Pretty-printing

[joelburget]

The three classic pretty-printer papers:

• Pretty Printing -- Derek C. Oppen, 1979.
• The Design of a Pretty-printing Library -- John Hughes, 1995.
• A prettier printer -- Philip Wadler, 1997. This algorithm is fairly widely used, eg Prettier uses it.

There's an adaptation of Wadler's algorithm to OCaml -- Strictly Pretty -- Christian Linding, 2000.

Wadler's algorithm is presented as a set of combinators

(<>) :: Doc -> Doc -> Doc
nil :: Doc
text :: String -> Doc
line :: Doc
nest :: Int -> Doc -> Doc
group :: Doc -> Doc
(<|>) :: Doc -> Doc -> Doc
flatten :: Doc -> Doc

However, we want to specify layout as part of the concrete syntax declaration. Probably with boxes. Eg:

com :=
  | "skip" { skip() }
  | [<hv 1,3,0> [<h 1> name ":="] iexp] { assign($1; $2) }
  | [<hov 1, 0, 0> 
      [<h 1> "if" bexp]
	  [<h 1> "then" com]
      [<h 1> "else" com]
    ] { if($2; $4; $6) }
  ...

This example is taken with only minor modifications from Syn: a single language for specifying abstract syntax trees, lexical analysis, parsing and pretty-printing -- Richard J Boulton, 1996.

This is also quite similar to Ocaml's Format (they even have almost exactly the same types of boxes):

• Format module.
• Format Unraveled - Richard Bonichon, Pierre Weis, 2017.

The Syn declaration seems a little heavy-weight. Ocaml also has break hints. This seems like roughly the right direction -- I'm still weighing the tradeoffs.

Other:

• A Pretty But Not Greedy Printer (Functional Pearl) Jean-Philippe Bernardy, 2017. Possibly useful while implementing the layout algorithm.