Wadler-style pretty printer for MoonBit: documents, groups and layout that fits a line width

pretty-printer

wadler

layout

document

Download zip

Author

moonrockz

Version

0.1.0

License

Apache-2.0

Repository

github.com/moonrockz/pretty

Last updated

7 hours ago

Downloads

Dependencies

moonrockz/expect0.6.0

#moonrockz/pretty

A Wadler-style pretty printer for MoonBit. Build a document from text, line breaks and groups; render lays it out to fit a line width.

let items = [@pretty.text("a"), @pretty.text("b"), @pretty.text("c")]
let doc = @pretty.group(
  @pretty.text("[") +
  @pretty.nest(2, @pretty.line() + @pretty.join(items, @pretty.text(",") + @pretty.line())) +
  @pretty.line() +
  @pretty.text("]"),
)
@pretty.render(doc) // "[ a, b, c ]"
@pretty.render(doc, width=6) // "[\n  a,\n  b,\n  c\n]"

Function	Flat	Broken
text(s)	s	s
verbatim(s)	s	s, no indentation after its line feeds
line()	a space	a line break
softline()	nothing	a line break
hardline()	always a line break
nest(n, d)	d	d, indented by n more
align(d)	d	d, indented to its start column
tab(n, d)	d	d, indented to the next multiple of n
group(d)	flat if it fits	else broken
if_break(b, f)	f	b

A group is flat when it fits together with the rest of its line, up to the next possible break (the fit rule of Wadler's "A prettier printer").
Evaluation is strict (Lindig, "Strictly Pretty") and uses explicit stacks, so deep documents render on wasm, wasm-gc, js and native.
The engine writes no trailing whitespace and no indentation on empty lines. Width counts code points.

Doc

source

type Doc

A document: pieces of text and places where render may break a line. Build documents with the functions of this package and join them with +. The representation is private, so it can change without a breaking release.

impl Add for Doc

align

source

fn align(d : Doc) -> Doc

d with the indentation of its line breaks set to the column where d starts.

concat

source

fn concat(docs : Array[Doc]) -> Doc

The documents one after the other.

empty

source

fn empty() -> Doc

The empty document. empty() + d and d + empty() render as d.

group

source

fn group(d : Doc) -> Doc

d on one line when it fits (with the rest of the line up to the next possible break), otherwise d with its line breaks.

test {
  let d = @pretty.group(@pretty.text("a") + @pretty.line() + @pretty.text("b"))
  inspect(@pretty.render(d, width=3), content="a b")
  inspect(@pretty.render(d, width=2), content="a\nb")
}

hardline

source

fn hardline() -> Doc

Always a line break and the indentation. Every group that holds it breaks.

if_break

source

fn if_break(broken : Doc, flat : Doc) -> Doc

broken when the enclosing group breaks, flat when it is flat. Only flat decides whether the group can be flat.

join

source

fn join(docs : Array[Doc], sep : Doc) -> Doc

The documents with sep between them.

line

source

fn line() -> Doc

A space in flat mode; a line break and the indentation in break mode.

nest

source

fn nest(n : Int, d : Doc) -> Doc

d with the indentation of its line breaks increased by n.

render

source

fn render(doc : Doc, width? : Int) -> String

Lay out doc in lines of at most width columns where its groups allow. A group is flat when it fits, together with the rest of its line up to the next possible break. Text wider than width stays whole. Uses explicit stacks, so deep documents render on every target.

test {
  let items = [@pretty.text("a"), @pretty.text("b")]
  let d = @pretty.group(
    @pretty.text("[") +
    @pretty.nest(
      2,
      @pretty.line() + @pretty.join(items, @pretty.text(",") + @pretty.line()),
    ) +
    @pretty.line() +
    @pretty.text("]"),
  )
  inspect(@pretty.render(d), content="[ a, b ]")
  inspect(@pretty.render(d, width=4), content="[\n  a,\n  b\n]")
}

softline

source

fn softline() -> Doc

Nothing in flat mode; a line break and the indentation in break mode.

tab

source

fn tab(n : Int, d : Doc) -> Doc

d with the indentation of its line breaks set to the next multiple of n that is greater than the current indentation (a tab stop). With n <= 0 the indentation does not change. Use it after align to indent to a fixed grid: indentation 6 becomes 8, indentation 8 becomes 12.

test {
  let body = @pretty.text("x") +
    @pretty.tab(4, @pretty.hardline() + @pretty.text("y"))
  inspect(
    @pretty.render(@pretty.text("ab") + @pretty.align(body)),
    content="abx\n    y",
  )
  inspect(
    @pretty.render(@pretty.text("abcd") + @pretty.align(body)),
    content="abcdx\n        y",
  )
}

text

source

fn text(s : String) -> Doc

The text s. A line feed in s is a hardline(), so the lines after it get the current indentation. Width counts code points.

test {
  inspect(
    @pretty.render(@pretty.nest(2, @pretty.text("a\nb"))),
    content="a\n  b",
  )
}

verbatim

source

fn verbatim(s : String) -> Doc

The text s as it is: a line feed in s starts a new line with no indentation. Use it for text whose bytes must not change, such as a doc comment. A group that holds a line feed this way always breaks.

Source Files

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pretty

#moonrockz/pretty

#Doc

#align

#concat

#empty

#group

#hardline

#if_break

#join

#line

#nest

#render

#softline

#tab

#text

#verbatim

Source Files

Doc

align

concat

empty

group

hardline

if_break

join

line

nest

render

softline

tab

text

verbatim