Format(3) OCaml library Format(3)NAMEFormat - Pretty printing.
Module
Module FormatDocumentation
Module Format
: sig end
Pretty printing.
This module implements a pretty-printing facility to format text within
'pretty-printing boxes'. The pretty-printer breaks lines at specified
break hints, and indents lines according to the box structure.
For a gentle introduction to the basics of pretty-printing using Format
, read http://caml.inria.fr/resources/doc/guides/format.en.html.
You may consider this module as providing an extension to the printf
facility to provide automatic line breaking. The addition of
pretty-printing annotations to your regular printf formats gives you
fancy indentation and line breaks. Pretty-printing annotations are
described below in the documentation of the function Format.fprintf .
You may also use the explicit box management and printing functions
provided by this module. This style is more basic but more verbose than
the fprintf concise formats.
For instance, the sequence open_box 0; print_string x = ; print_space
(); print_int 1; close_box (); print_newline () that prints x = 1
within a pretty-printing box, can be abbreviated as printf @[%s@ %i@]@.
x = 1 , or even shorter printf @[x =@ %i@]@. 1 .
Rule of thumb for casual users of this library:
-use simple boxes (as obtained by open_box 0 );
-use simple break hints (as obtained by print_cut () that outputs a
simple break hint, or by print_space () that outputs a space indicating
a break hint);
-once a box is opened, display its material with basic printing func‐
tions (e. g. print_int and print_string );
-when the material for a box has been printed, call close_box () to
close the box;
-at the end of your routine, flush the pretty-printer to display all
the remaining material, e.g. evaluate print_newline () .
The behaviour of pretty-printing commands is unspecified if there is no
opened pretty-printing box. Each box opened via one of the open_ func‐
tions below must be closed using close_box for proper formatting. Oth‐
erwise, some of the material printed in the boxes may not be output, or
may be formatted incorrectly.
In case of interactive use, the system closes all opened boxes and
flushes all pending text (as with the print_newline function) after
each phrase. Each phrase is therefore executed in the initial state of
the pretty-printer.
Warning: the material output by the following functions is delayed in
the pretty-printer queue in order to compute the proper line breaking.
Hence, you should not mix calls to the printing functions of the basic
I/O system with calls to the functions of this module: this could
result in some strange output seemingly unrelated with the evaluation
order of printing commands.
=== Boxes ===
val open_box : int -> unit
open_box d opens a new pretty-printing box with offset d . This box is
the general purpose pretty-printing box. Material in this box is dis‐
played 'horizontal or vertical': break hints inside the box may lead to
a new line, if there is no more room on the line to print the remainder
of the box, or if a new line may lead to a new indentation (demonstrat‐
ing the indentation of the box). When a new line is printed in the
box, d is added to the current indentation.
val close_box : unit -> unit
Closes the most recently opened pretty-printing box.
=== Formatting functions ===
val print_string : string -> unit
print_string str prints str in the current box.
val print_as : int -> string -> unit
print_as len str prints str in the current box. The pretty-printer for‐
mats str as if it were of length len .
val print_int : int -> unit
Prints an integer in the current box.
val print_float : float -> unit
Prints a floating point number in the current box.
val print_char : char -> unit
Prints a character in the current box.
val print_bool : bool -> unit
Prints a boolean in the current box.
=== Break hints ===
val print_space : unit -> unit
print_space () is used to separate items (typically to print a space
between two words). It indicates that the line may be split at this
point. It either prints one space or splits the line. It is equivalent
to print_break 1 0 .
val print_cut : unit -> unit
print_cut () is used to mark a good break position. It indicates that
the line may be split at this point. It either prints nothing or splits
the line. This allows line splitting at the current point, without
printing spaces or adding indentation. It is equivalent to print_break
0 0 .
val print_break : int -> int -> unit
Inserts a break hint in a pretty-printing box. print_break nspaces
offset indicates that the line may be split (a newline character is
printed) at this point, if the contents of the current box does not fit
on the current line. If the line is split at that point, offset is
added to the current indentation. If the line is not split, nspaces
spaces are printed.
val print_flush : unit -> unit
Flushes the pretty printer: all opened boxes are closed, and all pend‐
ing text is displayed.
val print_newline : unit -> unit
Equivalent to print_flush followed by a new line.
val force_newline : unit -> unit
Forces a newline in the current box. Not the normal way of
pretty-printing, you should prefer break hints.
val print_if_newline : unit -> unit
Executes the next formatting command if the preceding line has just
been split. Otherwise, ignore the next formatting command.
