ElixirFM-1.1: Functional Arabic MorphologyContentsIndex
Text.PrettyPrint.Leijen
Portabilityportable
Stabilityprovisional
Maintainerstefanor@cox.net
Contents
Documents
Basic combinators
Alignment
Operators
List combinators
Fillers
Bracketing combinators
Character documents
Primitive type documents
Pretty class
Rendering
Undocumented
Description

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.

Synopsis
data Doc
putDoc :: Doc -> IO ()
hPutDoc :: Handle -> Doc -> IO ()
empty :: Doc
char :: Char -> Doc
text :: String -> Doc
(<>) :: Doc -> Doc -> Doc
nest :: Int -> Doc -> Doc
line :: Doc
linebreak :: Doc
group :: Doc -> Doc
softline :: Doc
softbreak :: Doc
align :: Doc -> Doc
hang :: Int -> Doc -> Doc
indent :: Int -> Doc -> Doc
encloseSep :: Doc -> Doc -> Doc -> [Doc] -> Doc
list :: [Doc] -> Doc
tupled :: [Doc] -> Doc
semiBraces :: [Doc] -> Doc
(<+>) :: Doc -> Doc -> Doc
(<$>) :: Doc -> Doc -> Doc
(</>) :: Doc -> Doc -> Doc
(<$$>) :: Doc -> Doc -> Doc
(<//>) :: Doc -> Doc -> Doc
hsep :: [Doc] -> Doc
vsep :: [Doc] -> Doc
fillSep :: [Doc] -> Doc
sep :: [Doc] -> Doc
hcat :: [Doc] -> Doc
vcat :: [Doc] -> Doc
fillCat :: [Doc] -> Doc
cat :: [Doc] -> Doc
punctuate :: Doc -> [Doc] -> [Doc]
fill :: Int -> Doc -> Doc
fillBreak :: Int -> Doc -> Doc
enclose :: Doc -> Doc -> Doc -> Doc
squotes :: Doc -> Doc
dquotes :: Doc -> Doc
parens :: Doc -> Doc
angles :: Doc -> Doc
braces :: Doc -> Doc
brackets :: Doc -> Doc
lparen :: Doc
rparen :: Doc
langle :: Doc
rangle :: Doc
lbrace :: Doc
rbrace :: Doc
lbracket :: Doc
rbracket :: Doc
squote :: Doc
dquote :: Doc
semi :: Doc
colon :: Doc
comma :: Doc
space :: Doc
dot :: Doc
backslash :: Doc
equals :: Doc
string :: String -> Doc
int :: Int -> Doc
integer :: Integer -> Doc
float :: Float -> Doc
double :: Double -> Doc
rational :: Rational -> Doc
class Pretty a where
pretty :: a -> Doc
prettyList :: [a] -> Doc
data SimpleDoc
= SEmpty
| SChar Char SimpleDoc
| SText !Int String SimpleDoc
| SLine !Int SimpleDoc
renderPretty :: Float -> Int -> Doc -> SimpleDoc
renderCompact :: Doc -> SimpleDoc
displayS :: SimpleDoc -> ShowS
displayIO :: Handle -> SimpleDoc -> IO ()
bool :: Bool -> Doc
nesting :: (Int -> Doc) -> Doc
width :: Doc -> (Int -> Doc) -> Doc
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
show/hide Instances
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
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
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, "=".
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
show/hide Instances
Pretty Bool
Pretty Char
Pretty Double
Pretty Float
Pretty Int
Pretty Integer
Pretty String
Pretty ()
Pretty Doc
Pretty TagsType
Pretty ParaType
Pretty (Wrap Nest) => Pretty Cluster
Pretty ([] ([] ([] (Wrap Tokens)))) => Pretty ([] ([] ([] ([] (Wrap Tokens)))))
Pretty ([] ([] (Wrap Tokens))) => Pretty ([] ([] ([] (Wrap Tokens))))
Pretty ([] (Wrap Tokens)) => Pretty ([] ([] (Wrap Tokens)))
(Show a, Template a) => Pretty ([] ((,) TagsType ([] ((,) Form (Lexeme a)))))
(Show a, Template a) => Pretty ([] ((,) ParaType ([] ((,) Root (Morphs a)))))
Pretty a => Pretty ([] a)
Pretty (Wrap Tokens) => Pretty ([] (Wrap Tokens))
Pretty (Lexeme a) => Pretty ([] (Lexeme a))
Pretty (Wrap Nest) => Pretty ([] Lexicon)
Pretty a => Pretty (Maybe a)
Show a => Pretty (Entity a)
(Show a, Pretty (Entity a)) => Pretty (Entry a)
(Pretty (a String), Pretty (a PatternT), Pretty (a PatternQ), Pretty (a PatternL)) => Pretty (Wrap a)
(Pretty (Lexeme PatternT), Pretty (Lexeme PatternQ), Pretty (Lexeme String), Pretty (Lexeme PatternL)) => Pretty (Wrap Lexeme)
(Pretty (Nest PatternT), Pretty (Nest PatternQ), Pretty (Nest String), Pretty (Nest PatternL)) => Pretty (Wrap Nest)
Pretty (Entry a) => Pretty (Lexeme a)
(Show a, Pretty (Entry a)) => Pretty (Nest a)
(Show a, Template a) => Pretty (Derived a)
(Show a, Template a) => Pretty (Inflected a)
(Eq a, Morphing a a, Forming a, Show a, Template a, Pretty ([] a)) => Pretty (Tokens a)
Show a => Pretty (Either ((,) Root (Morphs a)) (Morphs a))
Show a => Pretty ((,) ([] TagsType) ([] (Morphs a)))
(Pretty a, Pretty b) => Pretty ((,) a b)
Show a => Pretty ((,) TagsType ([] ((,) ([] TagsType) ([] (Morphs a)))))
(Show a, Template a) => Pretty ((,) TagsType ([] ((,) Form (Lexeme a))))
(Show a, Template a) => Pretty ((,) ParaType ([] ((,) Root (Morphs a))))
(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
nesting :: (Int -> Doc) -> Doc
width :: Doc -> (Int -> Doc) -> Doc
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