Copyright Google, Inc. (c) 2013, Edward Kmett (c) 2011, Daan Leijen (c) 2000 BSD-style (see the file LICENSE) ekmett@gmail.com experimental portable Safe-Inferred Haskell98

Text.PrettyPrint.Free.Internal

Description

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.
```

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.

Synopsis

# Documents

data Doc e Source

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
```

Constructors

 Fail Empty Char !Char Text !Int String Line FlatAlt (Doc e) (Doc e) Cat (Doc e) (Doc e) Nest !Int (Doc e) Union (Doc e) (Doc e) Effect e Column (Int -> Doc e) Nesting (Int -> Doc e) Columns (Int -> Doc e) Ribbon (Int -> Doc e)

Instances

 Alternative Doc Monad Doc Functor Doc MonadPlus Doc Applicative Doc Plus Doc Alt Doc Apply Doc Bind Doc Show (Doc e) IsString (Doc e) Monoid (Doc e) Semigroup (Doc e) Pretty (Doc a)

putDoc :: Doc e -> IO () Source

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
```

hPutDoc :: Handle -> Doc e -> IO () Source

`(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
}```

# Basic combinators

char :: Char -> Doc e Source

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.

text :: String -> Doc e Source

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.

nest :: Int -> Doc e -> Doc e Source

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
!
```

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`.

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`.

group :: Doc e -> Doc e Source

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.

The document `softline` behaves like `space` if the resulting output fits the page, otherwise it behaves like `line`.

`softline = group line`

The document `softbreak` behaves like `empty` if the resulting output fits the page, otherwise it behaves like `line`.

`softbreak  = group linebreak`

A linebreak that can not be flattened; it is guaranteed to be rendered as a newline.

flatAlt :: Doc e -> Doc e -> Doc e Source

`flatAlt` creates a document that changes when flattened; normally it is rendered as the first argument, but when flattened is rendered as the second.

flatten :: Doc e -> Doc e Source

# Alignment

align :: Doc e -> Doc e Source

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
```

hang :: Int -> Doc e -> Doc e Source

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)`

indent :: Int -> Doc e -> Doc e Source

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 !
```

encloseSep :: Foldable f => Doc e -> Doc e -> Doc e -> f (Doc e) -> Doc e Source

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 ]
```

list :: Foldable f => f (Doc e) -> Doc e Source

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.

tupled :: Foldable f => f (Doc e) -> Doc e Source

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.

semiBraces :: Foldable f => f (Doc e) -> Doc e Source

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.

# Operators

(<+>) :: Doc e -> Doc e -> Doc e infixr 6 Source

above :: Doc e -> Doc e -> Doc e infixr 5 Source

The document `above x y` concatenates document `x` and `y` with a `line` in between. (infixr 5)

(</>) :: Doc e -> Doc e -> Doc e infixr 5 Source

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)

aboveBreak :: Doc e -> Doc e -> Doc e infixr 5 Source

The document `aboveBreak x y` concatenates document `x` and `y` with a `linebreak` in between. (infixr 5)

(<//>) :: Doc e -> Doc e -> Doc e infixr 5 Source

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)

# List combinators

hsep :: Foldable f => f (Doc e) -> Doc e Source

The document `(hsep xs)` concatenates all documents `xs` horizontally with `(<+>)`.

vsep :: Foldable f => f (Doc e) -> Doc e Source

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
```

fillSep :: Foldable f => f (Doc e) -> Doc e Source

The document `(fillSep xs)` concatenates documents `xs` horizontally with `(<+>)` as long as its fits the page, then inserts a `line` and continues doing that for all documents in `xs`.

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

sep :: Foldable f => f (Doc e) -> Doc e Source

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)`

hcat :: Foldable f => f (Doc e) -> Doc e Source

The document `(hcat xs)` concatenates all documents `xs` horizontally with `(<>)`.

vcat :: Foldable f => f (Doc e) -> Doc e Source

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.

fillCat :: Foldable f => f (Doc e) -> Doc e Source

The document `(fillCat xs)` concatenates documents `xs` horizontally with `(<>)` as long as its fits the page, then inserts a `linebreak` and continues doing that for all documents in `xs`.

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

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

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)`

punctuate :: Traversable f => Doc e -> f (Doc e) -> f (Doc e) Source

`(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`.)

# Fillers

fill :: Int -> Doc e -> Doc e Source

The document `(fill i x)` renders document `x`. It then 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 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
```

fillBreak :: Int -> Doc e -> Doc e Source

The document `(fillBreak i x)` first renders document `x`. It then 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 e
nest   :: Int -> Doc e -> Doc e
linebreak
:: Doc e
```

# Bracketing combinators

enclose :: Doc e -> Doc e -> Doc e -> Doc e Source

The document `(enclose l r x)` encloses document `x` between documents `l` and `r` using `(<>)`.

