    {-# LANGUAGE CPP #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Text.PrettyPrint.ANSI.Leijen.Internal
    -- Copyright   :  Daan Leijen (c) 2000, http://www.cs.uu.nl/~daan
    --                Max Bolingbroke (c) 2008, http://blog.omega-prime.co.uk
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  Edward Kmett <ekmett@gmail.com>
    -- Stability   :  provisional
    -- Portability :  portable
    --
    --
    -- __WARNING:__ Internal module. The contents of this file may vary arbitrarily
    -- between any two versions. Use the public API if you care about stability.
    module Text.PrettyPrint.ANSI.Leijen.Internal where

    import System.IO (Handle,hPutStr,hPutChar,stdout)

    import System.Console.ANSI (Color(..), ColorIntensity(..), ConsoleLayer(..),
                                Underlining(..), ConsoleIntensity(..),
                                SGR(..), hSetSGR, setSGRCode)

    import Data.String (IsString(..))
    import Data.Maybe (catMaybes)

    -- NB: if you import more from Data.Semigroup make sure the
    --     build-depends version range is still accurate
    -- NB2: if you consider re-exporting Semigroup((<>)) take into account
    --      that only starting with semigroup-0.8 `infixr 6 <>` was used!
    import qualified Data.Semigroup as Semi (Semigroup((<>)))

    #if __GLASGOW_HASKELL__ >= 710
    import Data.Monoid ((<>))
    #elif __GLASGOW_HASKELL__ >= 704
    import Data.Monoid (Monoid, mappend, mconcat, mempty, (<>))
    #else
    import Data.Monoid (Monoid, mappend, mconcat, mempty)
    infixr 6 <>
    #endif

    infixr 6 <+>
    infixr 5 </>,<//>,<$>,<$$>

    -----------------------------------------------------------
    -- list, tupled and semiBraces pretty print a list of
    -- documents either horizontally or vertically aligned.
    -----------------------------------------------------------

    -- | 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.
    list :: [Doc] -> Doc
    list            = encloseSep lbracket rbracket comma

    -- | 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.
    tupled :: [Doc] -> Doc
    tupled          = encloseSep lparen   rparen  comma

    -- | The document @(semiBraces xs)@ separates the documents @xs@ with
    -- semicolons and encloses them in braces. The documents are rendered
    -- horizontally if that fits the page. Otherwise they are aligned
    -- vertically. All semicolons are put in front of the elements.
    semiBraces :: [Doc] -> Doc
    semiBraces      = encloseSep lbrace   rbrace  semi

    -- | 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]
    -- @
    encloseSep :: Doc -> Doc -> Doc -> [Doc] -> Doc
    encloseSep left right sep ds
        = case ds of
            []  -> left <> right
            [d] -> left <> d <> right
            _   -> align (cat (zipWith (<>) (left : repeat sep) ds) <> right)

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

    -- | @(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'.)
    punctuate :: Doc -> [Doc] -> [Doc]
    punctuate p []      = []
    punctuate p [d]     = [d]
    punctuate p (d:ds)  = (d <> p) : punctuate p ds

    -----------------------------------------------------------
    -- high-level combinators
    -----------------------------------------------------------

    -- | The document @(sep xs)@ concatenates all documents @xs@ either
    -- horizontally with @(\<+\>)@, if it fits the page, or vertically with
    -- @(\<$\>)@.
    --
    -- > sep xs  = group (vsep xs)
    sep :: [Doc] -> Doc
    sep             = group . vsep

    -- | 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
    fillSep :: [Doc] -> Doc
    fillSep         = fold (</>)

    -- | The document @(hsep xs)@ concatenates all documents @xs@
    -- horizontally with @(\<+\>)@.
    hsep :: [Doc] -> Doc
    hsep            = fold (<+>)

    -- | The document @(vsep xs)@ concatenates all documents @xs@
    -- vertically with @(\<$\>)@. If a 'group' undoes the line breaks
    -- inserted by @vsep@, all documents are separated with a space.
    --
    -- > someText = map text (words ("text to lay out"))
    -- >
    -- > test     = text "some" <+> vsep someText
    --
    -- This is layed out as:
    --
    -- @
    -- some text
    -- to
    -- lay
    -- out
    -- @
    --
    -- The 'align' combinator can be used to align the documents under
    -- their first element
    --
    -- > test     = text "some" <+> align (vsep someText)
    --
    -- Which is printed as:
    --
    -- @
    -- some text
    --      to
    --      lay
    --      out
    -- @
    vsep :: [Doc] -> Doc
    vsep            = fold (Text.PrettyPrint.ANSI.Leijen.Internal.<$>)

    -- | The document @(cat xs)@ concatenates all documents @xs@ either
    -- horizontally with @(\<\>)@, if it fits the page, or vertically with
    -- @(\<$$\>)@.
    --
    -- > cat xs  = group (vcat xs)
    cat :: [Doc] -> Doc
    cat             = group . vcat

