 5.0
- Alert unstable.

    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 `Format.formatter_out_functions` for the meaning of argument `out_funs`.

- Since 4.06

### 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 =

`|` | `Output_flush` | `(*` | symbolic flush command | `*)`
`|` | `Output_newline` | `(*` | symbolic newline command | `*)`
`|` | `Output_string of `string`` | `(*` | `Output_string s`: symbolic output for string `s` | `*)`
`|` | `Output_spaces of `int`` | `(*` | `Output_spaces n`: symbolic command to output `n` spaces | `*)`
`|` | `Output_indent of `int`` | `(*` | `Output_indent i`: symbolic indentation of size `i` | `*)`

Items produced by symbolic pretty-printers

- Since 4.06

    type symbolic_output_buffer

The output buffer of a symbolic pretty-printer.

- Since 4.06

    val make_symbolic_output_buffer : unit -> symbolic_output_buffer

`make_symbolic_output_buffer ()` returns a fresh buffer for symbolic output.

- Since 4.06

    val clear_symbolic_output_buffer : symbolic_output_buffer -> unit

`clear_symbolic_output_buffer sob` resets buffer `sob`.

- Since 4.06

    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

    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

    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

    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

## Convenience formatting functions.

    val pp_print_iter : ?pp_sep:(formatter -> unit -> unit) ->
           (('a -> unit) -> 'b -> unit) ->
           (formatter -> 'a -> unit) -> formatter -> 'b -> unit

`pp_print_iter ~pp_sep iter pp_v ppf v` formats on `ppf` the iterations of `iter` over a collection `v` of values using `pp_v`. Iterations are separated by `pp_sep` (defaults to `Format.pp_print_cut`).

- Since 5.1

    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 `Format.pp_print_cut`). Does nothing on empty lists.

- Since 4.02

    val pp_print_array : ?pp_sep:(formatter -> unit -> unit) ->
           (formatter -> 'a -> unit) -> formatter -> 'a array -> unit

`pp_print_array ?pp_sep pp_v ppf a` prints items of array `a`, using `pp_v` to print each item, and calling `pp_sep` between items (`pp_sep` defaults to `Format.pp_print_cut`). Does nothing on empty arrays.

If `a` is mutated after `pp_print_array` is called, the printed values may not be what is expected because `Format` can delay the printing. This can be avoided by flushing `ppf`.

- Since 5.1

    val pp_print_seq : ?pp_sep:(formatter -> unit -> unit) ->
           (formatter -> 'a -> unit) ->
           formatter -> 'a Seq.t -> unit

`pp_print_seq ?pp_sep pp_v ppf s` prints items of sequence `s`, using `pp_v` to print each item, and calling `pp_sep` between items (`pp_sep` defaults to `Format.pp_print_cut`. Does nothing on empty sequences.

This function does not terminate on infinite sequences.

- Since 4.12

    val pp_print_text : formatter -> string -> unit

`pp_print_text ppf s` prints `s` with spaces and newlines respectively printed using `Format.pp_print_space` and `Format.pp_force_newline`.

- Since 4.02

    val format_text : ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
           ('a, 'b, 'c, 'd, 'e, 'f) format6

`format_text fmt` replaces spaces and newlines in the format string literal `fmt` with hint breaks or forced newlines:

- Blank lines (lines made only of spaces (`U+0020`)) are replaced by `"@n"`.
- Sequences of spaces and a newline (`U+000A`) preceding a blank line are replaced by `"@n"`: blank lines remain blank lines in the output.
- Remaining sequences made of `k` spaces and newlines are replaced by `"@;<0 k'>"` where `k' = max 1 k` is at least `1`.
- Breaks can be avoided using non-breaking space characters (`U+00A0`).
- Lines can be forced by using `"@\n"`

- Since 5.4

    val pp_print_option : ?none:(formatter -> unit -> unit) ->
           (formatter -> 'a -> unit) -> formatter -> 'a option -> unit

`pp_print_option ?none pp_v ppf o` prints `o` on `ppf` using `pp_v` if `o` is `Some v` and `none` if it is `None`. `none` prints nothing by default.

- Since 4.08

    val pp_print_result : ok:(formatter -> 'a -> unit) ->
           error:(formatter -> 'e -> unit) ->
           formatter -> ('a, 'e) result -> unit

`pp_print_result ~ok ~error ppf r` prints `r` on `ppf` using `ok` if `r` is `Ok _` and `error` if `r` is `Error _`.

- Since 4.08

    val pp_print_either : left:(formatter -> 'a -> unit) ->
           right:(formatter -> 'b -> unit) ->
           formatter -> ('a, 'b) Either.t -> unit

`pp_print_either ~left ~right ppf e` prints `e` on `ppf` using `left` if `e` is `Either.Left _` and `right` if `e` is `Either.Right _`.

- Since 4.13

## 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.

    val fprintf : formatter -> ('a, formatter, unit) 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 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 full 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 `Format.open_stag` and `Format.close_stag`.
- `@}`: 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 `get_std_formatter ()`.

It is defined similarly to `fun fmt -> fprintf (get_std_formatter ()) fmt` but delays calling `get_std_formatter` until after the final argument required by the `format` is received. When used with multiple domains, the output from the domains will be interleaved with each other at points where the formatter is flushed, such as with `Format.print_flush`.

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

Same as `fprintf` above, but output on `get_err_formatter ()`.

It is defined similarly to `fun fmt -> fprintf (get_err_formatter ()) fmt` but delays calling `get_err_formatter` until after the final argument required by the `format` is received. When used with multiple domains, the output from the domains will be interleaved with each other at points where the formatter is flushed, such as with `Format.print_flush`.

    val sprintf : ('a, unit, string) 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`. Note that if your format string contains a `%a`, you should use `asprintf`.

