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

Overloaded conversion to Doc.

Laws:

1. output should be pretty. :-)

Convert a Showable value to a Doc. If the String does not contain newlines, consider using the more performant unsafeViaShow.

Convert a Showable value that must not contain newlines to a Doc. If there may be newlines, use viaShow instead.

(unsafeTextWithoutNewlines s) contains the literal string s.

The string must not contain any newline characters, since this is an invariant of the Text constructor.

The empty document behaves like (pretty ""), so it has a height of 1. This may lead to surprising behaviour if we expect it to bear no weight inside e.g. vcat, where we get an empty line of output from it (parens for visibility only):

>>> vsep ["hello", parens emptyDoc, "world"]
hello
()
world

Together with <>, emptyDoc forms the Monoid Doc.

(nest i x) lays out the document x with the current indentation level 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, align and indent.

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

line' is like line, but behaves like mempty if the line break is undone by group (instead of space).

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

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

softline' is like softline, but behaves like mempty if the resulting output does not fit on the page (instead of space). In other words, line is to line' how softline is to softline'.

With enough space, we get direct concatenation:

>>> let doc = "ThisWord" <> softline' <> "IsWayTooLong"
>>> putDocW 80 doc
ThisWordIsWayTooLong

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

>>> putDocW 10 doc
ThisWord
IsWayTooLong

softline' = group line'

A hardline is always laid out as a line break, even when grouped or when there is plenty of space. Note that it might still be simply discarded if it is part of a flatAlt inside a group.

>>> let doc = "lorem ipsum" <> hardline <> "dolor sit amet"
>>> putDocW 1000 doc
lorem ipsum
dolor sit amet

>>> 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, 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.

Choose the first element of each Union, and discard the first field of all FlatAlts.

The result is Just if the element might change depending on the layout algorithm (i.e. contains differently renderable sub-documents), and Nothing if the document is static (e.g. contains only a plain Empty node). See [Group: special flattening] for further explanations.

(flatAlt x fallback) renders as x by default, but falls back to fallback when grouped. Since the layout algorithms rely on group having an effect of shortening the width of the contained text, careless usage of flatAlt with wide fallbacks might lead to unappealingly long lines.

flatAlt is particularly useful for defining conditional separators such as

softHyphen = flatAlt mempty "-"
softline   = flatAlt space line

We can use this to render Haskell's do-notation nicely:

>>> let open        = flatAlt "" "{ "
>>> let close       = flatAlt "" " }"
>>> let separator   = flatAlt "" "; "
>>> let prettyDo xs = group ("do" <+> encloseSep open close separator xs)
>>> let statements  = ["name:_ <- getArgs", "let greet = \"Hello, \" <> name", "putStrLn greet"]

This is put into a single line with {;} style if it fits,

>>> putDocW 80 (prettyDo statements)
do { name:_ <- getArgs; let greet = "Hello, " <> name; putStrLn greet }

When there is not enough space the statements are broken up into lines nicely,

>>> putDocW 10 (prettyDo statements)
do name:_ <- getArgs
   let greet = "Hello, " <> name
   putStrLn greet

(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 with i spaces, 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" <+> 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 aligned vertically, with separators put in the front:

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

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

Concatenate all documents element-wise with a binary function.

concatWith _ [] = mempty
concatWith (**) [x,y,z] = x ** y ** z

Multiple convenience definitions based on concatWith are alredy predefined, for example

hsep    = concatWith (<+>)
fillSep = concatWith (\x y -> x <> softline <> y)

This is also useful to define customized joiners,

>>> concatWith (surround dot) ["Data", "Text", "Prettyprint", "Doc"]
Data.Text.Prettyprint.Doc

(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.

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

Layout a document depending on the page width, if one has been specified.

>>> let prettyPageWidth (AvailablePerLine l r) = "Width:" <+> pretty l <> ", ribbon fraction:" <+> pretty r
>>> let doc = "prefix" <+> pageWidth (brackets . prettyPageWidth)
>>> putDocW 32 (vsep [indent n doc | n <- [0,4,8]])
prefix [Width: 32, ribbon fraction: 1.0]
    prefix [Width: 32, ribbon fraction: 1.0]
        prefix [Width: 32, ribbon fraction: 1.0]

(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

Insert a number of spaces. Negative values count as 0.

(plural n one many) is one if n is 1, and many otherwise. A typical use case is adding a plural "s".

>>> let things = [True]
>>> let amount = length things
>>> pretty things <+> "has" <+> pretty amount <+> plural "entry" "entries" amount
[True] has 1 entry

(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

(surround x l r) surrounds document x with l and r.

>>> surround "·" "A" "Z"
A·Z

This is merely an argument reordering of enclose, but allows for definitions like

>>> concatWith (surround ".") ["Data", "Text", "Prettyprint", "Doc"]
Data.Text.Prettyprint.Doc

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 Data.Text.Prettyprint.Doc.Render.Tutorials.StackMachineTutorial or Data.Text.Prettyprint.Doc.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. Data.Text.Prettyprint.Doc.Render.Text, should be enough for the most common needs.

Remove all annotations.

Although unAnnotate is idempotent with respect to rendering,

unAnnotate . unAnnotate = unAnnotate

it should not be used without caution, for each invocation traverses the entire contained document. If possible, it is preferrable to unannotate after producing the layout by using unAnnotateS.

Change the annotation of a Document.

Useful in particular to embed documents with one form of annotation in a more generlly annotated document.

Since this traverses the entire Doc tree, including parts that are not rendered due to other layouts fitting better, it is preferrable to reannotate after producing the layout by using reAnnotateS.