    -- | 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
    fillCat :: [Doc] -> Doc
    fillCat         = fold (<//>)

    -- | The document @(hcat xs)@ concatenates all documents @xs@
    -- horizontally with @(\<\>)@.
    hcat :: [Doc] -> Doc
    hcat            = fold (<>)

    -- | The document @(vcat xs)@ concatenates all documents @xs@
    -- vertically with @(\<$$\>)@. If a 'group' undoes the line breaks
    -- inserted by @vcat@, all documents are directly concatenated.
    vcat :: [Doc] -> Doc
    vcat            = fold (<$$>)

    fold :: (Doc -> Doc -> Doc) -> [Doc] -> Doc
    fold f []       = empty
    fold f ds       = foldr1 f ds

    #if __GLASGOW_HASKELL__ < 704
    -- | 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)
    (<>) :: Doc -> Doc -> Doc
    x <> y          = x `beside` y
    #endif

    -- | The document @(x \<+\> y)@ concatenates document @x@ and @y@ with a
    -- @space@ in between.  (infixr 6)
    (<+>) :: Doc -> Doc -> Doc
    x <+> y         = x <> space <> y

    -- | 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)
    (</>) :: Doc -> Doc -> Doc
    x </> y         = x <> softline <> y

    -- | 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)
    (<//>) :: Doc -> Doc -> Doc
    x <//> y        = x <> softbreak <> y

    -- | The document @(x \<$\> y)@ concatenates document @x@ and @y@ with a
    -- 'line' in between. (infixr 5)
    (<$>) :: Doc -> Doc -> Doc
    x <$> y         = x <> line <> y

    -- | The document @(x \<$$\> y)@ concatenates document @x@ and @y@ with
    -- a @linebreak@ in between. (infixr 5)
    (<$$>) :: Doc -> Doc -> Doc
    x <$$> y        = x <> linebreak <> y

    -- | The document @softline@ behaves like 'space' if the resulting
    -- output fits the page, otherwise it behaves like 'line'.
    --
    -- > softline = group line
    softline :: Doc
    softline        = group line

    -- | The document @softbreak@ behaves like 'empty' if the resulting
    -- output fits the page, otherwise it behaves like 'line'.
    --
    -- > softbreak  = group linebreak
    softbreak :: Doc
    softbreak       = group linebreak

    -- | Document @(squotes x)@ encloses document @x@ with single quotes
    -- \"'\".
    squotes :: Doc -> Doc
    squotes         = enclose squote squote

    -- | Document @(dquotes x)@ encloses document @x@ with double quotes
    -- '\"'.
    dquotes :: Doc -> Doc
    dquotes         = enclose dquote dquote

    -- | Document @(braces x)@ encloses document @x@ in braces, \"{\" and
    -- \"}\".
    braces :: Doc -> Doc
    braces          = enclose lbrace rbrace

    -- | Document @(parens x)@ encloses document @x@ in parenthesis, \"(\"
    -- and \")\".
    parens :: Doc -> Doc
    parens          = enclose lparen rparen

    -- | Document @(angles x)@ encloses document @x@ in angles, \"\<\" and
    -- \"\>\".
    angles :: Doc -> Doc
    angles          = enclose langle rangle

    -- | Document @(brackets x)@ encloses document @x@ in square brackets,
    -- \"[\" and \"]\".
    brackets :: Doc -> Doc
    brackets        = enclose lbracket rbracket

    -- | The document @(enclose l r x)@ encloses document @x@ between
    -- documents @l@ and @r@ using @(\<\>)@.
    --
    -- > enclose l r x   = l <> x <> r
    enclose :: Doc -> Doc -> Doc -> Doc
    enclose l r x   = l <> x <> r

    -- | The document @lparen@ contains a left parenthesis, \"(\".
    lparen :: Doc
    lparen          = char '('
    -- | The document @rparen@ contains a right parenthesis, \")\".
    rparen :: Doc
    rparen          = char ')'
    -- | The document @langle@ contains a left angle, \"\<\".
    langle :: Doc
    langle          = char '<'
    -- | The document @rangle@ contains a right angle, \">\".
    rangle :: Doc
    rangle          = char '>'
    -- | The document @lbrace@ contains a left brace, \"{\".
    lbrace :: Doc
    lbrace          = char '{'
    -- | The document @rbrace@ contains a right brace, \"}\".
    rbrace :: Doc
    rbrace          = char '}'
    -- | The document @lbracket@ contains a left square bracket, \"[\".
    lbracket :: Doc
    lbracket        = char '['
    -- | The document @rbracket@ contains a right square bracket, \"]\".
    rbracket :: Doc
    rbracket        = char ']'