=== Margin ===
val set_margin : int -> unit
set_margin d sets the value of the right margin to d (in characters):
this value is used to detect line overflows that leads to split lines.
Nothing happens if d is smaller than 2. If d is too large, the right
margin is set to the maximum admissible value (which is greater than
10^9 ).
val get_margin : unit -> int
Returns the position of the right margin.
=== Maximum indentation limit ===
val set_max_indent : int -> unit
set_max_indent d sets the value of the maximum indentation limit to d
(in characters): once this limit is reached, boxes are rejected to the
left, if they do not fit on the current line. Nothing happens if d is
smaller than 2. If d is too large, the limit is set to the maximum
admissible value (which is greater than 10^9 ).
val get_max_indent : unit -> int
Return the value of the maximum indentation limit (in characters).
=== Formatting depth: maximum number of boxes allowed before ellipsis
===
val set_max_boxes : int -> unit
set_max_boxes max sets the maximum number of boxes simultaneously
opened. Material inside boxes nested deeper is printed as an ellipsis
(more precisely as the text returned by get_ellipsis_text () ). Noth‐
ing happens if max is smaller than 2.
val get_max_boxes : unit -> int
Returns the maximum number of boxes allowed before ellipsis.
val over_max_boxes : unit -> bool
Tests if the maximum number of boxes allowed have already been opened.
=== Advanced formatting ===
val open_hbox : unit -> unit
open_hbox () opens a new pretty-printing box. This box is 'horizon‐
tal': the line is not split in this box (new lines may still occur
inside boxes nested deeper).
val open_vbox : int -> unit
open_vbox d opens a new pretty-printing box with offset d . This box
is 'vertical': every break hint inside this box leads to a new line.
When a new line is printed in the box, d is added to the current inden‐
tation.
val open_hvbox : int -> unit
open_hvbox d opens a new pretty-printing box with offset d . This box
is 'horizontal-vertical': it behaves as an 'horizontal' box if it fits
on a single line, otherwise it behaves as a 'vertical' box. When a new
line is printed in the box, d is added to the current indentation.
val open_hovbox : int -> unit
open_hovbox d opens a new pretty-printing box with offset d . This box
is 'horizontal or vertical': break hints inside this box may lead to a
new line, if there is no more room on the line to print the remainder
of the box. When a new line is printed in the box, d is added to the
current indentation.
=== Tabulations ===
val open_tbox : unit -> unit
Opens a tabulation box.
val close_tbox : unit -> unit
Closes the most recently opened tabulation box.
val print_tbreak : int -> int -> unit
Break hint in a tabulation box. print_tbreak spaces offset moves the
insertion point to the next tabulation ( spaces being added to this
position). Nothing occurs if insertion point is already on a tabula‐
tion mark. If there is no next tabulation on the line, then a newline
is printed and the insertion point moves to the first tabulation of the
box. If a new line is printed, offset is added to the current indenta‐
tion.
val set_tab : unit -> unit
Sets a tabulation mark at the current insertion point.
val print_tab : unit -> unit
print_tab () is equivalent to print_tbreak 0 0 .
=== Ellipsis ===
val set_ellipsis_text : string -> unit
Set the text of the ellipsis printed when too many boxes are opened (a
single dot, . , by default).
val get_ellipsis_text : unit -> string
Return the text of the ellipsis.
