Scala Prettyprint

A prettyprinting library for Scala following paper A prettier printer by Philip Wadler.

• Automatic indentation
• Automatic line length limit

Include Library

Use Jitpack to include the library.

Basic usage

Import the library:

import com.github.peterzeller.prettyprint.PrettyPrintDoc._

Create a document of type Doc.

val d: Doc = group("hello(" <> nested(2, line <> sep("," <> lineOrSpace, 
                       (0 to 20).map(_.toString))) <> ")")

Print the document with a maximum line length of 50 characters:

val s: String = d.prettyStr(50)

Example: Printing a Tree

case class Tree(text: String, children: List[Tree])

def tree(text: String, children: Tree*): Tree = Tree(text, children.toList)

def showTree(t: Tree): Doc =
  t.text <> nested(t.text.length, showBracket(t.children))

def showBracket(ts: List[Tree]): Doc =
  if (ts.isEmpty) ""
  else "[" <> nested(1, sep("," <> lineOrSpace, ts.map(showTree))) <> "]"

val t = tree("aaa",
  tree("bbbb",
    tree("ccc"),
    tree("dd")
  ),
  tree("eee"),
  tree("fff",
    tree("gg"),
    tree("hhh"),
    tree("ii")
  )
)

val d: Doc = showTree(t)
val s: String = d.prettyStr(30)
println(s)

Output:

aaa[bbbb[ccc,
         dd],
    eee,
    fff[gg,
        hhh,
        ii]]

Document API

Strings are implicitly converted to Doc:

val d: Doc = "Hello"

Use the following operators and function to create bigger documents:

• A <> B Concatenation of two documents.
• A :<|> B First try rendering A, if it exceeds maximum line length try B.
• A.flatten Remove all linebreaks from A.
• line A linebreak.
• lineOrSpace A linebreak that is changed to a space when removed by the formatter (flatten).
• group(A) Short for A.flatten :<|> (_ => A). First try to print in one line. If that is to long print in several lines.
• A <+> B Concatenation with a space between the documents. Short for A <> " " <> B.
• A </> B Concatenation with a linebreak or space between the documents. Short for A <> lineOrSpace <> B.
• sep(S, Ds) Documents Ds separated by S.