    -- | The document @squote@ contains a single quote, \"'\".
    squote :: Doc
    squote          = char '\''
    -- | The document @dquote@ contains a double quote, '\"'.
    dquote :: Doc
    dquote          = char '"'
    -- | The document @semi@ contains a semicolon, \";\".
    semi :: Doc
    semi            = char ';'
    -- | The document @colon@ contains a colon, \":\".
    colon :: Doc
    colon           = char ':'
    -- | The document @comma@ contains a comma, \",\".
    comma :: Doc
    comma           = char ','
    -- | The document @space@ contains a single space, \" \".
    --
    -- > x <+> y   = x <> space <> y
    space :: Doc
    space           = char ' '
    -- | The document @dot@ contains a single dot, \".\".
    dot :: Doc
    dot             = char '.'
    -- | The document @backslash@ contains a back slash, \"\\\".
    backslash :: Doc
    backslash       = char '\\'
    -- | The document @equals@ contains an equal sign, \"=\".
    equals :: Doc
    equals          = char '='

    -----------------------------------------------------------
    -- Combinators for prelude types
    -----------------------------------------------------------

    -- string is like "text" but replaces '\n' by "line"

    -- | 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.
    string :: String -> Doc
    string ""       = empty
    string ('\n':s) = line <> string s
    string s        = case (span (/='\n') s) of
                        (xs,ys) -> text xs <> string ys

    -- | The document @(bool b)@ shows the literal bool @b@ using 'text'.
    bool :: Bool -> Doc
    bool b          = text (show b)

    -- | The document @(int i)@ shows the literal integer @i@ using 'text'.
    int :: Int -> Doc
    int i           = text (show i)

    -- | The document @(integer i)@ shows the literal integer @i@ using 'text'.
    integer :: Integer -> Doc
    integer i       = text (show i)

    -- | The document @(float f)@ shows the literal float @f@ using 'text'.
    float :: Float -> Doc
    float f         = text (show f)

    -- | The document @(double d)@ shows the literal double @d@ using 'text'.
    double :: Double -> Doc
    double d        = text (show d)

    -- | The document @(rational r)@ shows the literal rational @r@ using 'text'.
    rational :: Rational -> Doc
    rational r      = text (show r)

    -----------------------------------------------------------
    -- overloading "pretty"
    -----------------------------------------------------------

    -- | The member @prettyList@ is only used to define the @instance Pretty
    -- a => Pretty [a]@. In normal circumstances only the @pretty@ function
    -- is used.
    class Pretty a where
      pretty        :: a -> Doc
      prettyList    :: [a] -> Doc
      prettyList    = list . map pretty

    instance Pretty a => Pretty [a] where
      pretty        = prettyList

    instance Pretty Doc where
      pretty        = id

    instance Pretty () where
      pretty ()     = text "()"

    instance Pretty Bool where
      pretty b      = bool b

    instance Pretty Char where
      pretty c      = char c
      prettyList s  = string s

    instance Pretty Int where
      pretty i      = int i

    instance Pretty Integer where
      pretty i      = integer i

    instance Pretty Float where
      pretty f      = float f

    instance Pretty Double where
      pretty d      = double d

    --instance Pretty Rational where
    --  pretty r      = rational r

    instance (Pretty a,Pretty b) => Pretty (a,b) where
      pretty (x,y)  = tupled [pretty x, pretty y]

    instance (Pretty a,Pretty b,Pretty c) => Pretty (a,b,c) where
      pretty (x,y,z)= tupled [pretty x, pretty y, pretty z]

    instance Pretty a => Pretty (Maybe a) where
      pretty Nothing        = empty
      pretty (Just x)       = pretty x

    -----------------------------------------------------------
    -- semi primitive: fill and fillBreak
    -----------------------------------------------------------

    -- | The document @(fillBreak i x)@ first renders document @x@. It
    -- than appends @space@s 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
    -- @
    fillBreak :: Int -> Doc -> Doc
    fillBreak f x   = width x (\w ->
                      if (w > f) then nest f linebreak
                                 else text (spaces (f - w)))

    -- | The document @(fill i x)@ renders document @x@. It than appends
    -- @space@s 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
    -- @
    fill :: Int -> Doc -> Doc
    fill f d        = width d (\w ->
                      if (w >= f) then empty
                                  else text (spaces (f - w)))

    width :: Doc -> (Int -> Doc) -> Doc
    width d f       = column (\k1 -> d <> column (\k2 -> f (k2 - k1)))

    -----------------------------------------------------------
    -- semi primitive: Alignment and indentation
    -----------------------------------------------------------

    -- | 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 !
    -- @
    indent :: Int -> Doc -> Doc
    indent i d      = hang i (text (spaces i) <> d)

    -- | 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)
    hang :: Int -> Doc -> Doc
    hang i d        = align (nest i d)

    -- | 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
    -- @
    align :: Doc -> Doc
    align d         = column (\k ->
                      nesting (\i -> nest (k - i) d))   --nesting might be negative :-)

    -----------------------------------------------------------
    -- Primitives
    -----------------------------------------------------------

