ocaml-base-compiler
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Pretty-printing.

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 std_formatter and the generic version prefixed by pp_ that takes a formatter as its first argument.

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

Introduction

You may consider this module as providing an extension to the printf facility to provide automatic line splitting. The addition of pretty-printing annotations to your regular printf format strings gives you fancy indentation and line breaks. Pretty-printing annotations are described below in the documentation of the function Format.fprintf.

You may also use the explicit pretty-printing box management and printing functions provided by this module. This style is more basic but more verbose than the concise fprintf format strings.

For instance, the sequence open_box 0; print_string "x ="; print_space (); print_int 1; close_box (); print_newline () that prints x = 1 within a pretty-printing box, can be abbreviated as printf "@[%s@ %i@]@." "x =" 1, or even shorter printf "@[x =@ %i@]@." 1.

Rule of thumb for casual users of this library:

  • use simple pretty-printing boxes (as obtained by open_box 0);
  • use simple break hints as obtained by print_cut () that outputs a simple break hint, or by print_space () that outputs a space indicating a break hint;
  • once a pretty-printing box is open, display its material with basic printing functions (e. g. print_int and print_string);
  • when the material for a pretty-printing box has been printed, call close_box () to close the box;
  • at the end of pretty-printing, flush the pretty-printer to display all the remaining material, e.g. evaluate print_newline ().

The behavior of pretty-printing commands is unspecified if there is no open pretty-printing box. Each box opened by one of the open_ functions below must be closed using close_box for proper formatting. Otherwise, some of the material printed in the boxes may not be output, or may be formatted incorrectly.

In case of interactive use, each phrase is executed in the initial state of the standard pretty-printer: after each phrase execution, the interactive system closes all open pretty-printing boxes, flushes all pending text, and resets the standard pretty-printer.

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

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

Formatters

type formatter

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

Pretty-printing boxes

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

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

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

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

Moreover, opening a box after the maximum indentation limit splits the line whether or not the box would end up fitting on the line.

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

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

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

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

Within this box, the pretty-printer emphasizes the box structure: if a structural box does not fit fully on a simple line, a break hint also splits the line if the splitting ``moves to the left'' (i.e. the new line gets an indentation smaller than the one of the current line).

This box is the general purpose pretty-printing box.

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

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

Closes the most recently open pretty-printing box.

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

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

This box prints material on a single line.

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

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

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

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

Every break hint in a vertical box splits the line.

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

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

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

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

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

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

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

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

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

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

Formatting functions

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

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

val pp_print_bytes : formatter -> bytes -> unit
val print_bytes : bytes -> unit

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

  • since 4.13.0
val pp_print_as : formatter -> int -> string -> unit
val print_as : int -> string -> unit

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

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

Print an integer in the current pretty-printing box.

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

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

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

Print a character in the current pretty-printing box.

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

Print a boolean in the current pretty-printing box.

Break hints

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

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

Simple break hints are:

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

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

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

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

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

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

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

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

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

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

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

val pp_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 4.08.0
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-printin