A modern, extensible and well-documented prettyprinter.
A prettyprinter/text rendering engine. Easy to use, well-documented, ANSI terminal backend exists, HTML backend is trivial to implement, no name clashes,
let prettyType = align . sep . zipWith () ("::" : repeat "->") prettySig name ty = pretty name prettyType ty in prettySig "example" ["Int", "Bool", "Char", "IO ()"]
-- Output for wide enough formats: example :: Int -> Bool -> Char -> IO ()
-- Output for narrow formats: example :: Int -> Bool -> Char -> IO ()
This package defines a prettyprinter to format text in a flexible and convenient way. The idea is to combine a document out of many small components, then using a layouter to convert it to an easily renderable simple document, which can then be rendered to a variety of formats, for example plain
Text, or Markdown. What you are reading right now was generated by this library (see
Haskell, more specifically Hackage, has a zoo of Wadler/Leijen based prettyprinters already. Each of them addresses a different concern with the classic
wl-pprintpackage. This package solves all these issues, and then some.
Stringhas exactly one use, and that’s showing Hello World in tutorials. For all other uses,
Textis what people should be using. The prettyprinter uses no
Stringdefinitions anywhere; using a
Stringmeans an immediate conversion to the internal
The library is stuffed with runnable examples, showing use cases for the vast majority of exported values. Many things reference related definitions, everything comes with at least a sentence explaining its purpose.
Many prettyprinters use the legacy API of the first Wadler/Leijen prettyprinter, which used e.g.
()to separate lines, which clashes with the ubiquitous synonym for
fmapthat’s been in Base for ages. These definitions were either removed or renamed, so there are no name clashes with standard libraries anymore.
Text is not all letters and newlines. Often, we want to add more information, the simplest kind being some form of styling. An ANSI terminal supports coloring, a web browser a plethora of different formattings.
More complex uses of annotations include e.g. adding type annotations for mouse-over hovers when printing a syntax tree, adding URLs to documentation, or adding source locations to show where a certain piece of output comes from. Idris is a project that makes extensive use of such a feature.
Special care has been applied to make annotations unobtrusive, so that if you don’t need or care about them there is no overhead, neither in terms of usability nor performance.
A document can be rendered in many different ways, for many different clients. There is plain text, there is the ANSI terminal, there is the browser. Each of these speak different languages, and the backend is responsible for the translation to those languages. Backends should be readily available, or easy to implement if a custom solution is desired.
As a result, each backend requires only minimal dependencies; if you don’t want to print to an ANSI terminal for example, there is no need to have a dependency on a terminal library.
Rendering large documents should be done efficiently, and the library should make it easy to optimize common use cases for the programmer.
The type of documents is abstract in most of the other Wadler/Leijen prettyprinters, making it hard to impossible to write adaptors from one library to another. The type should be exposed for such purposes so it is possible to write adaptors from library to library, or each of them is doomed to live on its own small island of incompatibility. For this reason, the
Doctype is fully exposed in a semi-internal module for this specific use case.
prettyprinterfamily of packages consists of:
prettyprinteris the core package. It defines the language to generate nicely laid out documents, which can then be given to renderers to display them in various ways, e.g. HTML, or plain text.
prettyprinter-ansi-terminalprovides a renderer suitable for ANSI terminal output including colors (at the cost of a dependency more).
prettyprinter-compat-wl-pprintprovides a drop-in compatibility layer for previous users of the
wl-pprintpackage. Use it for easy adaption of the new
prettyprinter, but don't develop anything new with it.
prettyprinter-compat-ansi-wl-pprintis the same, but for previous users of
prettyprinter-compat-annotated-wl-pprintis the same, but for previous users of
prettyprinter-convert-ansi-wl-pprintis a converter, not a drop-in replacement, for documents generated by
ansi-wl-pprint. Useful for interfacing with other libraries that use the other format, like Trifecta and Optparse-Applicative.
The library originally started as a fork of
ansi-wl-pprintuntil every line had been touched. The result is still in the same spirit as its predecessors, but modernized to match the current ecosystem and needs.
The most significant changes are:
()is removed as an operator, since it clashes with the common alias for
<>and operators were removed or replaced by ordinary names.
fusefunction to optimize often-used documents before rendering for efficiency.
SimpleDocStream, to contrast the new
bgColorDullfunctions, which can be found in the ANSI terminal specific
This module is based on previous work by Daan Leijen and Max Bolingbroke, who implemented and significantly extended the prettyprinter given by a paper by Phil Wadler in his 1997 paper »A Prettier Printer«, by adding lots of convenience functions, styling, and new functionality. Their package, ansi-wl-pprint is widely used in the Haskell ecosystem, and is at the time of writing maintained by Edward Kmett.