Text.PrettyPrint.ANSI.Leijen

Pretty print module based on Philip Wadler's "prettier printer"

      "A prettier printer"
      Draft paper, April 1997, revised March 1998.
      http://cm.bell-labs.com/cm/cs/who/wadler/papers/prettier/prettier.ps

PPrint is an implementation of the pretty printing combinators described by Philip Wadler (1997). In their bare essence, the combinators of Wadler are not expressive enough to describe some commonly occurring layouts. The PPrint library adds new primitives to describe these layouts and works well in practice.

The library is based on a single way to concatenate documents, which is associative and has both a left and right unit. This simple design leads to an efficient and short implementation. The simplicity is reflected in the predictable behaviour of the combinators which make them easy to use in practice.

A thorough description of the primitive combinators and their implementation can be found in Philip Wadler's paper (1997). Additions and the main differences with his original paper are:

• The nil document is called empty.
• The above combinator is called <$>. The operator </> is used for soft line breaks.
• There are three new primitives: align, fill and fillBreak. These are very useful in practice.
• Lots of other useful combinators, like fillSep and list.
• There are two renderers, renderPretty for pretty printing and renderCompact for compact output. The pretty printing algorithm also uses a ribbon-width now for even prettier output.
• There are two displayers, displayS for strings and displayIO for file based output.
• There is a Pretty class.
• The implementation uses optimised representations and strictness annotations.

Full documentation for the original wl-pprint library available at http://www.cs.uu.nl/~daan/download/pprint/pprint.html.

The library has been extended to allow formatting text for output to ANSI style consoles. New combinators allow:

• Control of foreground and background color of text
• The abliity to make parts of the text bold or underlined

This functionality is, as far as possible, portable across platforms with their varying terminals. However, one thing to be particularly wary of is that console colors will not be displayed on Windows unless the Doc value is output using the putDoc function or one of it's friends. Rendering the Doc to a String and then outputing that will only work on Unix-style operating systems.

Documents

data Doc

The abstract data type Doc represents pretty documents.

Doc is an instance of the Show class. (show doc) pretty prints document doc with a page width of 100 characters and a ribbon width of 40 characters.

 show (text "hello" <$> text "world")

Which would return the string "hello\nworld", i.e.

 hello
 world

Instances

Show Doc

IsString Doc

Monoid Doc

Pretty Doc

putDoc :: Doc -> IO ()

The action (putDoc doc) pretty prints document doc to the standard output, with a page width of 100 characters and a ribbon width of 40 characters.

 main :: IO ()
 main = do{ putDoc (text "hello" <+> text "world") }

Which would output

 hello world

Any ANSI colorisation in doc will be output.

hPutDoc :: Handle -> Doc -> IO ()

(hPutDoc handle doc) pretty prints document doc to the file handle handle with a page width of 100 characters and a ribbon width of 40 characters.

 main = do{ handle <- openFile "MyFile" WriteMode
          ; hPutDoc handle (vcat (map text
                            ["vertical","text"]))
          ; hClose handle
          }

Any ANSI colorisation in doc will be output.

Basic combinators

empty :: Doc

The empty document is, indeed, empty. Although empty has no content, it does have a 'height' of 1 and behaves exactly like (text "") (and is therefore not a unit of <$>).

char :: Char -> Doc

The document (char c) contains the literal character c. The character shouldn't be a newline ('\n'), the function line should be used for line breaks.

text :: String -> Doc

The document (text s) contains the literal string s. The string shouldn't contain any newline ('\n') characters. If the string contains newline characters, the function string should be used.

