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Text.PrettyPrint.ANSI.Leijen | Portability | portable | Stability | provisional | Maintainer | batterseapower@hotmail.com |
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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 <$>. The operator </> is used
for soft line breaks.
- There are three new primitives: align, fill and
fillBreak. These are very useful in practice.
- Lots of other useful combinators, like fillSep and list.
- There are two renderers, renderPretty for pretty printing and
renderCompact for compact output. The pretty printing algorithm
also uses a ribbon-width now for even prettier output.
- There are two displayers, displayS for strings and displayIO for
file based output.
- There is a Pretty class.
- The implementation uses optimised representations and strictness
annotations.
Full documentation for the original wl-pprint library available at
http://www.cs.uu.nl/~daan/download/pprint/pprint.html.
The library has been extended to allow formatting text for output
to ANSI style consoles. New combinators allow:
- Control of foreground and background color of text
- The abliity to make parts of the text bold or underlined
This functionality is, as far as possible, portable across platforms
with their varying terminals. However, one thing to be particularly
wary of is that console colors will not be displayed on Windows unless
the Doc value is output using the putDoc function or one of it's
friends. Rendering the Doc to a String and then outputing that
will only work on Unix-style operating systems.
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Synopsis |
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Documents
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The abstract data type Doc represents pretty documents.
Doc is an instance of the Show class. (show doc) pretty
prints document doc with a page width of 100 characters and a
ribbon width of 40 characters.
show (text "hello" <$> text "world")
Which would return the string "hello\nworld", i.e.
hello
world
| Instances | |
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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
Any ANSI colorisation in doc will be output.
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(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
}
Any ANSI colorisation in doc will be output.
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Basic combinators
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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 <$>).
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The document (char c) contains the literal character c. The
character shouldn't be a newline ('\n'), the function line
should be used for line breaks.
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The document (text s) contains the literal string s. The
string shouldn't contain any newline ('\n') characters. If the
string contains newline characters, the function string should be
used.
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The document (x <> y) concatenates document x and document
y. It is an associative operation having empty as a left and
right unit. (infixr 6)
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The document (nest i x) renders document x with the current
indentation level increased by i (See also hang, align and
indent).
nest 2 (text "hello" <$> text "world") <$> text "!"
outputs as:
hello
world
!
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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.
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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.
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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.
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The document softline behaves like space if the resulting
output fits the page, otherwise it behaves like line.
softline = group line
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The document softbreak behaves like empty if the resulting
output fits the page, otherwise it behaves like line.
softbreak = group linebreak
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Alignment
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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
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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)
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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 !
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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]
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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.
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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.
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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.
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Operators
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The document (x <+> y) concatenates document x and y with a
space in between. (infixr 6)
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The document (x <$> y) concatenates document x and y with a
line in between. (infixr 5)
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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)
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The document (x <$$> y) concatenates document x and y with
a linebreak in between. (infixr 5)
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The document (x <//> y) concatenates document x and y with
a softbreak in between. This effectively puts x and y either
right next to each other or underneath each other. (infixr 5)
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List combinators
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The document (hsep xs) concatenates all documents xs
horizontally with (<+>).
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The document (vsep xs) concatenates all documents xs
vertically with (<$>). If a group undoes the line breaks
inserted by vsep, all documents are separated with a space.
someText = map text (words ("text to lay out"))
test = text "some" <+> vsep someText
This is layed out as:
some text
to
lay
out
The align combinator can be used to align the documents under
their first element
test = text "some" <+> align (vsep someText)
Which is printed as:
some text
to
lay
out
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The document (fillSep xs) concatenates documents xs
horizontally with (<+>) as long as its fits the page, than
inserts a line and continues doing that for all documents in
xs.
fillSep xs = foldr (\<\/\>) empty xs
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The document (sep xs) concatenates all documents xs either
horizontally with (<+>), if it fits the page, or vertically with
(<$>).
sep xs = group (vsep xs)
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The document (hcat xs) concatenates all documents xs
horizontally with (<>).
