GHC.Utils.Ppr

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.cse.chalmers.se/~rjmh/Papers/pretty.ps

The document type

data Doc

The abstract type of documents. A Doc represents a *set* of layouts. A Doc with no occurrences of Union or NoDoc represents just one layout.

Instances

Show Doc

Defined in GHC.Utils.Ppr

Methods

showsPrec :: Int -> Doc -> ShowS

show :: Doc -> String

showList :: [Doc] -> ShowS

data TextDetails

The TextDetails data type

A TextDetails represents a fragment of text that will be output at some point.

Constructors

Chr !Char

A single Char fragment

Str String

A whole String fragment

PStr FastString

ZStr FastZString

LStr !PtrString

RStr !Int !Char

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.

ftext :: FastString -> Doc

ptext :: PtrString -> Doc

ztext :: FastZString -> Doc

sizedText :: Int -> String -> Doc

Some text with any width. (text s = sizedText (length s) s)

zeroWidthText :: String -> Doc

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

emptyText :: Doc

Empty text (one line high but no width). (emptyText = text "")

int

Arguments

:: Int

-> Doc

int n = text (show n)

integer

Arguments

:: Integer

-> Doc

integer n = text (show n)

float

Arguments

:: Float

-> Doc

float n = text (show n)

double

Arguments

:: Double

-> Doc

double n = text (show n)

rational

Arguments

:: Rational

-> Doc

rational n = text (show n)

hex

Arguments

:: Integer

-> Doc

See Note [Print Hexadecimal Literals]

Simple derived documents

semi

Arguments

:: Doc

A ';' character

comma

Arguments

:: Doc

A ',' character

colon

Arguments

:: Doc

A : character

space

Arguments

:: Doc

A space character

equals

Arguments

:: Doc

A '=' character

lparen

Arguments

:: Doc

A '(' character

rparen

Arguments

:: Doc

A ')' character

lbrack

Arguments

:: Doc

A '[' character

rbrack

Arguments

:: Doc

A ']' character

lbrace

Arguments

:: Doc

A '{' character

rbrace

Arguments

:: Doc

A '}' character

Wrapping documents in delimiters

parens

Arguments

:: Doc

-> Doc

Wrap document in (...)

brackets

Arguments

:: Doc

-> Doc

Wrap document in [...]

braces

Arguments

:: Doc

-> Doc

Wrap document in {...}

quotes

Arguments

:: Doc

-> Doc

Wrap document in `...'

squotes

Arguments

:: Doc

-> Doc

Wrap document in '...'

quote :: Doc -> Doc

doubleQuotes

Arguments

:: Doc

-> Doc

Wrap document in "..."

maybeParens :: Bool -> Doc -> Doc

Apply parens to Doc if boolean is true.

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]

hangNotEmpty :: Doc -> Int -> Doc -> Doc

Apply hang to the arguments if the first Doc is not empty.

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

docHead :: Doc -> (Maybe Char, Doc)

Get the first character of a document. We also return a new document, equivalent to the original one but faster to render. Use it to avoid work duplication.

Rendering documents

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 line length to ribbon length

style :: Style

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

renderStyle :: Style -> Doc -> String

Render the Doc to a String using the given Style.

data Mode

Rendering mode.

Constructors

PageMode

Normal

Fields

• asciiSpace :: Bool

ZigZagMode

With zig-zag cuts

LeftMode

No indentation, infinitely long lines

OneLineMode

All on one line

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.

txtPrinter :: TextDetails -> String -> String

Default TextDetails printer

GHC-specific rendering

printDoc :: Mode -> Int -> Handle -> Doc -> IO ()

printDoc_ :: Mode -> Int -> Handle -> Doc -> IO ()

bufLeftRender :: BufHandle -> Doc -> IO ()

printLeftRender :: Handle -> Doc -> IO ()