Data.Pretty

A Leijen-style pretty printer.

Build a layout-independent Doc from the combinators below, then render it to a string at a chosen page width.

Rendering is a pure function of the document and the width, so a document lays out identically on every backend and replays byte for byte. The strict renderer carries a Flat/Brk mode through a work list; its fits lookahead stops at the next forced break or as soon as the width is exceeded.

Import explicitly (import Data.Pretty); these names are not in Base’s unqualified scope. Widths count Unicode code points, matching str_len; they are not terminal display-cell measurements.

Types

`Doc`

type Doc
  = DNil
  | DText(String)
  | DSoft(String)
  | DHard
  | DCat(Doc, Doc)
  | DNest(Int, Doc)
  | DGroup(Doc)

A layout document. Its representation is hidden so indentation and break invariants are established only by the public combinators.

Functions and Values

`render`

render : (Int, Data.Pretty.Doc) -> String

Render a document to a string at page width w.

import Data.Pretty (render, group, cat, text, line)
render(80, group(cat(text("hello"), cat(line(), text("world")))))

hello world

`pretty`

pretty : (Data.Pretty.Doc) -> String

Render at the conventional width of 80 columns.

`text`

text : (String) -> Data.Pretty.Doc

Literal text. The string must not contain a newline; use hardline for a forced break.

`empty`

empty : () -> Data.Pretty.Doc

The empty document.

`line`

line : () -> Data.Pretty.Doc

A break that is a space when flat and a newline when broken.

`linebreak`

linebreak : () -> Data.Pretty.Doc

A break that is nothing when flat and a newline when broken.

`softline`

softline : () -> Data.Pretty.Doc

A line inside its own group: a space if it fits, a newline otherwise.

`softbreak`

softbreak : () -> Data.Pretty.Doc

A linebreak inside its own group: nothing if it fits, a newline otherwise.

`hardline`

hardline : () -> Data.Pretty.Doc

A newline that no group can flatten away.

`cat`

cat : (Data.Pretty.Doc, Data.Pretty.Doc) -> Data.Pretty.Doc

One document then another.

`nest`

nest : (Int, Data.Pretty.Doc) -> Data.Pretty.Doc

Indent every break inside d by at least zero columns. Negative indentation is clamped to zero.

`group`

group : (Data.Pretty.Doc) -> Data.Pretty.Doc

Try to lay d out flat; fall back to its broken form if it does not fit.

`concat_docs`

concat_docs : (List(Data.Pretty.Doc)) -> Data.Pretty.Doc

Concatenate a list of documents end to end.

`join_docs`

join_docs : (Data.Pretty.Doc, List(Data.Pretty.Doc)) -> Data.Pretty.Doc

Put separator between each pair of documents. Unlike hsep and vsep, the caller selects the exact separator document, which is useful for small target-language emitters (commas plus soft breaks, operators, or hard lines).

`hsep`

hsep : (List(Data.Pretty.Doc)) -> Data.Pretty.Doc

Documents separated by a single space, all on one line.

`vsep`

vsep : (List(Data.Pretty.Doc)) -> Data.Pretty.Doc

Documents separated by line breaks (spaces or newlines under a group).

`sep`

sep : (List(Data.Pretty.Doc)) -> Data.Pretty.Doc

vsep wrapped in a group: one line if it fits, one per element otherwise.

`enclose`

enclose : (Data.Pretty.Doc, Data.Pretty.Doc, Data.Pretty.Doc) -> Data.Pretty.Doc

d between the opening l and closing r.

`parens`

parens : (Data.Pretty.Doc) -> Data.Pretty.Doc

d in parentheses.

`brackets`

brackets : (Data.Pretty.Doc) -> Data.Pretty.Doc

d in square brackets.

`braces`

braces : (Data.Pretty.Doc) -> Data.Pretty.Doc

d in curly braces.

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