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The document (vcat xs) concatenates all documents xs
vertically with (<$$>). If a group undoes the line breaks
inserted by vcat, all documents are directly concatenated.
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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
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The document (cat xs) concatenates all documents xs either
horizontally with (<>), if it fits the page, or vertically with
(<$$>).
cat xs = group (vcat xs)
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(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.)
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Fillers
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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
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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
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Bracketing combinators
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The document (enclose l r x) encloses document x between
documents l and r using (<>).
enclose l r x = l <> x <> r
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Document (squotes x) encloses document x with single quotes
"'".
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Document (dquotes x) encloses document x with double quotes
'"'.
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Document (parens x) encloses document x in parenthesis, "("
and ")".
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Document (angles x) encloses document x in angles, "<" and
">".
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Document (braces x) encloses document x in braces, "{" and
"}".
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Document (brackets x) encloses document x in square brackets,
"[" and "]".
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Character documents
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The document lparen contains a left parenthesis, "(".
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The document rparen contains a right parenthesis, ")".
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The document langle contains a left angle, "<".
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The document rangle contains a right angle, ">".
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The document lbrace contains a left brace, "{".
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The document rbrace contains a right brace, "}".
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The document lbracket contains a left square bracket, "[".
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The document rbracket contains a right square bracket, "]".
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The document squote contains a single quote, "'".
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The document dquote contains a double quote, '"'.
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The document semi contains a semi colon, ";".
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The document colon contains a colon, ":".
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The document comma contains a comma, ",".
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The document space contains a single space, " ".
x <+> y = x <> space <> y
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The document dot contains a single dot, ".".
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The document backslash contains a back slash, "\".
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The document equals contains an equal sign, "=".
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Colorisation combinators
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Displays a document with the black forecolor
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Displays a document with the red forecolor
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Displays a document with the green forecolor
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Displays a document with the yellow forecolor
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Displays a document with the blue forecolor
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Displays a document with the magenta forecolor
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Displays a document with the cyan forecolor
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Displays a document with the white forecolor
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Displays a document with the dull black forecolor
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Displays a document with the dull red forecolor
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Displays a document with the dull green forecolor
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Displays a document with the dull yellow forecolor
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Displays a document with the dull blue forecolor
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Displays a document with the dull magenta forecolor
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Displays a document with the dull cyan forecolor
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Displays a document with the dull white forecolor
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Displays a document with the black backcolor
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Displays a document with the red backcolor
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Displays a document with the green backcolor
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Displays a document with the yellow backcolor
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Displays a document with the blue backcolor
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Displays a document with the magenta backcolor
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Displays a document with the cyan backcolor
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Displays a document with the white backcolor
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Displays a document with the dull block backcolor
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Displays a document with the dull red backcolor
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Displays a document with the dull green backcolor
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Displays a document with the dull yellow backcolor
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Displays a document with the dull blue backcolor
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Displays a document with the dull magenta backcolor
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Displays a document with the dull cyan backcolor
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Displays a document with the dull white backcolor
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Emboldening combinators
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Displays a document in a heavier font weight
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Displays a document in the normal font weight
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Underlining combinators
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Displays a document with underlining
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Displays a document with no underlining
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Removing formatting
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Removes all colorisation, emboldening and underlining from a document
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Primitive type documents
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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.
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The document (int i) shows the literal integer i using
text.
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The document (integer i) shows the literal integer i using
text.
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The document (float f) shows the literal float f using
text.
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The document (double d) shows the literal double d using
text.
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The document (rational r) shows the literal rational r using
text.
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Pretty class
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The member prettyList is only used to define the instance Pretty
a => Pretty [a]. In normal circumstances only the pretty function
is used.
| | Methods | | | Instances | |
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Rendering
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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 | |
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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.
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(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.
This rendering function does not add any colorisation information.
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(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) ""
ANSI color information will be discarded by this function unless
you are running on a Unix-like operating system. This is due to
a technical limitation in Windows ANSI support.
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(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)
Any ANSI colorisation in simpleDoc will be output.
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Undocumented
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Produced by Haddock version 2.4.2 |