package containers

  1. Overview
  2. Docs

Pretty printing of documents.

A document is a structured tree of text with formatting instructions.

It can be rendered into a string ("pretty printed"), see Pretty.

This follows Wadler's paper "A prettier printer", but with some changes in the rendering part because we can't rely on lazyness to make the algebraic implementation efficient.

Some general considerations: the type t is the type of documents, a tree with text leaves that is pretty printed within a given width.

Layout is controlled via the combination of a few primitives:

  • newline will either print a space or a newline. It is similar to Format's "@ " in that sense. A big difference with Format is that by default newline is actually a newline. It only becomes a space if it's in a group small enough to fit in the remainder of the current line.
  • group d tries to write d on a single line if there's room. If not, it has no effect.
  • nest n d increases the indentation level inside d. Any newline that is rendered as a new line is indented by n more spaces (which are cumulative with surrounding nest calls).
  • append a b (or a ^ b) just prints a followed by b.
  • fill d is a bit like group but it will try to cram as much as possible on each line. It is not all-or-nothing like group.


type t

The type of documents

val nil : t

Empty document

val char : char -> t

Single char.

val text : string -> t

Text. The string will be split on '\n', which are replaced by newline.

val textpf : ('a, unit, string, t) Stdlib.format4 -> 'a

Text, with a Printf-compatible format.

For example, textpf "%d-%d" 4 2 is like text "4-2".

val textf : ('a, Stdlib.Format.formatter, unit, t) Stdlib.format4 -> 'a

Text, with a Format-compatible format.

Note that this will bake-in any formatting done by Format. Newlines introduced by format will become hard newlines in the resulting document.

val nest : int -> t -> t

nest n d increases indentation by n inside d. If current indentation is m, then every newline inside d will be followed by n + m leading spaces.

val group : t -> t

Group the documents inside this.

Newlines immediately inside this group will either render as new lines or as spaces, depending on the width available.

val append : t -> t -> t


val newline : t

A line break.

val newline_or_spaces : int -> t

newline_or_spaces n either prints a newline (respecting indentation), or prints n spaces. newline is basically newline_or_spaces 1.

  • raises Invalid_argument

    if n < 0.

  • since 3.13
val nl : t

Alias for newline

val fill : t -> t list -> t

fill sep l resembles group (append_l ~sep l), except it tries to put as many items of l as possible on each line.

In terms of Format, this is like the hov box.

Output device

module Out : sig ... end

Arbitrary output.


module Ext : sig ... end

Extension node.

val ext : 'a Ext.t -> 'a -> t -> t

ext e v d wraps d with value v.

It is a document that has the same shape (and size) as d, except that additional data will be output when it is rendered using extension e.

When this is rendered, first e.pre out v is called; then d is printed; then out v is called. Here out is the output buffer/stream for rendering.

Pretty print and rendering

module Pretty : sig ... end


module Flatten : sig ... end

Trivial printing, on a single line.

val pp : Stdlib.Format.formatter -> t -> unit

Pretty-print, using Pretty and an unspecified margin.

val debug : Stdlib.Format.formatter -> t -> unit

Debug printer. This prints the structure of the document, it does not pretty-print it. See pp or Pretty.

Convenience functions

module Infix : sig ... end
include module type of Infix
val (^) : t -> t -> t

Alias of append.

val (^+) : t -> t -> t

x ^+ y is x ^ text " " ^ y

val (^/) : t -> t -> t

x ^/ y is x ^ newline ^ y

val sp : t

A single space

val append_l : ?sep:t -> t list -> t

append_l ?sep l is the concatenation of elements of l, separated by sep (default nil)

val append_sp : t list -> t

append_sp l is the concatenation of elements of l, separated by ' '

val append_nl : t list -> t

Same as append_l with sep=nl

val fill_map : t -> ('a -> t) -> 'a list -> t

fill_map sep f l is fill sep ( f l)

val bool : bool -> t
val int : int -> t
val float : float -> t
val float_hex : float -> t
val text_quoted : string -> t

text_quoted s is text (spf "%S" s)

val text_zero_width : string -> t

Text but we assume it takes no space on screen.

val of_list : ?sep:t -> ('a -> t) -> 'a list -> t

of_list f l maps each element of l to a document and concatenates them.

  • parameter sep

    separator inserted between elements (default nil)

val of_seq : ?sep:t -> ('a -> t) -> 'a Stdlib.Seq.t -> t

Same as of_list but with sequences.

val bracket : string -> t -> string -> t

bracket l d r groups d, between brackets l and r

val bracket2 : string -> t -> string -> t

bracket2 l d r groups d, indented by 2, between brackets l and r

val sexp_apply : string -> t list -> t

sexp_apply a l is the S-expr "(text a …l)", pretty-printed

val sexp_l : t list -> t

sexp_l [l1;…ln] is the S-expr "(l1 l2…ln)", pretty-printed

module Dump : sig ... end

Printers that correspond closely to OCaml's syntax.

module Term_color : sig ... end

Simple colors in terminals


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