Text.PrettyPrint.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 available at http://www.cs.uu.nl/~daan/download/pprint/pprint.html.

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

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

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
         }

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 infixr 6

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

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 infixr 6

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

(<$>) :: Doc -> Doc -> Doc infixr 5

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

(</>) :: Doc -> Doc -> Doc infixr 5

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 infixr 5

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

(<//>) :: Doc -> Doc -> Doc infixr 5

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, "=".

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.

Minimal complete definition

pretty

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

SEmpty

SChar Char SimpleDoc

SText !Int String SimpleDoc

SLine !Int 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.

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) ""

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)

Undocumented

bool :: Bool -> Doc

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

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

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