Provided by: ocaml-man_5.2.0-3_all 
NAME
Format - Pretty-printing.
Module
Module Format
Documentation
Module Format
: sig end
Pretty-printing.
If you are new to this module, see the Format.examples below.
This module implements a pretty-printing facility to format values within Format.boxes and Format.tags
combined with a set of Format.fpp . The pretty-printer splits lines at specified Format.breaks , and
indents lines according to the box structure. Similarly, Format.tags can be used to decouple text
presentation from its contents.
This pretty-printing facility is implemented as an overlay on top of abstract Format.formatter which
provide basic output functions. Some formatters are predefined, notably:
- Format.std_formatter outputs to stdout
- Format.err_formatter outputs to stderr
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 Format.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 Format.get_std_formatter is delayed until the last argument is
received.
More formatters can be created with Format.formatter_of_out_channel , Format.formatter_of_buffer ,
Format.formatter_of_symbolic_output_buffer or using Format.formatter .
Warning: Since Format.formatter 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
Format.get_std_formatter or Format.get_err_formatter ), the output from the domains will be interleaved
with each other at points where the formatters are flushed, such as with Format.print_flush . This
synchronization is not performed by formatters obtained from Format.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:
-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
Format.formatter .
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 Format.maxindent 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.
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 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 10^9.
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: Format.tag 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 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_flush : unit -> unit ;
out_newline : unit -> unit ;
out_spaces : int -> unit ;
out_indent : int -> unit ; (* .B "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. output_string and flush for a
out_channel device, or Buffer.add_substring and 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 = {
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 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.