`enclose l r x   = l <> x <> r`

squotes :: Doc e -> Doc e Source

Document `(squotes x)` encloses document `x` with single quotes "'".

dquotes :: Doc e -> Doc e Source

Document `(dquotes x)` encloses document `x` with double quotes '"'.

parens :: Doc e -> Doc e Source

Document `(parens x)` encloses document `x` in parenthesis, "(" and ")".

angles :: Doc e -> Doc e Source

Document `(angles x)` encloses document `x` in angles, "<" and ">".

braces :: Doc e -> Doc e Source

Document `(braces x)` encloses document `x` in braces, "{" and "}".

brackets :: Doc e -> Doc e Source

Document `(brackets x)` encloses document `x` in square brackets, "[" and "]".

# Character documents

The document `lparen` contains a left parenthesis, "(".

The document `rparen` contains a right parenthesis, ")".

The document `langle` contains a left angle, "<".

The document `rangle` contains a right angle, ">".

The document `lbrace` contains a left brace, "{".

The document `rbrace` contains a right brace, "}".

The document `lbracket` contains a left square bracket, "[".

The document `rbracket` contains a right square bracket, "]".

The document `squote` contains a single quote, "'".

The document `dquote` contains a double quote, '"'.

The document `semi` contains a semi colon, ";".

The document `colon` contains a colon, ":".

The document `comma` contains a comma, ",".

The document `space` contains a single space, " ".

`x <+> y   = x <> space <> y`

dot :: Doc e Source

The document `dot` contains a single dot, ".".

The document `backslash` contains a back slash, "\".

The document `equals` contains an equal sign, "=".

# Pretty class

class Pretty a where Source

The member `prettyList` is only used to define the ```instance Pretty a => Pretty [a]```. In normal circumstances only the `pretty` function is used.

Minimal complete definition

pretty

Methods

pretty :: a -> Doc e Source

prettyList :: [a] -> Doc e Source

Instances

 Pretty Bool Pretty Char Pretty Double Pretty Float Pretty Int Pretty Int8 Pretty Int16 Pretty Int32 Pretty Int64 Pretty Integer Pretty Word Pretty Word8 Pretty Word16 Pretty Word32 Pretty Word64 Pretty () Pretty ByteString Pretty ByteString Pretty Natural Pretty Text Pretty Text Pretty a => Pretty [a] Pretty a => Pretty (Maybe a) Pretty a => Pretty (Seq a) Pretty a => Pretty (NonEmpty a) Pretty (Doc a) (Pretty a, Pretty b) => Pretty (a, b) (Pretty a, Pretty b, Pretty c) => Pretty (a, b, c)

# Rendering

data SimpleDoc e Source

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

 SFail SEmpty SChar !Char (SimpleDoc e) SText !Int String (SimpleDoc e) SLine !Int (SimpleDoc e) SEffect e (SimpleDoc e)

Instances

 Functor SimpleDoc Foldable SimpleDoc Traversable SimpleDoc

renderPretty :: Float -> Int -> Doc e -> SimpleDoc e Source

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.

`(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.

renderSmart :: Int -> Doc e -> SimpleDoc e Source

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 20c 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 20c mark. In contrast, `renderSmart` will continue to check 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. mark. In addition, `renderSmart` uses this lookahead to minimize the number of lines printed, leading to more compact and visually appealing output. Consider this example using the same syntax as above: `aaaaaaaaaaa([abc, def, ghi])` When rendered with `renderPretty` and a page width of 20c, we get: ``` aaaaaaaaaaa([ abc , def , ghi ]) ``` Whereas when rendered with `renderSmart` and a page width of 20c, we get: ``` aaaaaaaaaaa( [abc, def, ghi]) ```

`(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) ""```

displayIO :: Handle -> SimpleDoc e -> IO () Source

`(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)`

# Undocumented

column :: (Int -> Doc e) -> Doc e Source

nesting :: (Int -> Doc e) -> Doc e Source

width :: Doc e -> (Int -> Doc e) -> Doc e Source

columns :: (Int -> Doc e) -> Doc e Source

ribbon :: (Int -> Doc e) -> Doc e Source

# Re-exported standard functions

empty :: Alternative f => forall a. f a

The identity of `<|>`

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

An associative operation.

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

If `a` is also a `Monoid` we further require

```(`<>`) = `mappend`
```