    -- | The abstract data type @Doc@ represents pretty documents.
    --
    -- More specifically, a value of type @Doc@ represents a non-empty set of
    -- possible renderings of a document.  The rendering functions select one of
    -- these possibilities.
    --
    -- @Doc@ is an instance of the 'Show' class. @(show doc)@ pretty
    -- prints document @doc@ with a page width of 80 characters and a
    -- ribbon width of 32 characters.
    --
    -- > show (text "hello" <$> text "world")
    --
    -- Which would return the string \"hello\\nworld\", i.e.
    --
    -- @
    -- hello
    -- world
    -- @
    data Doc        = Fail
                    | Empty
                    | Char Char             -- invariant: char is not '\n'
                    | Text !Int String      -- invariant: text doesn't contain '\n'
                    | Line
                    | FlatAlt Doc Doc       -- Render the first doc, but when
                                            -- flattened, render the second.
                    | Cat Doc Doc
                    | Nest !Int Doc
                    | Union Doc Doc         -- invariant: first lines of first doc longer than the first lines of the second doc
                    | Column  (Int -> Doc)
                    | Columns (Maybe Int -> Doc)
                    | Nesting (Int -> Doc)
                    | Color ConsoleLayer ColorIntensity -- Introduces coloring /around/ the embedded document
                            Color Doc
                    | Intensify ConsoleIntensity Doc
                    | Italicize Bool Doc
                    | Underline Underlining Doc
                    | RestoreFormat (Maybe (ColorIntensity, Color))  -- Only used during the rendered phase, to signal a SGR should be issued to restore the terminal formatting.
                                    (Maybe (ColorIntensity, Color))  -- These are the colors to revert the current forecolor/backcolor to (i.e. those from before the start of the Color block).
                                    (Maybe ConsoleIntensity)         -- Intensity to revert to.
                                    (Maybe Bool)                     -- Italicization to revert to.
                                    (Maybe Underlining)              -- Underlining to revert to.

    -- | The data type @SimpleDoc@ represents rendered documents and is
    -- used by the display functions.
    --
    -- Whereas values of the data type 'Doc' represent non-empty sets of possible
    -- renderings of a document, values of the data type @SimpleDoc@ represent
    -- single renderings of a document.
    --
    -- 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.
    data SimpleDoc  = SFail
                    | SEmpty
                    | SChar Char SimpleDoc
                    | SText !Int String SimpleDoc
                    | SLine !Int SimpleDoc
                    | SSGR [SGR] SimpleDoc

    -- MCB: Not in the wl-pprint package that we forked from. I added this when the "pretty" package
    -- from base gained a Monoid instance (<http://hackage.haskell.org/trac/ghc/ticket/4378>):
    instance Monoid Doc where
        mempty = empty
        mappend = (Semi.<>)
        mconcat = hcat

    instance Semi.Semigroup Doc where
        (<>) = beside

    -- MCB: also added when "pretty" got the corresponding instances:
    instance IsString Doc where
        fromString = text

    -- | 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 @\<$\>@).
    empty :: Doc
    empty           = Empty

    -- | 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.
    char :: Char -> Doc
    char '\n'       = line
    char c          = Char c

    -- | 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.
    text :: String -> Doc
    text ""         = Empty
    text s          = Text (length s) s

    -- | 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'.
    line :: Doc
    line            = FlatAlt Line space

    -- | 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'.
    linebreak :: Doc
    linebreak       = FlatAlt Line empty

    -- | A linebreak that will never be flattened; it is guaranteed to render
    -- as a newline.
    hardline :: Doc
    hardline = Line

    beside :: Doc -> Doc -> Doc
    beside x y      = Cat x y

    -- | 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
    -- !
    -- @
    nest :: Int -> Doc -> Doc
    nest i x        = Nest i x

    column, nesting :: (Int -> Doc) -> Doc
    column f        = Column f
    nesting f       = Nesting f

    columns :: (Maybe Int -> Doc) -> Doc
    columns f       = Columns f

    -- | 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.
    group :: Doc -> Doc
    group x         = Union (flatten x) x

    -- | A document that is normally rendered as the first argument, but
    -- when flattened, is rendered as the second document.
    flatAlt :: Doc -> Doc -> Doc
    flatAlt = FlatAlt

    flatten :: Doc -> Doc
    flatten (FlatAlt x y)    = y
    flatten (Cat x y)        = Cat (flatten x) (flatten y)
    flatten (Nest i x)       = Nest i (flatten x)
    flatten  Line            = Fail
    flatten (Union x y)      = flatten x
    flatten (Column f)       = Column (flatten . f)
    flatten (Columns f)      = Columns (flatten . f)
    flatten (Nesting f)      = Nesting (flatten . f)
    flatten (Color l i c x)  = Color l i c (flatten x)
    flatten (Intensify i x)  = Intensify i (flatten x)
    flatten (Italicize b x)  = Italicize b (flatten x)
    flatten (Underline u x)  = Underline u (flatten x)
    flatten other            = other                     --Empty,Char,Text,RestoreFormat

