Documentation

The abstract data type Doc ann represents pretty documents that have been annotated with data of type ann.

More specifically, a value of type Doc represents a non-empty set of possible layouts of a document. The layout functions select one of these possibilities, taking into account things like the width of the output document.

The annotation is an arbitrary piece of data associated with (part of) a document. Annotations may be used by the rendering backends in order to display output differently, such as

• color information (e.g. when rendering to the terminal)
• mouseover text (e.g. when rendering to rich HTML)
• whether to show something or not (to allow simple or detailed versions)

The simplest way to display a Doc is via the Show class.

>>> putStrLn (show (vsep ["hello", "world"]))
hello
world

An associative operation.

>>> [1,2,3] <> [4,5,6]
[1,2,3,4,5,6]

(nest i x) lays out the document x with the current nesting level (indentation of the following lines) increased by i. Negative values are allowed, and decrease the nesting level accordingly.

>>> vsep [nest 4 (vsep ["lorem", "ipsum", "dolor"]), "sit", "amet"]
lorem
    ipsum
    dolor
sit
amet

See also

• hang (nest relative to current cursor position instead of current nesting level)
• align (set nesting level to current cursor position)
• indent (increase indentation on the spot, padding with spaces).

The line document advances to the next line and indents to the current nesting level.

>>> let doc = "lorem ipsum" <> line <> "dolor sit amet"
>>> doc
lorem ipsum
dolor sit amet

line behaves like space if the line break is undone by group:

>>> group doc
lorem ipsum dolor sit amet

(group x) tries laying out x into a single line by removing the contained line breaks; if this does not fit the page, or when a hardline within x prevents it from being flattened, x is laid out without any changes.

The group function is key to layouts that adapt to available space nicely.

See vcat, line, or flatAlt for examples that are related, or make good use of it.

softline behaves like space if the resulting output fits the page, otherwise like line.

Here, we have enough space to put everything in one line:

>>> let doc = "lorem ipsum" <> softline <> "dolor sit amet"
>>> putDocW 80 doc
lorem ipsum dolor sit amet

If we narrow the page to width 10, the layouter produces a line break:

>>> putDocW 10 doc
lorem ipsum
dolor sit amet

softline = group line

(align x) lays out the 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. Without alignment, the second line is put simply below everything we've had so far,

>>> "lorem" <+> vsep ["ipsum", "dolor"]
lorem ipsum
dolor

If we add an align to the mix, the vsep's contents all start in the same column,

>>> "lorem" <+> align (vsep ["ipsum", "dolor"])
lorem ipsum
      dolor

(hang i x) lays out the document x with a nesting level set to the current column plus i. Negative values are allowed, and decrease the nesting level accordingly.

>>> let doc = reflow "Indenting these words with hang"
>>> putDocW 24 ("prefix" <+> hang 4 doc)
prefix Indenting these
           words with
           hang

This differs from nest, which is based on the current nesting level plus i. When you're not sure, try the more efficient nest first. In our example, this would yield

>>> let doc = reflow "Indenting these words with nest"
>>> putDocW 24 ("prefix" <+> nest 4 doc)
prefix Indenting these
    words with nest

hang i doc = align (nest i doc)

(indent i x) indents document x by i columns, starting from the current cursor position.

>>> let doc = reflow "The indent function indents these words!"
>>> putDocW 24 ("prefix" <> indent 4 doc)
prefix    The indent
          function
          indents these
          words!

indent i d = hang i ({i spaces} <> d)

(encloseSep l r sep xs) concatenates the documents xs separated by sep, and encloses the resulting document by l and r.

The documents are laid out horizontally if that fits the page,

>>> let doc = "list" <+> align (encloseSep lbracket rbracket comma (map pretty [1,20,300,4000]))
>>> putDocW 80 doc
list [1,20,300,4000]

If there is not enough space, then the input is split into lines entry-wise therwise they are laid out vertically, with separators put in the front:

>>> putDocW 10 doc
list [1
     ,20
     ,300
     ,4000]

Note that doc contains an explicit call to align so that the list items are aligned vertically.

For putting separators at the end of entries instead, have a look at punctuate.

Haskell-inspired variant of encloseSep with braces and comma as separator.

>>> let doc = list (map pretty [1,20,300,4000])

>>> putDocW 80 doc
[1, 20, 300, 4000]

>>> putDocW 10 doc
[ 1
, 20
, 300
, 4000 ]

Haskell-inspired variant of encloseSep with parentheses and comma as separator.