=== Semantics Tags ===
type tag = string
=== Semantics tags (or simply tags) are used to decorate printed enti‐
ties for user's defined purposes, e.g. setting font and giving size
indications for a display device, or marking delimitation of semantics
entities (e.g. HTML or TeX elements or terminal escape sequences). By
default, those tags do not influence line breaking calculation: the tag
'markers' are not considered as part of the printing material that
drives line breaking (in other words, the length of those strings is
considered as zero for line breaking). Thus, tag handling is in some
sense transparent to pretty-printing and does not interfere with usual
indentation. Hence, a single pretty printing routine can output both
simple 'verbatim' material or richer decorated output depending on the
treatment of tags. By default, tags are not active, hence the output is
not decorated with tag information. Once set_tags is set to true, the
pretty printer engine honours tags and decorates the output accord‐
ingly. When a tag has been opened (or closed), it is both and succes‐
sively 'printed' and 'marked'. Printing a tag means calling a formatter
specific function with the name of the tag as argument: that 'tag
printing' function can then print any regular material to the formatter
(so that this material is enqueued as usual in the formatter queue for
further line-breaking computation). Marking a tag means to output an
arbitrary string (the 'tag marker'), directly into the output device of
the formatter. Hence, the formatter specific 'tag marking' function
must return the tag marker string associated to its tag argument. Being
flushed directly into the output device of the formatter, tag marker
strings are not considered as part of the printing material that drives
line breaking (in other words, the length of the strings corresponding
to tag markers is considered as zero for line breaking). In addition,
advanced users may take advantage of the specificity of tag markers to
be precisely output when the pretty printer has already decided where
to break the lines, and precisely when the queue is flushed into the
output device. In the spirit of HTML tags, the default tag marking
functions output tags enclosed in < and > : hence, the opening marker
of tag t is <t> and the closing marker </t> . Default tag printing
functions just do nothing. Tag marking and tag printing functions are
user definable and can be set by calling set_formatter_tag_functions.
===
val open_tag : tag -> unit
open_tag t opens the tag named t ; the print_open_tag function of the
formatter is called with t as argument; the tag marker mark_open_tag t
will be flushed into the output device of the formatter.
val close_tag : unit -> unit
close_tag () closes the most recently opened tag t . In addition, the
print_close_tag function of the formatter is called with t as argument.
The marker mark_close_tag t will be flushed into the output device of
the formatter.
val set_tags : bool -> unit
set_tags b turns on or off the treatment of tags (default is off).
val set_print_tags : bool -> unit
val set_mark_tags : bool -> unit
set_print_tags b turns on or off the printing of tags, while
set_mark_tags b turns on or off the output of tag markers.
val get_print_tags : unit -> bool
val get_mark_tags : unit -> bool
Return the current status of tags printing and tags marking.
=== Redirecting the standard formatter output ===
val set_formatter_out_channel : Pervasives.out_channel -> unit
Redirect the pretty-printer output to the given channel. (All the out‐
put functions of the standard formatter are set to the default output
functions printing to the given channel.)
val set_formatter_output_functions : (string -> int -> int -> unit) ->
(unit -> unit) -> unit
set_formatter_output_functions out flush redirects the pretty-printer
output functions to the functions out and flush .
The out function performs all the pretty-printer string output. It is
called with a string s , a start position p , and a number of charac‐
ters n ; it is supposed to output characters p to p + n - 1 of s .
The flush function is called whenever the pretty-printer is flushed
(via conversion %! , or pretty-printing indications @? or @. , or
using low level functions print_flush or print_newline ).
val get_formatter_output_functions : unit -> (string -> int -> int ->
unit) * (unit -> unit)
Return the current output functions of the pretty-printer.
=== Changing the meaning of standard formatter pretty printing ===
=== The Format module is versatile enough to let you completely rede‐
fine the meaning of pretty printing: you may provide your own functions
to define how to handle indentation, line breaking, and even printing
of all the characters that have to be printed! ===
type formatter_out_functions = {
out_string : string -> int -> int -> unit ;
out_flush : unit -> unit ;
out_newline : unit -> unit ;
out_spaces : int -> unit ;
}
val set_formatter_out_functions : formatter_out_functions -> unit
set_formatter_out_functions out_funs Redirect the pretty-printer output
to the functions out_funs.out_string and out_funs.out_flush as
described in set_formatter_output_functions . In addition, the
pretty-printer function that outputs a newline is set to the function
out_funs.out_newline and the function that outputs indentation spaces
is set to the function out_funs.out_spaces .
This way, you can change the meaning of indentation (which can be some‐
thing else than just printing space characters) and the meaning of new
lines opening (which can be connected to any other action needed by the
application at hand). The two functions out_spaces and out_newline are
normally connected to out_string and out_flush : respective default
values for out_space and out_newline are out_string (String.make n ' ')
0 n and out_string \n 0 1 .
val get_formatter_out_functions : unit -> formatter_out_functions
Return the current output functions of the pretty-printer, including
line breaking and indentation functions. Useful to record the current
setting and restore it afterwards.