    -----------------------------------------------------------
    -- Colors
    -----------------------------------------------------------

    -- | Displays a document with the black forecolor
    black :: Doc -> Doc
    -- | Displays a document with the red forecolor
    red :: Doc -> Doc
    -- | Displays a document with the green forecolor
    green :: Doc -> Doc
    -- | Displays a document with the yellow forecolor
    yellow :: Doc -> Doc
    -- | Displays a document with the blue forecolor
    blue :: Doc -> Doc
    -- | Displays a document with the magenta forecolor
    magenta :: Doc -> Doc
    -- | Displays a document with the cyan forecolor
    cyan :: Doc -> Doc
    -- | Displays a document with the white forecolor
    white :: Doc -> Doc
    -- | Displays a document with the dull black forecolor
    dullblack :: Doc -> Doc
    -- | Displays a document with the dull red forecolor
    dullred :: Doc -> Doc
    -- | Displays a document with the dull green forecolor
    dullgreen :: Doc -> Doc
    -- | Displays a document with the dull yellow forecolor
    dullyellow :: Doc -> Doc
    -- | Displays a document with the dull blue forecolor
    dullblue :: Doc -> Doc
    -- | Displays a document with the dull magenta forecolor
    dullmagenta :: Doc -> Doc
    -- | Displays a document with the dull cyan forecolor
    dullcyan :: Doc -> Doc
    -- | Displays a document with the dull white forecolor
    dullwhite :: Doc -> Doc
    (black, dullblack)     = colorFunctions Black
    (red, dullred)         = colorFunctions Red
    (green, dullgreen)     = colorFunctions Green
    (yellow, dullyellow)   = colorFunctions Yellow
    (blue, dullblue)       = colorFunctions Blue
    (magenta, dullmagenta) = colorFunctions Magenta
    (cyan, dullcyan)       = colorFunctions Cyan
    (white, dullwhite)     = colorFunctions White

    -- | Displays a document with a forecolor given in the first parameter
    color :: Color -> Doc -> Doc
    -- | Displays a document with a dull forecolor given in the first parameter
    dullcolor :: Color -> Doc -> Doc
    color     = Color Foreground Vivid
    dullcolor = Color Foreground Dull

    colorFunctions :: Color -> (Doc -> Doc, Doc -> Doc)
    colorFunctions what = (color what, dullcolor what)

    -- | Displays a document with the black backcolor
    onblack :: Doc -> Doc
    -- | Displays a document with the red backcolor
    onred :: Doc -> Doc
    -- | Displays a document with the green backcolor
    ongreen :: Doc -> Doc
    -- | Displays a document with the yellow backcolor
    onyellow :: Doc -> Doc
    -- | Displays a document with the blue backcolor
    onblue :: Doc -> Doc
    -- | Displays a document with the magenta backcolor
    onmagenta :: Doc -> Doc
    -- | Displays a document with the cyan backcolor
    oncyan :: Doc -> Doc
    -- | Displays a document with the white backcolor
    onwhite :: Doc -> Doc
    -- | Displays a document with the dull black backcolor
    ondullblack :: Doc -> Doc
    -- | Displays a document with the dull red backcolor
    ondullred :: Doc -> Doc
    -- | Displays a document with the dull green backcolor
    ondullgreen :: Doc -> Doc
    -- | Displays a document with the dull yellow backcolor
    ondullyellow :: Doc -> Doc
    -- | Displays a document with the dull blue backcolor
    ondullblue :: Doc -> Doc
    -- | Displays a document with the dull magenta backcolor
    ondullmagenta :: Doc -> Doc
    -- | Displays a document with the dull cyan backcolor
    ondullcyan :: Doc -> Doc
    -- | Displays a document with the dull white backcolor
    ondullwhite :: Doc -> Doc
    (onblack, ondullblack)     = oncolorFunctions Black
    (onred, ondullred)         = oncolorFunctions Red
    (ongreen, ondullgreen)     = oncolorFunctions Green
    (onyellow, ondullyellow)   = oncolorFunctions Yellow
    (onblue, ondullblue)       = oncolorFunctions Blue
    (onmagenta, ondullmagenta) = oncolorFunctions Magenta
    (oncyan, ondullcyan)       = oncolorFunctions Cyan
    (onwhite, ondullwhite)     = oncolorFunctions White

    -- | Displays a document with a backcolor given in the first parameter
    oncolor :: Color -> Doc -> Doc
    -- | Displays a document with a dull backcolor given in the first parameter
    ondullcolor :: Color -> Doc -> Doc
    oncolor     = Color Background Vivid
    ondullcolor = Color Background Dull

    oncolorFunctions :: Color -> (Doc -> Doc, Doc -> Doc)
    oncolorFunctions what = (oncolor what, ondullcolor what)

    -----------------------------------------------------------
    -- Console Intensity
    -----------------------------------------------------------

    -- | Displays a document in a heavier font weight
    bold :: Doc -> Doc
    bold = Intensify BoldIntensity

    -- | Displays a document in the normal font weight
    debold :: Doc -> Doc
    debold = Intensify NormalIntensity

    -- NB: I don't support FaintIntensity here because it is not widely supported by terminals.