(<>) :: Doc -> Doc -> Doc

The document (x <> y) concatenates document x and document y. It is an associative operation having empty as a left and right unit. (infixr 6)

nest :: Int -> Doc -> Doc

The document (nest i x) renders document x with the current indentation level increased by i (See also hang, align and indent).

 nest 2 (text "hello" <$> text "world") <$> text "!"

outputs as:

 hello
   world
 !

line :: Doc

The line document advances to the next line and indents to the current nesting level. Document line behaves like (text " ") if the line break is undone by group.

linebreak :: Doc

The linebreak document advances to the next line and indents to the current nesting level. Document linebreak behaves like empty if the line break is undone by group.

group :: Doc -> Doc

The group combinator is used to specify alternative layouts. The document (group x) undoes all line breaks in document x. The resulting line is added to the current line if that fits the page. Otherwise, the document x is rendered without any changes.

softline :: Doc

The document softline behaves like space if the resulting output fits the page, otherwise it behaves like line.

 softline = group line

softbreak :: Doc

The document softbreak behaves like empty if the resulting output fits the page, otherwise it behaves like line.

 softbreak  = group linebreak

hardline :: Doc

A linebreak that will never be flattened; it is guaranteed to render as a newline.

flatAlt :: Doc -> Doc -> Doc

A document that is normally rendered as the first argument, but when flattened, is rendered as the second document.

renderSmart :: Float -> Int -> Doc -> SimpleDoc

A slightly smarter rendering algorithm with more lookahead. It provides provide earlier breaking on deeply nested structures For example, consider this python-ish pseudocode: fun(fun(fun(fun(fun([abcdefg, abcdefg]))))) If we put a softbreak (+ nesting 2) after each open parenthesis, and align the elements of the list to match the opening brackets, this will render with renderPretty and a page width of 20 as: fun(fun(fun(fun(fun([ | abcdef, | abcdef, ] ))))) | Where the 20c. boundary has been marked with |. Because renderPretty only uses one-line lookahead, it sees that the first line fits, and is stuck putting the second and third lines after the 20-c mark. In contrast, renderSmart will continue to check that the potential document up to the end of the indentation level. Thus, it will format the document as:

fun( | fun( | fun( | fun( | fun([ | abcdef, abcdef, ] | ))))) | Which fits within the 20c. boundary.

Alignment

align :: Doc -> Doc

The document (align x) renders document x with the nesting level set to the current column. It is used for example to implement hang.

As an example, we will put a document right above another one, regardless of the current nesting level:

 x $$ y  = align (x <$> y)

 test    = text "hi" <+> (text "nice" $$ text "world")

which will be layed out as:

 hi nice
    world

hang :: Int -> Doc -> Doc

The hang combinator implements hanging indentation. The document (hang i x) renders document x with a nesting level set to the current column plus i. The following example uses hanging indentation for some text:

 test  = hang 4 (fillSep (map text
         (words "the hang combinator indents these words !")))

Which lays out on a page with a width of 20 characters as:

 the hang combinator
     indents these
     words !

The hang combinator is implemented as:

 hang i x  = align (nest i x)

indent :: Int -> Doc -> Doc

The document (indent i x) indents document x with i spaces.

 test  = indent 4 (fillSep (map text
         (words "the indent combinator indents these words !")))

Which lays out with a page width of 20 as:

     the indent
     combinator
     indents these
     words !

encloseSep :: Doc -> Doc -> Doc -> [Doc] -> Doc

The document (encloseSep l r sep xs) concatenates the documents xs separated by sep and encloses the resulting document by l and r. The documents are rendered horizontally if that fits the page. Otherwise they are aligned vertically. All separators are put in front of the elements. For example, the combinator list can be defined with encloseSep:

 list xs = encloseSep lbracket rbracket comma xs
 test    = text "list" <+> (list (map int [10,200,3000]))

Which is layed out with a page width of 20 as:

 list [10,200,3000]

But when the page width is 15, it is layed out as:

 list [10
      ,200
      ,3000]

list :: [Doc] -> Doc

The document (list xs) comma separates the documents xs and encloses them in square brackets. The documents are rendered horizontally if that fits the page. Otherwise they are aligned vertically. All comma separators are put in front of the elements.

tupled :: [Doc] -> Doc

The document (tupled xs) comma separates the documents xs and encloses them in parenthesis. The documents are rendered horizontally if that fits the page. Otherwise they are aligned vertically. All comma separators are put in front of the elements.

semiBraces :: [Doc] -> Doc

The document (semiBraces xs) separates the documents xs with semi colons and encloses them in braces. The documents are rendered horizontally if that fits the page. Otherwise they are aligned vertically. All semi colons are put in front of the elements.

