Text.PrettyPrint.HughesPJ

John Hughes's and Simon Peyton Jones's Pretty Printer Combinators

Based on The Design of a Pretty-printing Library in Advanced Functional Programming, Johan Jeuring and Erik Meijer (eds), LNCS 925 http://www.cs.chalmers.se/~rjmh/Papers/pretty.ps

Heavily modified by Simon Peyton Jones, Dec 96

The document type

data Doc

The abstract type of documents. The Show instance is equivalent to using render.

Instances

Show Doc

Constructing documents

Converting values into documents

char :: Char -> Doc

A document of height and width 1, containing a literal character.

text :: String -> Doc

A document of height 1 containing a literal string. text satisfies the following laws:

• text s <> text t = text (s++t)

• text "" <> x = x, if x non-empty

The side condition on the last law is necessary because text "" has height 1, while empty has no height.

ptext :: String -> Doc

An obsolete function, now identical to text.

zeroWidthText :: String -> Doc

Some text, but without any width. Use for non-printing text such as a HTML or Latex tags

int :: Int -> Doc

int n = text (show n)

integer :: Integer -> Doc

integer n = text (show n)

float :: Float -> Doc

float n = text (show n)

double :: Double -> Doc

double n = text (show n)

rational :: Rational -> Doc

rational n = text (show n)

Simple derived documents

semi :: Doc

A ';' character

comma :: Doc

A ',' character

colon :: Doc

A : character

space :: Doc

A space character

equals :: Doc

A '=' character

lparen :: Doc

A '(' character

rparen :: Doc

A ')' character

lbrack :: Doc

A '[' character

rbrack :: Doc

A ']' character

lbrace :: Doc

A '{' character

rbrace :: Doc

A '}' character

Wrapping documents in delimiters

parens :: Doc -> Doc

Wrap document in (...)

brackets :: Doc -> Doc

Wrap document in [...]

braces :: Doc -> Doc

Wrap document in {...}

quotes :: Doc -> Doc

Wrap document in '...'

doubleQuotes :: Doc -> Doc

Wrap document in "..."

Combining documents

empty :: Doc

The empty document, with no height and no width. empty is the identity for <>, <+>, $$ and $+$, and anywhere in the argument list for sep, hcat, hsep, vcat, fcat etc.

(<>) :: Doc -> Doc -> Doc infixl 6

Beside. <> is associative, with identity empty.

(<+>) :: Doc -> Doc -> Doc infixl 6

Beside, separated by space, unless one of the arguments is empty. <+> is associative, with identity empty.

hcat :: [Doc] -> Doc

List version of <>.

hsep :: [Doc] -> Doc

List version of <+>.

($$) :: Doc -> Doc -> Doc infixl 5

Above, except that if the last line of the first argument stops at least one position before the first line of the second begins, these two lines are overlapped. For example:

   text "hi" $$ nest 5 (text "there")

lays out as

   hi   there

rather than

   hi
        there

$$ is associative, with identity empty, and also satisfies

• (x $$ y) <> z = x $$ (y <> z), if y non-empty.

($+$) :: Doc -> Doc -> Doc infixl 5

Above, with no overlapping. $+$ is associative, with identity empty.

vcat :: [Doc] -> Doc

List version of $$.

sep :: [Doc] -> Doc

Either hsep or vcat.

cat :: [Doc] -> Doc

Either hcat or vcat.

fsep :: [Doc] -> Doc

"Paragraph fill" version of sep.

fcat :: [Doc] -> Doc

"Paragraph fill" version of cat.

nest :: Int -> Doc -> Doc

Nest (or indent) a document by a given number of positions (which may also be negative). nest satisfies the laws:

• nest 0 x = x

• nest k (nest k' x) = nest (k+k') x

• nest k (x <> y) = nest k z <> nest k y

• nest k (x $$ y) = nest k x $$ nest k y

• nest k empty = empty

• x <> nest k y = x <> y, if x non-empty

The side condition on the last law is needed because empty is a left identity for <>.

hang :: Doc -> Int -> Doc -> Doc

hang d1 n d2 = sep [d1, nest n d2]

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

punctuate p [d1, ... dn] = [d1 <> p, d2 <> p, ... dn-1 <> p, dn]

Predicates on documents

isEmpty :: Doc -> Bool

Returns True if the document is empty

Rendering documents

Default rendering

render :: Doc -> String

Renders the document as a string using the default style.

Rendering with a particular style

data Style

A rendering style.

Constructors

Style

Fields

mode :: Mode

The rendering mode

lineLength :: Int

Length of line, in chars

ribbonsPerLine :: Float

Ratio of ribbon length to line length

style :: Style

The default style (mode=PageMode, lineLength=100, ribbonsPerLine=1.5).

renderStyle :: Style -> Doc -> String

Render the document as a string using a specified style.

General rendering

fullRender

Arguments

:: Mode

Rendering mode

-> Int

Line length

-> Float

Ribbons per line

-> (TextDetails -> a -> a)

What to do with text

-> a

What to do at the end

-> Doc

The document

-> a

Result

The general rendering interface.

data Mode

Rendering mode.

Constructors

PageMode

Normal

ZigZagMode

With zig-zag cuts

LeftMode

No indentation, infinitely long lines

OneLineMode

All on one line

data TextDetails

Constructors

Chr Char

Str String

PStr String