    -----------------------------------------------------------
    -- Italicization
    -----------------------------------------------------------

    {-

    I'm in two minds about providing these functions, since italicization is so rarely implemented.
    It is especially bad because "italicization" may cause the meaning of colors to flip, which will
    look a bit weird, to say the least...

    -- | Displays a document in italics. This is not widely supported, and it's use is not recommended
    italicize :: Doc -> Doc
    italicize = Italicize True

    -- | Displays a document with no italics
    deitalicize :: Doc -> Doc
    deitalicize = Italicize False

    -}

    -----------------------------------------------------------
    -- Underlining
    -----------------------------------------------------------

    -- | Displays a document with underlining
    underline :: Doc -> Doc
    underline = Underline SingleUnderline

    -- | Displays a document with no underlining
    deunderline :: Doc -> Doc
    deunderline = Underline NoUnderline

    -- NB: I don't support DoubleUnderline here because it is not widely supported by terminals.

    -----------------------------------------------------------
    -- Removing formatting
    -----------------------------------------------------------

    -- | Removes all colorisation, emboldening and underlining from a document
    plain :: Doc -> Doc
    plain Fail            = Fail
    plain e@Empty         = e
    plain c@(Char _)      = c
    plain t@(Text _ _)    = t
    plain l@Line          = l
    plain (FlatAlt x y)   = FlatAlt (plain x) (plain y)
    plain (Cat x y)       = Cat (plain x) (plain y)
    plain (Nest i x)      = Nest i (plain x)
    plain (Union x y)     = Union (plain x) (plain y)
    plain (Column f)      = Column (plain . f)
    plain (Columns f)     = Columns (plain . f)
    plain (Nesting f)     = Nesting (plain . f)
    plain (Color _ _ _ x) = plain x
    plain (Intensify _ x) = plain x
    plain (Italicize _ x) = plain x
    plain (Underline _ x) = plain x
    plain (RestoreFormat _ _ _ _ _) = Empty

    -----------------------------------------------------------
    -- Renderers
    -----------------------------------------------------------

    -----------------------------------------------------------
    -- renderPretty: the default pretty printing algorithm
    -----------------------------------------------------------

    -- list of indentation/document pairs; saves an indirection over [(Int,Doc)]
    data Docs   = Nil
                | Cons !Int Doc Docs

    -- | 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.
    renderPretty :: Float -> Int -> Doc -> SimpleDoc
    renderPretty = renderFits fits1

    -- | A slightly smarter rendering algorithm with more lookahead. It provides
    -- provide earlier breaking on deeply nested structures
    -- For example, consider this python-ish pseudocode:
    -- @fun(fun(fun(fun(fun([abcdefg, abcdefg])))))@
    -- If we put a softbreak (+ nesting 2) after each open parenthesis, and align
    -- the elements of the list to match the opening brackets, this will render with
    -- @renderPretty@ and a page width of 20 as:
    -- @
    -- fun(fun(fun(fun(fun([
    --                     | abcdef,
    --                     | abcdef,
    --                     ]
    --   )))))             |
    -- @
    -- Where the 20c. boundary has been marked with |.
    -- Because @renderPretty@ only uses one-line lookahead, it sees that the first
    -- line fits, and is stuck putting the second and third lines after the 20-c
    -- mark. In contrast, @renderSmart@ will continue to check that the potential
    -- document up to the end of the indentation level. Thus, it will format the
    -- document as:
    --
    -- @
    -- fun(                |
    --   fun(              |
    --     fun(            |
    --       fun(          |
    --         fun([       |
    --               abcdef,
    --               abcdef,
    --             ]       |
    --   )))))             |
    -- @
    -- Which fits within the 20c. boundary.
    renderSmart :: Float -> Int -> Doc -> SimpleDoc
    renderSmart = renderFits fitsR

    renderFits :: (Int -> Int -> Int -> SimpleDoc -> Bool)
               -> Float -> Int -> Doc -> SimpleDoc
    renderFits fits rfrac w x
        -- I used to do a @SSGR [Reset]@ here, but if you do that it will result
        -- in any rendered @Doc@ containing at least some ANSI control codes. This
        -- may be undesirable if you want to render to non-ANSI devices by simply
        -- not making use of the ANSI color combinators I provide.
        --
        -- What I "really" want to do here is do an initial Reset iff there is some
        -- ANSI color within the Doc, but that's a bit fiddly. I'll fix it if someone
        -- complains!
        = best 0 0 Nothing Nothing Nothing Nothing Nothing (Cons 0 x Nil)
        where
          -- r :: the ribbon width in characters
          r  = max 0 (min w (round (fromIntegral w * rfrac)))

