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

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

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

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)

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 laid out as:

  hi nice
     world
  

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

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

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

It is equivalent to <>.

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

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

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

cat xs = group (vcat xs)

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.

The document colon contains a colon, ":".

Specifies how to create the document based upon which column it is in.

The document comma contains a comma, ",".

(displayB simpleDoc) takes the output simpleDoc from a rendering function and transforms it to a Builder type (for further manipulation before converting to a lazy Text).

(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)

(displayT simpleDoc) takes the output simpleDoc from a rendering function and transforms it to a lazy Text value.

showWidth :: Int -> Doc -> Text
showWidth w x = displayT (renderPretty 0.4 w x)

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

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

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

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

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

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

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 laid out with a page width of 20 as:

  list [10,200,3000]
  

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

  list [10
       ,200
       ,3000]
  

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

The document (fill i x) renders document x. It then 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 laid out as:

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

The document (fillBreak i x) first renders document x. It then 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
  

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

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

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

fillSep xs = foldr (</>) empty xs

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

(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'
                          ['T.pack' "vertical", 'T.pack' "text"]))
          'hClose' handle

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)

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

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 !
  

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

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

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

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

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

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 or if rendered with renderOneLine.

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.

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.

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

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
  !
  

Specifies how to nest the document based upon which column it is being nested in.

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

(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 laid out on a page width of 20 as:

  (words,in,a,tuple)
  

But when the page width is 15, it is laid 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.)

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

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

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

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

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

(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.

(renderOneLine x) renders document x without adding any indentation or newlines.

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.

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

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

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.

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

sep xs = group (vsep xs)

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

softbreak = group linebreak

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

softline = group line

The document space contains a single space, " ".

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

The document spacebreak behaves like space when rendered normally but like empty when using renderCompact or renderOneLine.

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

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

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.

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.

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.

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.

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
  

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

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.