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) 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

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

Same as `Format.fprintf`, except the formatter is the last argument. `dprintf "..." a b c` is a function of type `formatter -> unit` which can be given to a format specifier `%t`.

This can be used as a replacement for `Format.asprintf` to delay formatting decisions. Using the string returned by `Format.asprintf` in a formatting context forces formatting decisions to be taken in isolation, and the final string may be created prematurely. `Format.dprintf` allows delay of formatting decisions until the final formatting context is known. For example:

      let t = Format.dprintf "%i@ %i@ %i" 1 2 3 in
      ...
      Format.printf "@[<v>%t@]" t

- Since 4.08

    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

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

Same as `Format.dprintf` above, but instead of returning immediately, passes the suspended printer to its first argument at the end of printing.

- Since 4.08

    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

    val ksprintf : (string -> 'a) -> ('b, unit, string, 'a) format4 -> 'b

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

    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

## Examples

A few warmup examples to get an idea of how Format is used.

We have a list `l` of pairs `(int * bool)`, which the toplevel prints for us:

    # let l = List.init 20 (fun n -> n, n mod 2 = 0)
      val l : (int * bool) list =
      [(0, true); (1, false); (2, true); (3, false); (4, true); (5, false);
       (6, true); (7, false); (8, true); (9, false); (10, true); (11, false);
       (12, true); (13, false); (14, true); (15, false); (16, true); (17, false);
       (18, true); (19, false)]

If we want to print it ourself without the toplevel magic, we can try this:

      # let pp_pair out (x,y) = Format.fprintf out "(%d, %b)" x y
      val pp_pair : Format.formatter -> int * bool -> unit = <fun>
      # Format.printf "l: [@[<hov>%a@]]@."
        Format.(pp_print_list ~pp_sep:(fun out () -> fprintf out ";@ ") pp_pair) l
        l: [(0, true); (1, false); (2, true); (3, false); (4, true); (5, false);
            (6, true); (7, false); (8, true); (9, false); (10, true); (11, false);
            (12, true); (13, false); (14, true); (15, false); (16, true);
            (17, false); (18, true); (19, false)]

What this does, briefly, is:

- `pp_pair` prints a pair `bool*int` surrounded in "(" ")". It takes a formatter (into which formatting happens), and the pair itself. When printing is done it returns `()`.

- `Format.printf "l = [@[<hov>%a@]]@." ... l` is like `printf`, but with additional formatting instructions (denoted with "@"). The pair "`@[<hov>`" and "`@]`" is a "horizontal-or-vertical box".

- "@." ends formatting with a newline. It is similar to "\n" but is also aware of the `Format.formatter`'s state. Do not use "\n" with `Format`.

- "%a" is a formatting instruction, like "%d" or "%s" for `printf`. However, where "%d" prints an integer and "%s" prints a string, "%a" takes a printer (of type `Format.formatter -> 'a -> unit`) and a value (of type `'a`) and applies the printer to the value. This is key to compositionality of printers.

- We build a list printer using `Format.pp_print_list ~pp_sep:(...) pp_pair`. `pp_print_list` takes an element printer and returns a list printer. The `?pp_sep` optional argument, if provided, is called in between each element to print a separator.

- Here, for a separator, we use `(fun out () -> Format.fprintf out ";@ ")`. It prints ";", and then "@ " which is a breaking space (either it prints " ", or it prints a newline if the box is about to overflow). This "@ " is responsible for the list printing splitting into several lines.

If we omit "@ ", we get an ugly single-line print:

    # Format.printf "l: [@[<hov>%a@]]@."
          Format.(pp_print_list ~pp_sep:(fun out () -> fprintf out "; ") pp_pair) l
      l: [(0, true); (1, false); (2, true); (* ... *); (18, true); (19, false)]
    - : unit = ()

Generally, it is good practice to define custom printers for important types in your program. If, for example, you were to define basic geometry types like so:

      type point = {
        x: float;
        y: float;
      }

      type rectangle = {
        ll: point; (* lower left *)
        ur: point; (* upper right *)
      }

For debugging purpose, or to display information in logs, or on the console, it would be convenient to define printers for these types. Here is an example of to do it. Note that "%.3f" is a `float` printer up to 3 digits of precision after the dot; "%f" would print as many digits as required, which is somewhat verbose; "%h" is an hexadecimal float printer.

      let pp_point out (p:point) =
        Format.fprintf out "{ @[x=%.3f;@ y=%.3f@] }" p.x p.y

      let pp_rectangle out (r:rectangle) =
        Format.fprintf out "{ @[ll=%a;@ ur=%a@] }"
          pp_point r.ll pp_point r.ur

In the `.mli` file, we could have:

        val pp_point : Format.formatter -> point -> unit

        val pp_rectangle : Format.formatter -> rectangle -> unit

These printers can now be used with "%a" inside other printers.

     # Format.printf "some rectangle: %a@."
            (Format.pp_print_option pp_rectangle)
            (Some {ll={x=1.; y=2.}; ur={x=42.; y=500.12345}})
      some rectangle: { l={ x=1.000; y=2.000 }; ur={ x=42.000; y=500.123 } }

      # Format.printf "no rectangle: %a@."
            (Format.pp_option pp_rectangle)
            None
      no rectangle:

See how we combine `pp_print_option` (option printer) and our newly defined rectangle printer, like we did with `pp_print_list` earlier.

For a more extensive tutorial, see "Using the Format module".

A final note: the `Format` module is a starting point. The OCaml ecosystem has libraries that makes formatting easier and more expressive, with more combinators, more concise names, etc. An example of such a library is Fmt.

Automatic deriving of pretty-printers from type definitions is also possible, using https://github.com/ocaml-ppx/ppx_deriving or similar ppx derivers.