Operators

(<+>) :: Doc -> Doc -> Doc

The document (x <+> y) concatenates document x and y with a space in between. (infixr 6)

(<$>) :: Doc -> Doc -> Doc

The document (x <$> y) concatenates document x and y with a line in between. (infixr 5)

(</>) :: Doc -> Doc -> Doc

The document (x </> y) concatenates document x and y with a softline in between. This effectively puts x and y either next to each other (with a space in between) or underneath each other. (infixr 5)

(<$$>) :: Doc -> Doc -> Doc

The document (x <$$> y) concatenates document x and y with a linebreak in between. (infixr 5)

(<//>) :: Doc -> Doc -> Doc

The document (x <//> y) concatenates document x and y with a softbreak in between. This effectively puts x and y either right next to each other or underneath each other. (infixr 5)

List combinators

hsep :: [Doc] -> Doc

The document (hsep xs) concatenates all documents xs horizontally with (<+>).

vsep :: [Doc] -> Doc

The document (vsep xs) concatenates all documents xs vertically with (<$>). If a group undoes the line breaks inserted by vsep, all documents are separated with a space.

 someText = map text (words ("text to lay out"))

 test     = text "some" <+> vsep someText

This is layed out as:

 some text
 to
 lay
 out

The align combinator can be used to align the documents under their first element

 test     = text "some" <+> align (vsep someText)

Which is printed as:

 some text
      to
      lay
      out

fillSep :: [Doc] -> Doc

The document (fillSep xs) concatenates documents xs horizontally with (<+>) as long as its fits the page, than inserts a line and continues doing that for all documents in xs.

 fillSep xs  = foldr (\<\/\>) empty xs

sep :: [Doc] -> Doc

The document (sep xs) concatenates all documents xs either horizontally with (<+>), if it fits the page, or vertically with (<$>).

 sep xs  = group (vsep xs)

hcat :: [Doc] -> Doc

The document (hcat xs) concatenates all documents xs horizontally with (<>).

vcat :: [Doc] -> Doc

The document (vcat xs) concatenates all documents xs vertically with (<$$>). If a group undoes the line breaks inserted by vcat, all documents are directly concatenated.

fillCat :: [Doc] -> Doc

The document (fillCat xs) concatenates documents xs horizontally with (<>) as long as its fits the page, than inserts a linebreak and continues doing that for all documents in xs.

 fillCat xs  = foldr (\<\/\/\>) empty xs

cat :: [Doc] -> Doc

The document (cat xs) concatenates all documents xs either horizontally with (<>), if it fits the page, or vertically with (<$$>).

 cat xs  = group (vcat xs)

punctuate :: Doc -> [Doc] -> [Doc]

(punctuate p xs) concatenates all documents in xs with document p except for the last document.

 someText = map text ["words","in","a","tuple"]
 test     = parens (align (cat (punctuate comma someText)))

This is layed out on a page width of 20 as:

 (words,in,a,tuple)

But when the page width is 15, it is layed out as:

 (words,
  in,
  a,
  tuple)

(If you want put the commas in front of their elements instead of at the end, you should use tupled or, in general, encloseSep.)

Fillers

fill :: Int -> Doc -> Doc

The document (fill i x) renders document x. It than appends spaces until the width is equal to i. If the width of x is already larger, nothing is appended. This combinator is quite useful in practice to output a list of bindings. The following example demonstrates this.

 types  = [("empty","Doc")
          ,("nest","Int -> Doc -> Doc")
          ,("linebreak","Doc")]

 ptype (name,tp)
        = fill 6 (text name) <+> text "::" <+> text tp

 test   = text "let" <+> align (vcat (map ptype types))

Which is layed out as:

 let empty  :: Doc
     nest   :: Int -> Doc -> Doc
     linebreak :: Doc

fillBreak :: Int -> Doc -> Doc

The document (fillBreak i x) first renders document x. It than appends spaces until the width is equal to i. If the width of x is already larger than i, the nesting level is increased by i and a line is appended. When we redefine ptype in the previous example to use fillBreak, we get a useful variation of the previous output:

 ptype (name,tp)
        = fillBreak 6 (text name) <+> text "::" <+> text tp

The output will now be:

 let empty  :: Doc
     nest   :: Int -> Doc -> Doc
     linebreak
            :: Doc