          -- best :: n = indentation of current line
          --         k = current column
          --        (ie. (k >= n) && (k - n == count of inserted characters)
          best n k mb_fc mb_bc mb_in mb_it mb_un Nil = SEmpty
          best n k mb_fc mb_bc mb_in mb_it mb_un (Cons i d ds)
            = case d of
                Fail          -> SFail
                Empty         -> best_typical n k ds
                Char c        -> let k' = k+1 in seq k' (SChar c (best_typical n k' ds))
                Text l s      -> let k' = k+l in seq k' (SText l s (best_typical n k' ds))
                Line          -> SLine i (best_typical i i ds)
                FlatAlt x _   -> best_typical n k (Cons i x ds)
                Cat x y       -> best_typical n k (Cons i x (Cons i y ds))
                Nest j x      -> let i' = i+j in seq i' (best_typical n k (Cons i' x ds))
                Union x y     -> nicest n k (best_typical n k (Cons i x ds))
                                            (best_typical n k (Cons i y ds))
                Column f      -> best_typical n k (Cons i (f k) ds)
                Columns f     -> best_typical n k (Cons i (f (Just w)) ds)
                Nesting f     -> best_typical n k (Cons i (f i) ds)
                Color l t c x -> SSGR [SetColor l t c] (best n k mb_fc' mb_bc' mb_in mb_it mb_un (Cons i x ds_restore))
                  where
                    mb_fc' = case l of { Background -> mb_fc; Foreground -> Just (t, c) }
                    mb_bc' = case l of { Background -> Just (t, c); Foreground -> mb_bc }
                Intensify t x -> SSGR [SetConsoleIntensity t] (best n k mb_fc mb_bc (Just t) mb_it mb_un (Cons i x ds_restore))
                Italicize t x -> SSGR [SetItalicized t] (best n k mb_fc mb_bc mb_in (Just t) mb_un (Cons i x ds_restore))
                Underline u x -> SSGR [SetUnderlining u] (best n k mb_fc mb_bc mb_in mb_it (Just u) (Cons i x ds_restore))
                RestoreFormat mb_fc' mb_bc' mb_in' mb_it' mb_un' -> SSGR sgrs (best n k mb_fc' mb_bc' mb_in' mb_it' mb_un' ds)
                  where
                    -- We need to be able to restore the entire SGR state, hence we carry around what we believe
                    -- that state should be in all the arguments to this function. Note that in some cases we could
                    -- avoid the Reset of the entire state, but not in general.
                    sgrs = Reset : catMaybes [
                        fmap (uncurry (SetColor Foreground)) mb_fc',
                        fmap (uncurry (SetColor Background)) mb_bc',
                        fmap SetConsoleIntensity mb_in',
                        fmap SetItalicized mb_it',
                        fmap SetUnderlining mb_un'
                      ]
            where
              best_typical n' k' ds' = best n' k' mb_fc mb_bc mb_in mb_it mb_un ds'
              ds_restore = Cons i (RestoreFormat mb_fc mb_bc mb_in mb_it mb_un) ds

          --nicest :: r = ribbon width, w = page width,
          --          n = indentation of current line, k = current column
          --          x and y, the (simple) documents to chose from.
          --          precondition: first lines of x are longer than the first lines of y.
          nicest n k x y    | fits w (min n k) width x  = x
                            | otherwise     = y
                            where
                              width = min (w - k) (r - k + n)

    -- @fits1@ does 1 line lookahead.
    fits1 :: Int -> Int -> Int -> SimpleDoc -> Bool
    fits1 _ _ w x        | w < 0         = False
    fits1 _ _ w SFail                    = False
    fits1 _ _ w SEmpty                   = True
    fits1 p m w (SChar c x)              = fits1 p m (w - 1) x
    fits1 p m w (SText l s x)            = fits1 p m (w - l) x
    fits1 _ _ w (SLine i x)              = True
    fits1 p m w (SSGR _ x)               = fits1 p m w x

    -- @fitsR@ has a little more lookahead: assuming that nesting roughly
    -- corresponds to syntactic depth, @fitsR@ checks that not only the current line
    -- fits, but the entire syntactic structure being formatted at this level of
    -- indentation fits. If we were to remove the second case for @SLine@, we would
    -- check that not only the current structure fits, but also the rest of the
    -- document, which would be slightly more intelligent but would have exponential
    -- runtime (and is prohibitively expensive in practice).
    -- p = pagewidth
    -- m = minimum nesting level to fit in
    -- w = the width in which to fit the first line
    fitsR :: Int -> Int -> Int -> SimpleDoc -> Bool
    fitsR p m w x        | w < 0         = False
    fitsR p m w SFail                    = False
    fitsR p m w SEmpty                   = True
    fitsR p m w (SChar c x)              = fitsR p m (w - 1) x
    fitsR p m w (SText l s x)            = fitsR p m (w - l) x
    fitsR p m w (SLine i x) | m < i      = fitsR p m (p - i) x
                            | otherwise  = True
    fitsR p m w (SSGR _ x)               = fitsR p m w x

