pprint
  1. Overview
  2. Docs

PPrint is an OCaml library for pretty-printing textual documents. It takes care of indentation and line breaks, and is typically used to pretty-print code.

Building Documents

type document

The abstract type of documents.

Atomic Documents

val empty : document

empty is the empty document.

val char : char -> document

char c is an atomic document that consists of the single character c. This character must not be a newline character.

val string : string -> document

string s is an atomic document that consists of the string s. This string must not contain a newline. The printing engine assumes that the ideal width of this string is String.length s. This assumption is safe if this is an ASCII string. Otherwise, fancystring or utf8string should be preferred.

val substring : string -> int -> int -> document

substring s ofs len is an atomic document that consists of the portion of the string s delimited by the offset ofs and the length len. This portion must not contain a newline. substring s ofs len is equivalent to string (String.sub s ofs len), but is expected to be more efficient, as the substring is not actually extracted.

val fancystring : string -> int -> document

fancystring s alen is an atomic document that consists of the string s. This string must not contain a newline. The string may contain fancy characters: color escape characters, UTF-8 characters, etc. Thus, its apparent length (which measures how many columns the text will take up on screen) differs from its length in bytes. The printing engine assumes that its apparent length is alen.

val fancysubstring : string -> int -> int -> int -> document

fancysubstring s ofs len alen is equivalent to fancystring (String.sub s ofs len) alen.

val utf8string : string -> document

utf8string s is an atomic document that consists of the UTF-8-encoded string s. This string must not contain a newline. utf8string s is equivalent to fancystring s (utf8_length s), where utf8_length s is the apparent length of the UTF-8-encoded string s.

val utf8format : ('a, unit, string, document) format4 -> 'a

utf8format format <args>... is equivalent to utf8string (Printf.sprintf format <args>...).

Blanks and Newlines

val hardline : document

The atomic document hardline represents a forced newline. This document has infinite ideal width: thus, if there is a choice between printing it in flat mode and printing it in normal mode, normal mode is preferred. In other words, when hardline is placed directly inside a group, this group is dissolved: group hardline is equivalent to hardline. This combinator should be seldom used; consider using break instead.

val blank : int -> document

The atomic document blank n consists of n blank characters. A blank character is like an ordinary ASCII space character char ' ', except that blank characters that appear at the end of a line are automatically suppressed.

val space : document

space is a synonym for blank 1. It consists of one blank character. It is therefore not equivalent to char ' '.

val break : int -> document

The document break n is a breakable blank of width n. It produces n blank characters if the printing engine is in flat mode, and a single newline character if the printing engine is in normal mode. break 1 is equivalent to ifflat (blank 1) hardline.

Composite Documents

val (^^) : document -> document -> document

doc1 ^^ doc2 is the concatenation of the documents doc1 and doc2.

val group : document -> document

group doc encodes a choice. If the document doc fits on the current line, then it is rendered on a single line, in flat mode. (All group combinators inside it are then ignored.) Otherwise, this group is dissolved, and doc is rendered in normal mode. There might be more groups within doc, whose presence leads to further choices being explored.

val ifflat : document -> document -> document

ifflat doc1 doc2 is rendered as doc1 if the printing engine is in flat mode, that is, if the printing engine has determined that some enclosing group fits on the current line. Otherwise, it is rendered as doc2. Use this combinator with caution! Because the printing engine is free to choose between doc1 and doc2, these documents must be semantically equivalent. It is up to the user to enforce this property.

val nest : int -> document -> document

To render the document nest j doc, the printing engine temporarily increases the current indentation level by j, then renders doc. The effect of the current indentation level is as follows: every time a newline character is emitted, it is immediately followed by n blank characters, where n is the current indentation level. Thus, one may think of nest j doc roughly as the document doc in which j blank characters have been inserted after every newline character.

val align : document -> document

To render align doc, the printing engine sets the current indentation level to the current column, then renders doc. In other words, the document doc is rendered within a box whose upper left corner is the current position of the printing engine.

type point = int * int

A point is a pair of a line number and a column number.

type range = point * point

A range is a pair of points.

val range : (range -> unit) -> document -> document

The document range hook doc is printed like the document doc, but allows the caller to register a hook that is applied, when the document is printed, to the range occupied by this document in the output text. This offers a way of mapping positions in the output text back to (sub)documents.

Rendering Documents

Three renderers are available. They offer the same API, described by the signature RENDERER, and differ only in the nature of the output channel that they use.

module type RENDERER = sig ... end

This signature describes the document renderers in a manner that is independent of the type of the output channel.

module ToChannel : RENDERER with type channel = out_channel and type document = document

This renderer sends its output into an output channel.

module ToBuffer : RENDERER with type channel = Buffer.t and type document = document

This renderer sends its output into a memory buffer.

This renderer sends its output into a formatter channel.

Defining Custom Documents

It is possible to define custom document constructors, provided they meet the expectations of the printing engine. In short, the custom document combinator custom expects an object of class custom. This object must provide three methods. The method requirement must compute the ideal width of the custom document. The methods pretty and compact must render the custom document. For this purpose, they have access to the output channel and to the state of the printing engine.

type requirement = int

A width requirement is expressed as an integer. The value max_int is reserved and represents infinity.

val infinity : requirement

infinity represents an infinite width requirement.

class type output = object ... end

An output channel is abstractly represented as an object equipped with methods for displaying one character and for displaying a substring.

type state = {
  1. width : int;
    (*

    The line width. This parameter is fixed throughout the execution of the renderer.

    *)
  2. ribbon : int;
    (*

    The ribbon width. This parameter is fixed throughout the execution of the renderer.

    *)
  3. mutable last_indent : int;
    (*

    The number of blanks that were printed at the beginning of the current line. This field is updated (only) when a hardline is emitted. It is used (only) to determine whether the ribbon width constraint is respected.

    *)
  4. mutable line : int;
    (*

    The current line. This field is updated (only) when a hardline is emitted. It is not used by the pretty-printing engine itself.

    *)
  5. mutable column : int;
    (*

    The current column. This field must be updated whenever something is sent to the output channel. It is used (only) to determine whether the width constraint is respected.

    *)
}

The internal state of the rendering engine is exposed to the user who wishes to define custom documents. However, its structure is subject to change in future versions of the library.

class type custom = object ... end

A custom document is defined by implementing an object of class custom.

val custom : custom -> document

custom constructs a custom document out an object of type custom.

Some of the key functions of the library are exposed, in the hope that they may be useful to authors of custom (leaf and composite) documents. In the case of a leaf document, they can help perform certain basic functions; for instance, applying the function pretty to the document hardline is a simple way of printing a hardline, while respecting the indentation parameters and updating the state in a correct manner. Similarly, applying pretty to the document blank n is a simple way of printing n blank characters. In the case of a composite document (one that contains subdocuments), these functions are essential: they allow computing the width requirement of a subdocument and displaying a subdocument.

val requirement : document -> requirement

requirement doc computes the width requirement of the document doc. It runs in constant time.

val pretty : output -> state -> int -> bool -> document -> unit

pretty output state indent flatten doc prints the document doc. See the documentation of the method pretty in the class custom.

val compact : output -> document -> unit

compact output doc prints the document doc. See the documentation of the method compact in the class custom.