>>> let doc = tupled (map pretty [1,20,300,4000])

>>> putDocW 80 doc
(1, 20, 300, 4000)

>>> putDocW 10 doc
( 1
, 20
, 300
, 4000 )

(x <+> y) concatenates document x and y with a space in between.

>>> "hello" <+> "world"
hello world

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

(hsep xs) concatenates all documents xs horizontally with <+>, i.e. it puts a space between all entries.

>>> let docs = Util.words "lorem ipsum dolor sit amet"

>>> hsep docs
lorem ipsum dolor sit amet

hsep does not introduce line breaks on its own, even when the page is too narrow:

>>> putDocW 5 (hsep docs)
lorem ipsum dolor sit amet

For automatic line breaks, consider using fillSep instead.

(vsep xs) concatenates all documents xs above each other. If a group undoes the line breaks inserted by vsep, the documents are separated with a space instead.

Using vsep alone yields

>>> "prefix" <+> vsep ["text", "to", "lay", "out"]
prefix text
to
lay
out

grouping a vsep separates the documents with a space if it fits the page (and does nothing otherwise). See the sep convenience function for this use case.

The align function can be used to align the documents under their first element:

>>> "prefix" <+> align (vsep ["text", "to", "lay", "out"])
prefix text
       to
       lay
       out

Since grouping a vsep is rather common, sep is a built-in for doing that.

(fillSep xs) concatenates the documents xs horizontally with <+> as long as it fits the page, then inserts a line and continues doing that for all documents in xs. (line means that if grouped, the documents are separated with a space instead of newlines. Use fillCat if you do not want a space.)

Let's print some words to fill the line:

>>> let docs = take 20 (cycle ["lorem", "ipsum", "dolor", "sit", "amet"])
>>> putDocW 80 ("Docs:" <+> fillSep docs)
Docs: lorem ipsum dolor sit amet lorem ipsum dolor sit amet lorem ipsum dolor
sit amet lorem ipsum dolor sit amet

The same document, printed at a width of only 40, yields

>>> putDocW 40 ("Docs:" <+> fillSep docs)
Docs: lorem ipsum dolor sit amet lorem
ipsum dolor sit amet lorem ipsum dolor
sit amet lorem ipsum dolor sit amet

(sep xs) tries laying out the documents xs separated with spaces, and if this does not fit the page, separates them with newlines. This is what differentiates it from vsep, which always lays out its contents beneath each other.

>>> let doc = "prefix" <+> sep ["text", "to", "lay", "out"]
>>> putDocW 80 doc
prefix text to lay out

With a narrower layout, the entries are separated by newlines:

>>> putDocW 20 doc
prefix text
to
lay
out

sep = group . vsep

(hcat xs) concatenates all documents xs horizontally with <> (i.e. without any spacing).

It is provided only for consistency, since it is identical to mconcat.

>>> let docs = Util.words "lorem ipsum dolor"
>>> hcat docs
loremipsumdolor

(vcat xs) vertically concatenates the documents xs. If it is grouped, the line breaks are removed.

In other words vcat is like vsep, with newlines removed instead of replaced by spaces.

>>> let docs = Util.words "lorem ipsum dolor"
>>> vcat docs
lorem
ipsum
dolor
>>> group (vcat docs)
loremipsumdolor

Since grouping a vcat is rather common, cat is a built-in shortcut for it.

(fillCat xs) concatenates documents xs horizontally with <> as long as it fits the page, then inserts a line' and continues doing that for all documents in xs. This is similar to how an ordinary word processor lays out the text if you just keep typing after you hit the maximum line length.

(line' means that if grouped, the documents are separated with nothing instead of newlines. See fillSep if you want a space instead.)

Observe the difference between fillSep and fillCat. fillSep concatenates the entries spaced when grouped,

>>> let docs = take 20 (cycle (["lorem", "ipsum", "dolor", "sit", "amet"]))
>>> putDocW 40 ("Grouped:" <+> group (fillSep docs))
Grouped: lorem ipsum dolor sit amet
lorem ipsum dolor sit amet lorem ipsum
dolor sit amet lorem ipsum dolor sit
amet

On the other hand, fillCat concatenates the entries directly when grouped,

>>> putDocW 40 ("Grouped:" <+> group (fillCat docs))
Grouped: loremipsumdolorsitametlorem
ipsumdolorsitametloremipsumdolorsitamet
loremipsumdolorsitamet

(cat xs) tries laying out the documents xs separated with nothing, and if this does not fit the page, separates them with newlines. This is what differentiates it from vcat, which always lays out its contents beneath each other.