Since reAnnotate has the right type and satisfies 'reAnnotate id = id', it is used to define the Functor instance of Doc.

Change the annotations of a Document. Individual annotations can be removed, changed, or replaced by multiple ones.

This is a general function that combines unAnnotate and reAnnotate, and it is useful for mapping semantic annotations (such as »this is a keyword«) to display annotations (such as »this is red and underlined«), because some backends may not care about certain annotations, while others may.

Annotations earlier in the new list will be applied earlier, i.e. returning [Bold, Green] will result in a bold document that contains green text, and not vice-versa.

Since this traverses the entire Doc tree, including parts that are not rendered due to other layouts fitting better, it is preferrable to reannotate after producing the layout by using alterAnnotationsS.

Remove all annotations. unAnnotate for SimpleDocStream.

Change the annotation of a document. reAnnotate for SimpleDocStream.

Change the annotation of a document to a different annotation, or none at all. alterAnnotations for SimpleDocStream.

Note that the Doc version is more flexible, since it allows changing a single annotation to multiple ones. (SimpleDocTree restores this flexibility again.)

Fusion depth parameter, used by fuse.

(fuse depth doc) combines text nodes so they can be rendered more efficiently. A fused document is always laid out identical to its unfused version.

When laying a Document out to a SimpleDocStream, every component of the input is translated directly to the simpler output format. This sometimes yields undesirable chunking when many pieces have been concatenated together.

For example

>>> "a" <> "b" <> pretty 'c' <> "d"
abcd

results in a chain of four entries in a SimpleDocStream, although this is fully equivalent to the tightly packed

>>> "abcd" :: Doc ann
abcd

which is only a single SimpleDocStream entry, and can be processed faster.

It is therefore a good idea to run fuse on concatenations of lots of small strings that are used many times,

>>> let oftenUsed = fuse Shallow ("a" <> "b" <> pretty 'c' <> "d")
>>> hsep (replicate 5 oftenUsed)
abcd abcd abcd abcd abcd

The data type SimpleDocStream represents laid out documents and is used by the display functions.

A simplified view is that Doc = [SimpleDocStream], and the layout functions pick one of the SimpleDocStreams based on which one fits the layout constraints best. This means that SimpleDocStream has all complexity contained in Doc resolved, making it very easy to convert it to other formats, such as plain text or terminal output.

To write your own Doc to X converter, it is therefore sufficient to convert from SimpleDocStream. The »Render« submodules provide some built-in converters to do so, and helpers to create own ones.

Decide whether a SimpleDocStream fits the constraints given, namely

• page width
• minimum nesting level to fit in
• width in which to fit the first line; Nothing is unbounded

List of nesting level/document pairs yet to be laid out.

Maximum number of characters that fit in one line. The layout algorithms will try not to exceed the set limit by inserting line breaks when applicable (e.g. via softline').

Options to influence the layout algorithms.

The default layout options, suitable when you just want some output, and don’t particularly care about the details. Used by the Show instance, for example.

>>> defaultLayoutOptions
LayoutOptions {layoutPageWidth = AvailablePerLine 80 0.4}

This is the default layout algorithm, and it is used by show, putDoc and hPutDoc.

layoutPretty commits to rendering something in a certain way if the next element fits the layout constraints; in other words, it has one SimpleDocStream element lookahead when rendering. Consider using the smarter, but a bit less performant, layoutSmart algorithm if the results seem to run off to the right before having lots of line breaks.

A layout algorithm with more lookahead than layoutPretty, that introduces line breaks earlier if the content does not (or will not, rather) fit into one line.

Considre the following python-ish document,

>>> let fun x = hang 2 ("fun(" <> softline' <> x) <> ")"
>>> let doc = (fun . fun . fun . fun . fun) (list ["abcdef", "ghijklm"])

which we’ll be rendering using the following pipeline (where the layout algorithm has been left open),

>>> import Data.Text.IO as T
>>> import Data.Text.Prettyprint.Doc.Render.Text
>>> let hr = pipe <> pretty (replicate (26-2) '-') <> pipe
>>> let go layouter x = (T.putStrLn . renderStrict . layouter (LayoutOptions (AvailablePerLine 26 1))) (vsep [hr, x, hr])

If we render this using layoutPretty with a page width of 26 characters per line, all the fun calls fit into the first line so they will be put there,

>>> go layoutPretty doc
|------------------------|
fun(fun(fun(fun(fun(
                  [ abcdef
                  , ghijklm ])))))
|------------------------|

Note that this exceeds the desired 26 character page width. The same document, rendered with layoutSmart, fits the layout contstraints:

>>> go layoutSmart doc
|------------------------|
fun(
  fun(
    fun(
      fun(
        fun(
          [ abcdef
          , ghijklm ])))))
|------------------------|

The key difference between layoutPretty and layoutSmart is that the latter will check the potential document up to the end of the current indentation level, instead of just having one element lookahead.

The Wadler/Leijen layout algorithm

(layoutCompact x) lays out the document x without adding any indentation. Since no 'pretty' printing is involved, this layouter is very fast. The resulting output contains fewer characters than a prettyprinted version and can be used for output that is read by other programs.

>>> let doc = hang 4 (vsep ["lorem", "ipsum", hang 4 (vsep ["dolor", "sit"])])
>>> doc
lorem
    ipsum
    dolor
        sit

>>> let putDocCompact = renderIO System.IO.stdout . layoutCompact
>>> putDocCompact doc
lorem
ipsum
dolor
sit

Render a SimpleDocStream to a ShowS, useful to write Show instances based on the prettyprinter.

instance Show MyType where
    showsPrec _ = renderShowS . layoutPretty defaultLayoutOptions . pretty