    -----------------------------------------------------------
    -- renderCompact: renders documents without indentation
    --  fast and fewer characters output, good for machines
    -----------------------------------------------------------

    -- | @(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.
    renderCompact :: Doc -> SimpleDoc
    renderCompact x
        = scan 0 [x]
        where
          scan k []     = SEmpty
          scan k (d:ds) = case d of
                            Fail                    -> SFail
                            Empty                   -> scan k ds
                            Char c                  -> let k' = k+1 in seq k' (SChar c (scan k' ds))
                            Text l s                -> let k' = k+l in seq k' (SText l s (scan k' ds))
                            FlatAlt x _             -> scan k (x:ds)
                            Line                    -> SLine 0 (scan 0 ds)
                            Cat x y                 -> scan k (x:y:ds)
                            Nest j x                -> scan k (x:ds)
                            Union x y               -> scan k (y:ds)
                            Column f                -> scan k (f k:ds)
                            Columns f               -> scan k (f Nothing:ds)
                            Nesting f               -> scan k (f 0:ds)
                            Color _ _ _ x           -> scan k (x:ds)
                            Intensify _ x           -> scan k (x:ds)
                            Italicize _ x           -> scan k (x:ds)
                            Underline _ x           -> scan k (x:ds)
                            RestoreFormat _ _ _ _ _ -> scan k ds

    -----------------------------------------------------------
    -- Displayers:  displayS and displayIO
    -----------------------------------------------------------

    -- | @(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.
    displayS :: SimpleDoc -> ShowS
    displayS SFail              = error $ "@SFail@ can not appear uncaught in a " ++
                                  "rendered @SimpleDoc@"
    displayS SEmpty             = id
    displayS (SChar c x)        = showChar c . displayS x
    displayS (SText l s x)      = showString s . displayS x
    displayS (SLine i x)        = showString ('\n':indentation i) . displayS x
    displayS (SSGR s x)         = showString (setSGRCode s) . displayS x

    -- | @(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 80 doc)
    --
    -- Any ANSI colorisation in @simpleDoc@ will be output.
    displayIO :: Handle -> SimpleDoc -> IO ()
    displayIO handle simpleDoc
        = display simpleDoc
        where
          display SFail         = error $ "@SFail@ can not appear uncaught in a " ++
                                  "rendered @SimpleDoc@"
          display SEmpty         = return ()
          display (SChar c x)    = do{ hPutChar handle c; display x}
          display (SText l s x)  = do{ hPutStr handle s; display x}
          display (SLine i x)    = do{ hPutStr handle ('\n':indentation i); display x}
          display (SSGR s x)     = do{ hSetSGR handle s; display x}

    -----------------------------------------------------------
    -- default pretty printers: show, putDoc and hPutDoc
    -----------------------------------------------------------
    instance Show Doc where
      showsPrec d doc       = displayS (renderPretty 0.4 80 doc)

    -- | The action @(putDoc doc)@ pretty prints document @doc@ to the
    -- standard output, with a page width of 80 characters and a ribbon
    -- width of 32 characters.
    --
    -- > main :: IO ()
    -- > main = do{ putDoc (text "hello" <+> text "world") }
    --
    -- Which would output
    --
    -- @
    -- hello world
    -- @
    --
    -- Any ANSI colorisation in @doc@ will be output.
    putDoc :: Doc -> IO ()
    putDoc doc              = hPutDoc stdout doc

    -- | @(hPutDoc handle doc)@ pretty prints document @doc@ to the file
    -- handle @handle@ with a page width of 80 characters and a ribbon
    -- width of 32 characters.
    --
    -- > main = do{ handle <- openFile "MyFile" WriteMode
    -- >          ; hPutDoc handle (vcat (map text
    -- >                            ["vertical","text"]))
    -- >          ; hClose handle
    -- >          }
    --
    -- Any ANSI colorisation in @doc@ will be output.
    hPutDoc :: Handle -> Doc -> IO ()
    hPutDoc handle doc  = displayIO handle (renderPretty 0.4 80 doc)

    -----------------------------------------------------------
    -- insert spaces
    -- "indentation" used to insert tabs but tabs seem to cause
    -- more trouble than they solve :-)
    -----------------------------------------------------------
    spaces :: Int -> String
    spaces n        | n <= 0    = ""
                    | otherwise = replicate n ' '

    indentation :: Int -> String
    indentation n   = spaces n

    --indentation n   | n >= 8    = '\t' : indentation (n-8)
    --                | otherwise = spaces n

    --  LocalWords:  PPrint combinators Wadler Wadler's encloseSep