>>> let docs = Util.words "lorem ipsum dolor"
>>> putDocW 80 ("Docs:" <+> cat docs)
Docs: loremipsumdolor

When there is enough space, the documents are put above one another,

>>> putDocW 10 ("Docs:" <+> cat docs)
Docs: lorem
ipsum
dolor

cat = group . vcat

(punctuate p xs) appends p to all but the last document in xs.

>>> let docs = punctuate comma (Util.words "lorem ipsum dolor sit amet")
>>> putDocW 80 (hsep docs)
lorem, ipsum, dolor, sit, amet

The separators are put at the end of the entries, which we can see if we position the result vertically:

>>> putDocW 20 (vsep docs)
lorem,
ipsum,
dolor,
sit,
amet

If you want put the commas in front of their elements instead of at the end, you should use tupled or, in general, encloseSep.

(fill i x) lays out the document x. It then appends spaces until the width is equal to i. If the width of x is already larger, nothing is appended.

This function is quite useful in practice to output a list of bindings:

>>> let types = [("empty","Doc"), ("nest","Int -> Doc -> Doc"), ("fillSep","[Doc] -> Doc")]
>>> let ptype (name, tp) = fill 5 (pretty name) <+> "::" <+> pretty tp
>>> "let" <+> align (vcat (map ptype types))
let empty :: Doc
    nest  :: Int -> Doc -> Doc
    fillSep :: [Doc] -> Doc

(fillBreak i x) first lays out the document x. It then 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 example given in fill to use fillBreak, we get a useful variation of the output:

>>> let types = [("empty","Doc"), ("nest","Int -> Doc -> Doc"), ("fillSep","[Doc] -> Doc")]
>>> let ptype (name, tp) = fillBreak 5 (pretty name) <+> "::" <+> pretty tp
>>> "let" <+> align (vcat (map ptype types))
let empty :: Doc
    nest  :: Int -> Doc -> Doc
    fillSep
          :: [Doc] -> Doc

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

>>> enclose "A" "Z" "·"
A·Z

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

>>> squotes "·"
'·'

>>> dquotes "·"
"·"

>>> parens "·"
(·)

>>> angles "·"
<·>

>>> braces "·"
{·}

>>> brackets "·"
[·]

>>> lparen
(

>>> rparen
)

>>> langle
<

>>> rangle
>

>>> lbrace
{

>>> rbrace
}

>>> lbracket
[

>>> rbracket
]

>>> squote
'

>>> dquote
"

>>> semi
;

>>> colon
:

>>> comma
,

>>> "a" <> space <> "b"
a b

This is mostly used via <+>,

>>> "a" <+> "b"
a b

>>> dot
.

>>> backslash
\

>>> equals
=

>>> pipe
|

Add an annotation to a Doc. This annotation can then be used by the renderer to e.g. add color to certain parts of the output. For a full tutorial example on how to use it, see the Prettyprinter.Render.Tutorials.StackMachineTutorial or Prettyprinter.Render.Tutorials.TreeRenderingTutorial modules.

This function is only relevant for custom formats with their own annotations, and not relevant for basic prettyprinting. The predefined renderers, e.g. Prettyprinter.Render.Text, should be enough for the most common needs.

Layout a document depending on which column it starts at. align is implemented in terms of column.

>>> column (\l -> "Columns are" <+> pretty l <> "-based.")
Columns are 0-based.

>>> let doc = "prefix" <+> column (\l -> "| <- column" <+> pretty l)
>>> vsep [indent n doc | n <- [0,4,8]]
prefix | <- column 7
    prefix | <- column 11
        prefix | <- column 15

Layout a document depending on the current nesting level. align is implemented in terms of nesting.

>>> let doc = "prefix" <+> nesting (\l -> brackets ("Nested:" <+> pretty l))
>>> vsep [indent n doc | n <- [0,4,8]]
prefix [Nested: 0]
    prefix [Nested: 4]
        prefix [Nested: 8]

(width doc f) lays out the document doc, and makes the column width of it available to a function.

>>> let annotate doc = width (brackets doc) (\w -> " <- width:" <+> pretty w)
>>> align (vsep (map annotate ["---", "------", indent 3 "---", vsep ["---", indent 4 "---"]]))
[---] <- width: 5
[------] <- width: 8
[   ---] <- width: 8
[---
    ---] <- width: 8