Bracketing combinators

enclose :: Doc -> Doc -> Doc -> Doc

The document (enclose l r x) encloses document x between documents l and r using (<>).

 enclose l r x   = l <> x <> r

squotes :: Doc -> Doc

Document (squotes x) encloses document x with single quotes "'".

dquotes :: Doc -> Doc

Document (dquotes x) encloses document x with double quotes '"'.

parens :: Doc -> Doc

Document (parens x) encloses document x in parenthesis, "(" and ")".

angles :: Doc -> Doc

Document (angles x) encloses document x in angles, "<" and ">".

braces :: Doc -> Doc

Document (braces x) encloses document x in braces, "{" and "}".

brackets :: Doc -> Doc

Document (brackets x) encloses document x in square brackets, "[" and "]".

Character documents

lparen :: Doc

The document lparen contains a left parenthesis, "(".

rparen :: Doc

The document rparen contains a right parenthesis, ")".

langle :: Doc

The document langle contains a left angle, "<".

rangle :: Doc

The document rangle contains a right angle, ">".

lbrace :: Doc

The document lbrace contains a left brace, "{".

rbrace :: Doc

The document rbrace contains a right brace, "}".

lbracket :: Doc

The document lbracket contains a left square bracket, "[".

rbracket :: Doc

The document rbracket contains a right square bracket, "]".

squote :: Doc

The document squote contains a single quote, "'".

dquote :: Doc

The document dquote contains a double quote, '"'.

semi :: Doc

The document semi contains a semi colon, ";".

colon :: Doc

The document colon contains a colon, ":".

comma :: Doc

The document comma contains a comma, ",".

space :: Doc

The document space contains a single space, " ".

 x <+> y   = x <> space <> y

dot :: Doc

The document dot contains a single dot, ".".

backslash :: Doc

The document backslash contains a back slash, "\".

equals :: Doc

The document equals contains an equal sign, "=".