=== Changing the meaning of printing semantics tags ===
type formatter_tag_functions = {
mark_open_tag : tag -> string ;
mark_close_tag : tag -> string ;
print_open_tag : tag -> unit ;
print_close_tag : tag -> unit ;
}
The tag handling functions specific to a formatter: mark versions are
the 'tag marking' functions that associate a string marker to a tag in
order for the pretty-printing engine to flush those markers as 0 length
tokens in the output device of the formatter. print versions are the
'tag printing' functions that can perform regular printing when a tag
is closed or opened.
val set_formatter_tag_functions : formatter_tag_functions -> unit
set_formatter_tag_functions tag_funs changes the meaning of opening and
closing tags to use the functions in tag_funs .
When opening a tag name t , the string t is passed to the opening tag
marking function (the mark_open_tag field of the record tag_funs ),
that must return the opening tag marker for that name. When the next
call to close_tag () happens, the tag name t is sent back to the clos‐
ing tag marking function (the mark_close_tag field of record tag_funs
), that must return a closing tag marker for that name.
The print_ field of the record contains the functions that are called
at tag opening and tag closing time, to output regular material in the
pretty-printer queue.
val get_formatter_tag_functions : unit -> formatter_tag_functions
Return the current tag functions of the pretty-printer.
=== Multiple formatted output ===
type formatter
Abstract data corresponding to a pretty-printer (also called a format‐
ter) and all its machinery.
Defining new pretty-printers permits unrelated output of material in
parallel on several output channels. All the parameters of a
pretty-printer are local to this pretty-printer: margin, maximum inden‐
tation limit, maximum number of boxes simultaneously opened, ellipsis,
and so on, are specific to each pretty-printer and may be fixed inde‐
pendently. Given a Pervasives.out_channel output channel oc , a new
formatter writing to that channel is simply obtained by calling format‐
ter_of_out_channel oc . Alternatively, the make_formatter function
allocates a new formatter with explicit output and flushing functions
(convenient to output material to strings for instance).
val formatter_of_out_channel : Pervasives.out_channel -> formatter
formatter_of_out_channel oc returns a new formatter that writes to the
corresponding channel oc .
val std_formatter : formatter
The standard formatter used by the formatting functions above. It is
defined as formatter_of_out_channel stdout .
val err_formatter : formatter
A formatter to use with formatting functions below for output to stan‐
dard error. It is defined as formatter_of_out_channel stderr .
val formatter_of_buffer : Buffer.t -> formatter
formatter_of_buffer b returns a new formatter writing to buffer b . As
usual, the formatter has to be flushed at the end of pretty printing,
using pp_print_flush or pp_print_newline , to display all the pending
material.
val stdbuf : Buffer.t
The string buffer in which str_formatter writes.
val str_formatter : formatter
A formatter to use with formatting functions below for output to the
stdbuf string buffer. str_formatter is defined as formatter_of_buffer
stdbuf .
val flush_str_formatter : unit -> string
Returns the material printed with str_formatter , flushes the formatter
and resets the corresponding buffer.
val make_formatter : (string -> int -> int -> unit) -> (unit -> unit)
-> formatter
make_formatter out flush returns a new formatter that writes according
to the output function out , and the flushing function flush . For
instance, a formatter to the Pervasives.out_channel oc is returned by
make_formatter (Pervasives.output oc) (fun () -> Pervasives.flush oc) .
