-- Hoogle documentation, generated by Haddock -- See Hoogle, http://www.haskell.org/hoogle/ -- | A free monad based on the Wadler/Leijen pretty printer -- -- A free monad based on the Wadler/Leijen pretty printer @package wl-pprint-extras @version 3.2 -- | Pretty print module based on Daan Leijen's implementation of Philip -- Wadler's "prettier printer" -- --

--   "A prettier printer"
--   Draft paper, April 1997, revised March 1998.
--   http://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf
--

-- -- This 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 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 display routines, displayS for strings and -- displayIO for file based output.
There is a Pretty class.
The implementation uses optimised representations and strictness -- annotations.
A type argument has been added and embedded effects can -- be seen in the SimpleDoc type.

module Text.PrettyPrint.Free.Internal -- | 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" `above` text "world")
--

-- -- Which would return the string "hello\nworld", i.e. -- --

--   hello
--   world
--

data Doc e Empty :: Doc e Char :: {-# UNPACK #-} !Char -> Doc e Text :: {-# UNPACK #-} !Int -> String -> Doc e Line :: !(Maybe Char) -> Doc e Cat :: (Doc e) -> (Doc e) -> Doc e Nest :: {-# UNPACK #-} !Int -> (Doc e) -> Doc e Union :: (Doc e) -> (Doc e) -> Doc e Effect :: e -> Doc e Column :: (Int -> Doc e) -> Doc e Nesting :: (Int -> Doc e) -> Doc e Columns :: (Int -> Doc e) -> Doc e Ribbon :: (Int -> Doc e) -> Doc e -- | 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
--

putDoc :: Doc e -> IO () -- | (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
--            }
--

hPutDoc :: Handle -> Doc e -> IO () -- | 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 e -- | 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 e -- | 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" `above` text "world") `above` text "!"
--

-- -- outputs as: -- --

--   hello
--     world
--   !
--

nest :: Int -> Doc e -> Doc e -- | 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 e -- | 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 e -- | 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 e -> Doc e -- | The document softline behaves like space if the -- resulting output fits the page, otherwise it behaves like line. -- --

--   softline = group line
--

softline :: Doc e -- | The document softbreak behaves like empty if the -- resulting output fits the page, otherwise it behaves like line. -- --

--   softbreak  = group linebreak
--

softbreak :: Doc e -- | 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 (above x y)
--

-- --

--   test    = text "hi" <+> (text "nice" $$ text "world")
--

-- -- which will be layed out as: -- --

--   hi nice
--      world
--

align :: Doc e -> Doc e -- | 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 e -> Doc e -- | 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 e -> Doc e -- | 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 :: Foldable f => Doc e -> Doc e -> Doc e -> f (Doc e) -> Doc e -- | 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 :: Foldable f => f (Doc e) -> Doc e -- | 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 :: Foldable f => f (Doc e) -> Doc e -- | 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. semiBraces :: Foldable f => f (Doc e) -> Doc e (<+>) :: Doc e -> Doc e -> Doc e -- | The document above x y concatenates document x and -- y with a line in between. (infixr 5) above :: Doc e -> Doc e -> Doc e -- | 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 e -> Doc e -> Doc e -- | The document aboveBreak x y concatenates document x -- and y with a linebreak in between. (infixr 5) aboveBreak :: Doc e -> Doc e -> Doc e -- | 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 e -> Doc e -> Doc e -- | The document (hsep xs) concatenates all documents xs -- horizontally with (<+>). hsep :: Foldable f => f (Doc e) -> Doc e -- | The document (vsep xs) concatenates all documents xs -- vertically with above. 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 :: Foldable f => f (Doc e) -> Doc e -- | 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 :: Foldable f => f (Doc e) -> Doc e -- | The document (sep xs) concatenates all documents xs -- either horizontally with (<+>), if it fits the page, or -- vertically with above. -- --

--   sep xs  = group (vsep xs)
--

sep :: Foldable f => f (Doc e) -> Doc e -- | The document (hcat xs) concatenates all documents xs -- horizontally with (<>). hcat :: Foldable f => f (Doc e) -> Doc e -- | The document (vcat xs) concatenates all documents xs -- vertically with aboveBreak. If a group undoes the line -- breaks inserted by vcat, all documents are directly -- concatenated. vcat :: Foldable f => f (Doc e) -> Doc e -- | 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 :: Foldable f => f (Doc e) -> Doc e -- | The document (cat xs) concatenates all documents xs -- either horizontally with (<>), if it fits the page, or -- vertically with aboveBreak. -- --