Colorisation combinators

black :: Doc -> Doc

Displays a document with the black forecolor

red :: Doc -> Doc

Displays a document with the red forecolor

green :: Doc -> Doc

Displays a document with the green forecolor

yellow :: Doc -> Doc

Displays a document with the yellow forecolor

blue :: Doc -> Doc

Displays a document with the blue forecolor

magenta :: Doc -> Doc

Displays a document with the magenta forecolor

cyan :: Doc -> Doc

Displays a document with the cyan forecolor

white :: Doc -> Doc

Displays a document with the white forecolor

dullblack :: Doc -> Doc

Displays a document with the dull black forecolor

dullred :: Doc -> Doc

Displays a document with the dull red forecolor

dullgreen :: Doc -> Doc

Displays a document with the dull green forecolor

dullyellow :: Doc -> Doc

Displays a document with the dull yellow forecolor

dullblue :: Doc -> Doc

Displays a document with the dull blue forecolor

dullmagenta :: Doc -> Doc

Displays a document with the dull magenta forecolor

dullcyan :: Doc -> Doc

Displays a document with the dull cyan forecolor

dullwhite :: Doc -> Doc

Displays a document with the dull white forecolor

onblack :: Doc -> Doc

Displays a document with the black backcolor

onred :: Doc -> Doc

Displays a document with the red backcolor

ongreen :: Doc -> Doc

Displays a document with the green backcolor

onyellow :: Doc -> Doc

Displays a document with the yellow backcolor

onblue :: Doc -> Doc

Displays a document with the blue backcolor

onmagenta :: Doc -> Doc

Displays a document with the magenta backcolor

oncyan :: Doc -> Doc

Displays a document with the cyan backcolor

onwhite :: Doc -> Doc

Displays a document with the white backcolor

ondullblack :: Doc -> Doc

Displays a document with the dull block backcolor

ondullred :: Doc -> Doc

Displays a document with the dull red backcolor

ondullgreen :: Doc -> Doc

Displays a document with the dull green backcolor

ondullyellow :: Doc -> Doc

Displays a document with the dull yellow backcolor

ondullblue :: Doc -> Doc

Displays a document with the dull blue backcolor

ondullmagenta :: Doc -> Doc

Displays a document with the dull magenta backcolor

ondullcyan :: Doc -> Doc

Displays a document with the dull cyan backcolor

ondullwhite :: Doc -> Doc

Displays a document with the dull white backcolor

Emboldening combinators

bold :: Doc -> Doc

Displays a document in a heavier font weight

debold :: Doc -> Doc

Displays a document in the normal font weight

Underlining combinators

underline :: Doc -> Doc

Displays a document with underlining

deunderline :: Doc -> Doc

Displays a document with no underlining

Removing formatting

plain :: Doc -> Doc

Removes all colorisation, emboldening and underlining from a document

Primitive type documents

string :: String -> Doc

The document (string s) concatenates all characters in s using line for newline characters and char for all other characters. It is used instead of text whenever the text contains newline characters.

int :: Int -> Doc

The document (int i) shows the literal integer i using text.

integer :: Integer -> Doc

The document (integer i) shows the literal integer i using text.

float :: Float -> Doc

The document (float f) shows the literal float f using text.

double :: Double -> Doc

The document (double d) shows the literal double d using text.

rational :: Rational -> Doc

The document (rational r) shows the literal rational r using text.

Pretty class

class Pretty a where

The member prettyList is only used to define the instance Pretty a => Pretty [a]. In normal circumstances only the pretty function is used.

Methods

pretty :: a -> Doc

prettyList :: [a] -> Doc

Instances

Pretty Bool

Pretty Char

Pretty Double

Pretty Float

Pretty Int

Pretty Integer

Pretty ()

Pretty Doc

Pretty a => Pretty [a]

Pretty a => Pretty (Maybe a)

(Pretty a, Pretty b) => Pretty (a, b)

(Pretty a, Pretty b, Pretty c) => Pretty (a, b, c)

Rendering

data SimpleDoc

The data type SimpleDoc represents rendered documents and is used by the display functions.

The Int in SText contains the length of the string. The Int in SLine contains the indentation for that line. The library provides two default display functions displayS and displayIO. You can provide your own display function by writing a function from a SimpleDoc to your own output format.

Constructors

SFail

SEmpty

SChar Char SimpleDoc

SText !Int String SimpleDoc

SLine !Int SimpleDoc

SSGR [SGR] SimpleDoc

renderPretty :: Float -> Int -> Doc -> SimpleDoc

This is the default pretty printer which is used by show, putDoc and hPutDoc. (renderPretty ribbonfrac width x) renders document x with a page width of width and a ribbon width of (ribbonfrac * width) characters. The ribbon width is the maximal amount of non-indentation characters on a line. The parameter ribbonfrac should be between 0.0 and 1.0. If it is lower or higher, the ribbon width will be 0 or width respectively.

renderCompact :: Doc -> SimpleDoc

(renderCompact x) renders document x without adding any indentation. Since no 'pretty' printing is involved, this renderer is very fast. The resulting output contains fewer characters than a pretty printed version and can be used for output that is read by other programs.

This rendering function does not add any colorisation information.

displayS :: SimpleDoc -> ShowS

(displayS simpleDoc) takes the output simpleDoc from a rendering function and transforms it to a ShowS type (for use in the Show class).

 showWidth :: Int -> Doc -> String
 showWidth w x   = displayS (renderPretty 0.4 w x) ""

ANSI color information will be discarded by this function unless you are running on a Unix-like operating system. This is due to a technical limitation in Windows ANSI support.

displayIO :: Handle -> SimpleDoc -> IO ()

(displayIO handle simpleDoc) writes simpleDoc to the file handle handle. This function is used for example by hPutDoc:

 hPutDoc handle doc  = displayIO handle (renderPretty 0.4 100 doc)

Any ANSI colorisation in simpleDoc will be output.

Undocumented

bool :: Bool -> Doc

column :: (Int -> Doc) -> Doc

columns :: (Maybe Int -> Doc) -> Doc

nesting :: (Int -> Doc) -> Doc

width :: Doc -> (Int -> Doc) -> Doc