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With Invertible Syntax, you write a description of your language similar to what you would write with parser combinators, and then can you use that same description to parse or to pretty-print. The results are guaranteed to be isomorphic by construction.

Inspired by Haskell's invertible-syntax and partial-isomorphisms.

Getting Started

Define your syntax by implementing Syntax[A] for your desired type. The apply method will be provided with a type for which the Transcriber typelass is defined, and uses the primitives and combinators defined in the Syntax companion to construct a P[A].

scala> import invertible._, Syntax._

scala> val ints = new Syntax[List[BigInt]] {
     |   def apply[P[_]: Transcriber] = int sepBy1 sepSpace
     | }
ints: invertible.Syntax[List[BigInt]] = $anon$1@506cb8df

To parse input text, simply call parse. If the parser fails, you get a nice error message identifying the location of the problem:

scala> ints.parse("1 20 30")
res0: scalaz.\/[invertible.ParseFailure,List[BigInt]] = \/-(List(1, 20, 30))

scala> ints.parse("1 abc").leftMap(println)
expected: " " or digit; found: 'a'
1 abc

To print:

scala> ints.print(List(1, 20, 300))
res2: Option[String] = Some(1 20 300)

Note: in a simple syntax like this, printing never fails because the syntax can represent every possible value. But it’s not uncommon for the syntax’s result type to include values that can’t be represented in the textual form. E.g. variable names may be arbitrary Strings according to the AST, but only alphanumeric names are actually parsed/printed. In that case, the printer will be unable to map the value to a valid text, and will return None.


A port of the simple parser presented in Tillmann Rendel and Klaus Ostermann. Invertible syntax descriptions: Unifying parsing and pretty printing. Haskell symposium, 2010 is in simple.scala.

A more complicated example parser, handling a richer language and providing source locations, is in example.scala.

The examples project contains a complete, verified JSON parser/printer.


Invertible syntax and partial isomorphisms for Scala



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