--   cat xs  = group (vcat xs)
--

cat :: Foldable f => f (Doc e) -> Doc e -- | (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 :: Traversable f => Doc e -> f (Doc e) -> f (Doc e) -- | 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 e")
--            ,("nest","Int -> Doc e -> Doc e")
--            ,("linebreak","Doc e")]
--   
--   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 e
--       nest   :: Int -> Doc e -> Doc e
--       linebreak :: Doc e
--

fill :: Int -> Doc e -> Doc e -- | 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 e
--       nest   :: Int -> Doc e -> Doc e 
--       linebreak
--              :: Doc e
--

fillBreak :: Int -> Doc e -> Doc e -- | The document (enclose l r x) encloses document x -- between documents l and r using (<>). -- --

--   enclose l r x   = l <> x <> r
--

enclose :: Doc e -> Doc e -> Doc e -> Doc e -- | Document (squotes x) encloses document x with single -- quotes "'". squotes :: Doc e -> Doc e -- | Document (dquotes x) encloses document x with double -- quotes '"'. dquotes :: Doc e -> Doc e -- | Document (parens x) encloses document x in -- parenthesis, "(" and ")". parens :: Doc e -> Doc e -- | Document (angles x) encloses document x in angles, -- "<" and ">". angles :: Doc e -> Doc e -- | Document (braces x) encloses document x in braces, -- "{" and "}". braces :: Doc e -> Doc e -- | Document (brackets x) encloses document x in square -- brackets, "[" and "]". brackets :: Doc e -> Doc e -- | The document lparen contains a left parenthesis, "(". lparen :: Doc e -- | The document rparen contains a right parenthesis, ")". rparen :: Doc e -- | The document langle contains a left angle, "<". langle :: Doc e -- | The document rangle contains a right angle, ">". rangle :: Doc e -- | The document lbrace contains a left brace, "{". lbrace :: Doc e -- | The document rbrace contains a right brace, "}". rbrace :: Doc e -- | The document lbracket contains a left square bracket, "[". lbracket :: Doc e -- | The document rbracket contains a right square bracket, "]". rbracket :: Doc e -- | The document squote contains a single quote, "'". squote :: Doc e -- | The document dquote contains a double quote, '"'. dquote :: Doc e -- | The document semi contains a semi colon, ";". semi :: Doc e -- | The document colon contains a colon, ":". colon :: Doc e -- | The document comma contains a comma, ",". comma :: Doc e -- | The document space contains a single space, " ". -- --

--   x <+> y   = x <> space <> y
--

space :: Doc e -- | The document dot contains a single dot, ".". dot :: Doc e -- | The document backslash contains a back slash, "\". backslash :: Doc e -- | The document equals contains an equal sign, "=". equals :: Doc e -- | 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 prettyList = list . map pretty pretty :: Pretty a => a -> Doc e prettyList :: Pretty a => [a] -> Doc e -- | 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. data SimpleDoc e SEmpty :: SimpleDoc e SChar :: {-# UNPACK #-} !Char -> (SimpleDoc e) -> SimpleDoc e SText :: {-# UNPACK #-} !Int -> String -> (SimpleDoc e) -> SimpleDoc e SLine :: {-# UNPACK #-} !Int -> (SimpleDoc e) -> SimpleDoc e SEffect :: e -> (SimpleDoc e) -> SimpleDoc e -- | 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 e -> SimpleDoc e -- | (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. renderCompact :: Doc e -> SimpleDoc e -- | (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) ""
--

displayS :: SimpleDoc e -> ShowS -- | (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)
--

displayIO :: Handle -> SimpleDoc e -> IO () column :: (Int -> Doc e) -> Doc e nesting :: (Int -> Doc e) -> Doc e width :: Doc e -> (Int -> Doc e) -> Doc e columns :: (Int -> Doc e) -> Doc e ribbon :: (Int -> Doc e) -> Doc e -- | The identity of <|> empty :: Alternative f => forall a. f a -- | An associative operation. -- --

