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Text.PrettyPrint.HughesPJ

Portability | portable
Stability | provisional
Maintainer | libraries@haskell.org

Contents

The document type
Constructing documents
  Converting values into documents
  Simple derived documents
  Wrapping documents in delimiters
  Combining documents
Predicates on documents
Rendering documents
  Default rendering
  Rendering with a particular style
  General rendering

Description
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
Synopsis

data Doc
char :: Char -> Doc
text :: String -> Doc
ptext :: String -> Doc
int :: Int -> Doc
integer :: Integer -> Doc
float :: Float -> Doc
double :: Double -> Doc
rational :: Rational -> Doc
semi :: Doc
comma :: Doc
colon :: Doc
space :: Doc
equals :: Doc
lparen :: Doc
rparen :: Doc
lbrack :: Doc
rbrack :: Doc
lbrace :: Doc
rbrace :: Doc
parens :: Doc -> Doc
brackets :: Doc -> Doc
braces :: Doc -> Doc
quotes :: Doc -> Doc
doubleQuotes :: Doc -> Doc
empty :: Doc
(<>) :: Doc -> Doc -> Doc
(<+>) :: Doc -> Doc -> Doc
hcat :: [Doc] -> Doc
hsep :: [Doc] -> Doc
($$) :: Doc -> Doc -> Doc
($+$) :: Doc -> Doc -> Doc
vcat :: [Doc] -> Doc
sep :: [Doc] -> Doc
cat :: [Doc] -> Doc
fsep :: [Doc] -> Doc
fcat :: [Doc] -> Doc
nest :: Int -> Doc -> Doc
hang :: Doc -> Int -> Doc -> Doc
punctuate :: Doc -> [Doc] -> [Doc]
isEmpty :: Doc -> Bool
render :: Doc -> String

data Style = Style {

mode :: Mode
lineLength :: Int
ribbonsPerLine :: Float

}

style :: Style
renderStyle :: Style -> Doc -> String
fullRender :: Mode -> Int -> Float -> (TextDetails -> a -> a) -> a -> Doc -> a

data Mode

= PageMode
| ZigZagMode
| LeftMode
| OneLineMode

data TextDetails

= Chr Char
| Str String
| PStr String

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`.
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 `<>`, `<+>`, `$$` and `$+$`, and anywhere in the argument list for `sep`, `hcat`, `hsep`, `vcat`, `fcat` etc.
(<>) :: Doc -> Doc -> Doc
Beside. `<>` is associative, with identity `empty`.
(<+>) :: Doc -> Doc -> Doc
Beside, separated by space, unless one of the arguments is `empty`. `<+>` is associative, with identity `empty`.
hcat :: [Doc] -> Doc
List version of `<>`.
hsep :: [Doc] -> Doc
List version of `<+>`.
($$) :: Doc -> Doc -> Doc

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
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 |

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

:: 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 |

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