=== Basic functions to use with formatters ===
val pp_open_hbox : formatter -> unit -> unit
val pp_open_vbox : formatter -> int -> unit
val pp_open_hvbox : formatter -> int -> unit
val pp_open_hovbox : formatter -> int -> unit
val pp_open_box : formatter -> int -> unit
val pp_close_box : formatter -> unit -> unit
val pp_open_tag : formatter -> string -> unit
val pp_close_tag : formatter -> unit -> unit
val pp_print_string : formatter -> string -> unit
val pp_print_as : formatter -> int -> string -> unit
val pp_print_int : formatter -> int -> unit
val pp_print_float : formatter -> float -> unit
val pp_print_char : formatter -> char -> unit
val pp_print_bool : formatter -> bool -> unit
val pp_print_break : formatter -> int -> int -> unit
val pp_print_cut : formatter -> unit -> unit
val pp_print_space : formatter -> unit -> unit
val pp_force_newline : formatter -> unit -> unit
val pp_print_flush : formatter -> unit -> unit
val pp_print_newline : formatter -> unit -> unit
val pp_print_if_newline : formatter -> unit -> unit
val pp_open_tbox : formatter -> unit -> unit
val pp_close_tbox : formatter -> unit -> unit
val pp_print_tbreak : formatter -> int -> int -> unit
val pp_set_tab : formatter -> unit -> unit
val pp_print_tab : formatter -> unit -> unit
val pp_set_tags : formatter -> bool -> unit
val pp_set_print_tags : formatter -> bool -> unit
val pp_set_mark_tags : formatter -> bool -> unit
val pp_get_print_tags : formatter -> unit -> bool
val pp_get_mark_tags : formatter -> unit -> bool
val pp_set_margin : formatter -> int -> unit
val pp_get_margin : formatter -> unit -> int
val pp_set_max_indent : formatter -> int -> unit
val pp_get_max_indent : formatter -> unit -> int
val pp_set_max_boxes : formatter -> int -> unit
val pp_get_max_boxes : formatter -> unit -> int
val pp_over_max_boxes : formatter -> unit -> bool
val pp_set_ellipsis_text : formatter -> string -> unit
val pp_get_ellipsis_text : formatter -> unit -> string
val pp_set_formatter_out_channel : formatter -> Pervasives.out_channel
-> unit
val pp_set_formatter_output_functions : formatter -> (string -> int ->
int -> unit) -> (unit -> unit) -> unit
val pp_get_formatter_output_functions : formatter -> unit -> (string ->
int -> int -> unit) * (unit -> unit)
val pp_set_formatter_tag_functions : formatter -> formatter_tag_func‐
tions -> unit
val pp_get_formatter_tag_functions : formatter -> unit -> format‐
ter_tag_functions
val pp_set_formatter_out_functions : formatter -> formatter_out_func‐
tions -> unit
val pp_get_formatter_out_functions : formatter -> unit -> format‐
ter_out_functions
These functions are the basic ones: usual functions operating on the
standard formatter are defined via partial evaluation of these primi‐
tives. For instance, print_string is equal to pp_print_string std_for‐
matter .
=== printf like functions for pretty-printing. ===
val fprintf : formatter -> ('a, formatter, unit) Pervasives.format ->
'a
=== fprintf ff fmt arg1 ... argN formats the arguments arg1 to argN
according to the format string fmt, and outputs the resulting string on
the formatter ff. The format fmt is a character string which contains
three types of objects: plain characters and conversion specifications
as specified in the Printf module, and pretty-printing indications spe‐
cific to the Format module. The pretty-printing indication characters
are introduced by a @ character, and their meanings are: - @[: open a
pretty-printing box. The type and offset of the box may be optionally
specified with the following syntax: the < character, followed by an
optional box type indication, then an optional integer offset, and the
closing > character. Box type is one of h, v, hv, b, or hov, which
stand respectively for an horizontal box, a vertical box, an 'horizon‐
tal-vertical' box, or an 'horizontal or vertical' box (b standing for
an 'horizontal or vertical' box demonstrating indentation and hov
standing for a regular'horizontal or vertical' box). For instance,
@[<hov 2> opens an 'horizontal or vertical' box with indentation 2 as
obtained with open_hovbox 2. For more details about boxes, see the
various box opening functions open_*box. - @]: close the most recently
opened pretty-printing box. - @,: output a good break hint, as with
print_cut (). - @ : output a good break space, as with print_space ().
- @;: output a fully specified good break as with print_break. The
nspaces and offset parameters of the break may be optionally specified
with the following syntax: the < character, followed by an integer
nspaces value, then an integer offset, and a closing > character. If
no parameters are provided, the good break defaults to a good break
space. - @.: flush the pretty printer and output a new line, as with
print_newline (). - @<n>: print the following item as if it were of
length n. Hence, printf @<0>%s arg prints arg as a zero length string.
If @<n> is not followed by a conversion specification, then the follow‐
ing character of the format is printed as if it were of length n. -
@{: open a tag. The name of the tag may be optionally specified with
the following syntax: the < character, followed by an optional string
specification, and the closing > character. The string specification is
any character string that does not contain the closing character '>'.