--   (a <> b) <> c = a <> (b <> c)
--

(<>) :: Semigroup a => a -> a -> a instance Show (Doc e) instance Traversable SimpleDoc instance Foldable SimpleDoc instance Functor SimpleDoc instance MonadPlus Doc instance Alternative Doc instance Plus Doc instance Alt Doc instance Monad Doc instance Bind Doc instance Applicative Doc instance Apply Doc instance Functor Doc instance Pretty a => Pretty (Maybe a) instance (Pretty a, Pretty b, Pretty c) => Pretty (a, b, c) instance (Pretty a, Pretty b) => Pretty (a, b) instance Pretty (Doc a) instance Pretty Double instance Pretty Float instance Pretty Natural instance Pretty Integer instance Pretty Word64 instance Pretty Word32 instance Pretty Word16 instance Pretty Word8 instance Pretty Word instance Pretty Int64 instance Pretty Int32 instance Pretty Int16 instance Pretty Int8 instance Pretty Int instance Pretty a => Pretty (NonEmpty a) instance Pretty a => Pretty (Seq a) instance Pretty Char instance Pretty Bool instance Pretty () instance Pretty ByteString instance Pretty ByteString instance Pretty a => Pretty [a] instance IsString (Doc e) instance Monoid (Doc e) instance Semigroup (Doc e) -- | Pretty print module based on Daan Leijen's implementation of Philip -- Wadler's "prettier printer" -- --

--   "A prettier printer"
--   Draft paper, April 1997, revised March 1998.
--   http://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf
--

The nil document is called empty.
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.
A type argument has been added and embedded effects can -- be seen in the SimpleDoc type.

module Text.PrettyPrint.Free -- | 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" `above` text "world")
--

-- -- Which would return the string "hello\nworld", i.e. -- --

--   hello
--   world
--

data Doc e -- | 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
--

putDoc :: Doc e -> IO () -- | (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
--            }
--

--   nest 2 (text "hello" `above` text "world") `above` text "!"
--

-- -- outputs as: -- --

--   hello
--     world
--   !
--

--   softline = group line
--

softline :: Doc e -- | The document softbreak behaves like empty if the -- resulting output fits the page, otherwise it behaves like line. -- --

--   softbreak  = group linebreak
--

--   x $$ y  = align (above x y)
--

-- --

--   test    = text "hi" <+> (text "nice" $$ text "world")
--

-- -- which will be layed out as: -- --

--   hi nice
--      world
--

--   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 e -> Doc e -- | 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 !
--

--   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]
--

--   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
--

--   fillSep xs  = foldr (</>) empty xs
--

fillSep :: Foldable f => f (Doc e) -> Doc e -- | The document (sep xs) concatenates all documents xs -- either horizontally with (<+>), if it fits the page, or -- vertically with above. -- --

--   sep xs  = group (vsep xs)
--

--   fillCat xs  = foldr (<//>) empty xs
--

--   cat xs  = group (vcat xs)
--

cat :: Foldable f => f (Doc e) -> Doc e -- | (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)
--

--   types  = [("empty","Doc e")
--            ,("nest","Int -> Doc e -> Doc e")
--            ,("linebreak","Doc e")]
--   
--   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 e
--       nest   :: Int -> Doc e -> Doc e
--       linebreak :: Doc e
--

--   ptype (name,tp)
--          = fillBreak 6 (text name) <+> text "::" <+> text tp
--

-- -- The output will now be: -- --

--   let empty  :: Doc e
--       nest   :: Int -> Doc e -> Doc e 
--       linebreak
--              :: Doc e
--

fillBreak :: Int -> Doc e -> Doc e -- | The document (enclose l r x) encloses document x -- between documents l and r using (<>). -- --

--   enclose l r x   = l <> x <> r
--

--   x <+> y   = x <> space <> y
--

--   showWidth :: Int -> Doc -> String
--   showWidth w x   = displayS (renderPretty 0.4 w x) ""
--

displayS :: SimpleDoc e -> ShowS -- | (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)
--

displayIO :: Handle -> SimpleDoc e -> IO () column :: (Int -> Doc e) -> Doc e nesting :: (Int -> Doc e) -> Doc e width :: Doc e -> (Int -> Doc e) -> Doc e columns :: (Int -> Doc e) -> Doc e -- | The identity of <|> empty :: Alternative f => forall a. f a -- | An associative operation. -- --

--   (a <> b) <> c = a <> (b <> c)
--

(<>) :: Semigroup a => a -> a -> a