wl-pprint-extras-1.4.1: A free monad based on the Wadler/Leijen pretty printer

Portabilityportable
Stabilityexperimental
Maintainerekmett@gmail.com

Text.PrettyPrint.Free

Contents

Description

Pretty print module based on Daan Leijen's implementation of Philip Wadler's "prettier printer"

      "A prettier printer"
      Draft paper, April 1997, revised March 1998.
      http://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf

This 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 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.
  • A type argument has been added and embedded effects can be seen in the SimpleDoc type.

Synopsis

Documents

data Doc e Source

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" `above` text "world")

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

 hello
 world

putDoc :: Doc e -> IO ()Source

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 e -> IO ()Source

(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

char :: Char -> Doc eSource

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 eSource

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.

nest :: Int -> Doc e -> Doc eSource

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" `above` text "world") `above` text "!"

outputs as:

 hello
   world
 !

line :: Doc eSource

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 eSource

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 e -> Doc eSource

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 eSource

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

 softline = group line

softbreak :: Doc eSource

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

 softbreak  = group linebreak

Alignment

align :: Doc e -> Doc eSource

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 $$ y  = align (above x y)
 test    = text "hi" <+> (text "nice" $$ text "world")

which will be layed out as:

 hi nice
    world

hang :: Int -> Doc e -> Doc eSource

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 e -> Doc eSource

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 :: Foldable f => Doc e -> Doc e -> Doc e -> f (Doc e) -> Doc eSource

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 :: Foldable f => f (Doc e) -> Doc eSource

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 :: Foldable f => f (Doc e) -> Doc eSource

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 :: Foldable f => f (Doc e) -> Doc eSource

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 e -> Doc e -> Doc eSource

above :: Doc e -> Doc e -> Doc eSource

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

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

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)

aboveBreak :: Doc e -> Doc e -> Doc eSource

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

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

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 :: Foldable f => f (Doc e) -> Doc eSource

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

vsep :: Foldable f => f (Doc e) -> Doc eSource

The document (vsep xs) concatenates all documents xs vertically with above. 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 :: Foldable f => f (Doc e) -> Doc eSource

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 :: Foldable f => f (Doc e) -> Doc eSource

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

 sep xs  = group (vsep xs)

hcat :: Foldable f => f (Doc e) -> Doc eSource

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

vcat :: Foldable f => f (Doc e) -> Doc eSource

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

fillCat :: Foldable f => f (Doc e) -> Doc eSource

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 :: Foldable f => f (Doc e) -> Doc eSource

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

 cat xs  = group (vcat xs)

punctuate :: Traversable f => Doc e -> f (Doc e) -> f (Doc e)Source

(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 e -> Doc eSource

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 e")
          ,("nest","Int -> Doc e -> Doc e")
          ,("linebreak","Doc e")]

 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 e
     nest   :: Int -> Doc e -> Doc e
     linebreak :: Doc e

fillBreak :: Int -> Doc e -> Doc eSource

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 e
     nest   :: Int -> Doc e -> Doc e 
     linebreak
            :: Doc e

Bracketing combinators

enclose :: Doc e -> Doc e -> Doc e -> Doc eSource

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

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

squotes :: Doc e -> Doc eSource

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

dquotes :: Doc e -> Doc eSource

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

parens :: Doc e -> Doc eSource

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

angles :: Doc e -> Doc eSource

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

braces :: Doc e -> Doc eSource

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

brackets :: Doc e -> Doc eSource

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

Character documents

lparen :: Doc eSource

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

rparen :: Doc eSource

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

langle :: Doc eSource

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

rangle :: Doc eSource

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

lbrace :: Doc eSource

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

rbrace :: Doc eSource

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

lbracket :: Doc eSource

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

rbracket :: Doc eSource

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

squote :: Doc eSource

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

dquote :: Doc eSource

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

semi :: Doc eSource

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

colon :: Doc eSource

The document colon contains a colon, ":".

comma :: Doc eSource

The document comma contains a comma, ",".

space :: Doc eSource

The document space contains a single space, " ".

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

dot :: Doc eSource

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

backslash :: Doc eSource

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

equals :: Doc eSource

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

Primitive type documents

string :: String -> Doc eSource

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 eSource

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

integer :: Integer -> Doc eSource

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

float :: Float -> Doc eSource

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

double :: Double -> Doc eSource

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

rational :: Rational -> Doc eSource

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

Pretty class

class Pretty a whereSource

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 eSource

prettyList :: [a] -> Doc eSource

Instances

Pretty Bool 
Pretty Char 
Pretty Double 
Pretty Float 
Pretty Int 
Pretty Integer 
Pretty () 
Pretty Natural 
Pretty a => Pretty [a] 
Pretty a => Pretty (Maybe a) 
Pretty a => Pretty (Seq a) 
Pretty a => Pretty (NonEmpty a) 
Pretty (Doc a) 
(Pretty a, Pretty b) => Pretty (a, b) 
(Pretty a, Pretty b, Pretty c) => Pretty (a, b, c) 

Rendering

data SimpleDoc e Source

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.

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

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 e -> SimpleDoc eSource

(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 e -> ShowSSource

(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 e -> IO ()Source

(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

column, nesting :: (Int -> Doc e) -> Doc eSource

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

columns :: (Int -> Doc e) -> Doc eSource

Re-exported standard functions

empty :: Alternative f => forall a. f a

The identity of <|>

(<>) :: Semigroup a => a -> a -> a

An associative operation.

 (a <> b) <> c = a <> (b <> c)