Module Stdlib.Format

Pretty-printing.

If you are new to this module, see the examples below.

This module implements a pretty-printing facility to format values within 'pretty-printing boxes' and 'semantic tags' combined with a set of printf-like functions. The pretty-printer splits lines at specified break hints, and indents lines according to the box structure. Similarly, semantic tags can be used to decouple text presentation from its contents.

This pretty-printing facility is implemented as an overlay on top of abstract formatters which provide basic output functions. Some formatters are predefined, notably:

Most functions in the Format module come in two variants: a short version that operates on the current domain's standard formatter as obtained using get_std_formatter and the generic version prefixed by pp_ that takes a formatter as its first argument. For the version that operates on the current domain's standard formatter, the call to get_std_formatter is delayed until the last argument is received.

More formatters can be created with formatter_of_out_channel, formatter_of_buffer, formatter_of_symbolic_output_buffer or using custom formatters.

Warning: Since formatters contain mutable state, it is not thread-safe to use the same formatter on multiple domains in parallel without synchronization.

If multiple domains write to the same output channel using the predefined formatters (as obtained by get_std_formatter or get_err_formatter), the output from the domains will be interleaved with each other at points where the formatters are flushed, such as with print_flush. This synchronization is not performed by formatters obtained from formatter_of_out_channel (on the standard out channels or others).

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:

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:

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_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_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:

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 Stdlib.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 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 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. 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 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.

See also 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 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.

See also 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 = {
  1. max_indent : int;
  2. margin : int;
}
  • since 4.08
val check_geometry : geometry -> bool

Check if the formatter geometry is valid: 1 < max_indent < margin

  • 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, 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 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 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 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 pp_open_stag and 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 set_formatter_stag_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.

  • since 4.08
type tag = string
type stag +=
  1. | 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 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 = {
  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;
    (*
    • 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_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. Stdlib.output_string and Stdlib.flush for a Stdlib.out_channel device, or Buffer.add_substring and Stdlib.ignore for a Buffer.t output device),
  • 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 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
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

Redefining semantic tag operations

type formatter_stag_functions = {
  1. mark_open_stag : stag -> string;
  2. mark_close_stag : stag -> string;
  3. print_open_stag : stag -> unit;
  4. 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, formatter_of_buffer b returns a new formatter using buffer b as its output device. Similarly, given a Stdlib.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 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 print_flush.

  • alert unstable
val std_formatter : formatter

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

It is defined as formatter_of_out_channel Stdlib.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 formatter_of_out_channel Stdlib.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 pp_print_flush or 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 stdbuf string buffer.

str_formatter is defined as formatter_of_buffer 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 oc)
  (fun () -> Stdlib.flush oc)

returns a formatter to the Stdlib.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 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 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:

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

type 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
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 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 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 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 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 pp_print_space and pp_force_newline.

  • since 4.02
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:

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 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 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 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 asprintf to delay formatting decisions. Using the string returned by asprintf in a formatting context forces formatting decisions to be taken in isolation, and the final string may be created prematurely. 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 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:

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.