If omitted, the tag name defaults to the empty string. For more
details about tags, see the functions open_tag and close_tag. - @}:
close the most recently opened tag. - @?: flush the pretty printer as
with print_flush (). This is equivalent to the conversion %!. - @\n:
force a newline, as with force_newline (). - @@: print a single @
character. Example: printf @[%s@ %d@]@. x = 1 is equivalent to
open_box (); print_string x = ; print_space (); print_int 1; close_box
(); print_newline (). It prints x = 1 within a pretty-printing box.
Note: If you need to prevent the interpretation of a @ character as a
pretty-printing indication, escape it with a % character, as usual in
format strings. ===
val printf : ('a, formatter, unit) Pervasives.format -> 'a
Same as fprintf above, but output on std_formatter .
val eprintf : ('a, formatter, unit) Pervasives.format -> 'a
Same as fprintf above, but output on err_formatter .
val sprintf : ('a, unit, string) Pervasives.format -> 'a
Same as printf above, but instead of printing on a formatter, returns a
string containing the result of formatting the arguments. Note that
the pretty-printer queue is flushed at the end of each call to sprintf
.
In case of multiple and related calls to sprintf to output material on
a single string, you should consider using fprintf with the predefined
formatter str_formatter and call flush_str_formatter () to get the
final result.
Alternatively, you can use Format.fprintf with a formatter writing to a
buffer of your own: flushing the formatter and the buffer at the end of
pretty-printing returns the desired string.
val asprintf : ('a, formatter, unit, string) Pervasives.format4 -> 'a
Same as printf above, but instead of printing on a formatter, returns a
string containing the result of formatting the arguments. The type of
asprintf is general enough to interact nicely with %a conversions.
Since 4.01.0
val ifprintf : formatter -> ('a, formatter, unit) Pervasives.format ->
'a
Same as fprintf above, but does not print anything. Useful to ignore
some material when conditionally printing.
Since 3.10.0
=== Formatted output functions with continuations. ===
val kfprintf : (formatter -> 'a) -> formatter -> ('b, formatter, unit,
'a) Pervasives.format4 -> 'b
Same as fprintf above, but instead of returning immediately, passes the
formatter to its first argument at the end of printing.
val ikfprintf : (formatter -> 'a) -> formatter -> ('b, formatter, unit,
'a) Pervasives.format4 -> 'b
Same as kfprintf above, but does not print anything. Useful to ignore
some material when conditionally printing.
Since 3.12.0
val ksprintf : (string -> 'a) -> ('b, unit, string, 'a) Pervasives.for‐
mat4 -> 'b
Same as sprintf above, but instead of returning the string, passes it
to the first argument.
=== Deprecated ===
val bprintf : Buffer.t -> ('a, formatter, unit) Pervasives.format -> 'a
A deprecated and error prone function. Do not use it.
If you need to print to some buffer b , you must first define a format‐
ter writing to b , using let to_b = formatter_of_buffer b ; then use
regular calls to Format.fprintf on formatter to_b .
val kprintf : (string -> 'a) -> ('b, unit, string, 'a) Pervasives.for‐
mat4 -> 'b
A deprecated synonym for ksprintf .
val set_all_formatter_output_functions : out:(string -> int -> int ->
unit) -> flush:(unit -> unit) -> newline:(unit -> unit) -> spaces:(int
-> unit) -> unit
Deprecated. Subsumed by set_formatter_out_functions .
Since 4.00.0
val get_all_formatter_output_functions : unit -> (string -> int -> int
-> unit) * (unit -> unit) * (unit -> unit) * (int -> unit)
Deprecated. Subsumed by get_formatter_out_functions .
Since 4.00.0
val pp_set_all_formatter_output_functions : formatter -> out:(string ->
int -> int -> unit) -> flush:(unit -> unit) -> newline:(unit -> unit)
-> spaces:(int -> unit) -> unit
Deprecated. Subsumed by pp_set_formatter_out_functions .
Since 4.01.0
val pp_get_all_formatter_output_functions : formatter -> unit ->
(string -> int -> int -> unit) * (unit -> unit) * (unit -> unit) * (int
-> unit)
Deprecated. Subsumed by pp_get_formatter_out_functions .
Since 4.01.0
OCamldoc 2013-10-24 Format(3)
