spartanz / parserz   0.2.4

Apache License 2.0 GitHub

A purely-functional library for creating both parsers, pretty-printers, and grammar definitions from a single, type-safe specification of a grammar

Scala versions: 2.13 2.12

Welcome to Parserz

zero dependency invertible parser combinators library for Scala

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Parserz is a purely-functional library for creating parsers, pretty-printers, and grammar definitions from a single, type-safe specification of a grammar.

The main idea of the library is to facilitate conversions between types A and B in both directions via single coherent implementation, as in contrast of having two separate implementations for A => B and B => A.

The theory behind this idea is outlined in the paper Invertible Syntax Descriptions but the library does not exactly follow the implementation proposed in paper.

Overview and highlights

The following are the abstractions provided by library

trait ParsersModule {
  type Input
  sealed abstract class Grammar[-SI, +SO, +E, A] {}

Grammar is a data type for modelling user-defined grammars and ParsersModule is a module that contains interpreters of Grammar into runnable functions, i.e. parsers and printers, etc.

Internally library follows the "free encoding" design where Grammar is defined as a generalized algebraic data type (GADT), and all interpreters are implemented as traversals of Grammar.


  • Input - the input type; values of this type are consumed by parser and produced by printer
  • A - the output type; values of this type are produced by parser and consumed by printer
  • E - the error type; values of this type are produced by both parser and printer if they cannot proceed normally
  • SI - the input state type and
  • SO - the output state type; values of these types represent state that is threaded through the execution of parser and printer

Note: for the printer or other interpreters, meaning of Input and A may be completely reversed. Unfortunately, Scala does not allow redefining type names based on one's interpretation, so we have chosen the parser direction to be the titular one simply because library name is parserz.

Getting the library

Simply add

libraryDependencies += "org.spartanz" %% "parserz" % "0.2.0"

to the SBT build.

Using the library

Instantiate the module

import org.spartanz.parserz.ParsersModule

object MyParser extends ParsersModule {
  override type Input = List[Char]

which also requires defining the input type.

Import combinators

import MyParser.Grammar._
import MyParser._

It is now possible to create grammars!

Let's create building blocks to parse the following program


into description on how it can be invoked

case class Call(name: ::[Char], args: List[Call])

Full working example is here

Consuming input

consume combinator and its variants are used to take a portion of input and return remainder of the input and the consumed value. This is how it's defined:

final def consume[E, A](to: Input => E \/ (Input, A), from: ((Input, A)) => E \/ Input): Grammar[Any, Nothing, E, A]

It requires two functions to be provided both returning either a value or an error, for example

val char: G[Char] = consume({
  case c :: cs => Right(cs -> c)
  case Nil     => Left("eoi")
}, {
  case (cs, c) => Right(c :: cs)

The G[A] is simply a type alias for Grammar[Any, Nothing, String, A], which means

  • state is not used (accepts any state and returns nothing)
  • error type is set to String
  • the value taken by this grammar is of type List[Char] (as defined above) and produced value is of type A


There are combinators that allow to assert the value (and return error if assertion failed), for example filter and its variants, which is defined as

final def filter[E1 >: E](e: E1)(f: A => Boolean): Grammar[SI, SO, E1, A]

It accepts a predicate function which is applied to the result of existing grammar. For example to test that consumed character is something specific we can do

val alpha: G[Char]  = char.filter("expected: alphabetical")(_.isLetter)
val comma: G[Char]  = char.filter("expected: comma")(_ == ',')

Resulting grammars are new bigger building blocks.

Sequencing, alterating and repeating

  • ~ or zip combines two grammars that produce A and B into a grammar that produces Tuple2[A, B]
  • | or alt combines two grammars that produce A and B into a grammar that produces Either[A, B]
  • rep repeats the grammar zero or more times while it can and
  • rep1 repeats the grammar one or more times while it can

In the program "a(bx(),cy(z()))", "bx(),cy(z())" is the list of arguments of function a. Here is how to parse them: it's a call to another function followed by zero or more calls to other functions prefixed by comma, or no calls at all:

val args: G[List[Call]] = ((call ~ (comma ~ call).rep) | succeed(Nil)).map({
  case Left((e1, en)) => e1 ::
  case Right(_)       => Nil
}, {
  case Nil            => Right(Nil)
  case e1 :: en       => Left((e1,',', _))))

Recursive grammars

In the previous example there is a call to call grammar that produces a value of Call. Call is also a recursive data structure, as it references itself:

case class Call(name: ::[Char], args: List[Call])

To allow recursion in grammar definitions, current version of library has the delay combinator. Here is how grammar for Call is defined:

lazy val call: G[Call] = delay {
 (alpha.rep1 ~ paren1 ~ args ~ paren2).map(
   { case (((name, _), exp), _) => Call(name, exp) },
   { case Call(name, exp)       => (((name, '('), exp), ')') }

Notice the lazy modifier as well which allows this grammar to be forward-referenced.


All grammars are now constructed and can be passed to interpreters.


The parser is simply a function constructed by the library from the given grammar:

val parser: (Unit, List[Char]) => (Unit, String \/ (List[Char], Call)) = MyParser.parser(call)

Passing program "a(bx(),cy(z()))" into this function (along with state of Unit) yields

Call(::('a', Nil), List(
  Call(::('b', List('x')), Nil), 
  Call(::('c', List('y')), List(
    Call(::('z', Nil), Nil)

which is indeed the structure representing function calls in the program.


The printer is also a function constructed by the library from the given grammar:

val printer: (Unit, (List[Char], Call)) => (Unit, String \/ List[Char]) = MyParser.printer(call)

Printing the value just received from the parser yields the program text back:

printer((), (Nil, value))

Here Nil is initial output, which is, of course, empty, and there is a reverse done on the output because of the consume implementation above which prepends to list.

Grammar definition

There is an interpreter that produces a description of the grammar in the Backus–Naur form (BNF).

val description: List[String] = MyParser.bnf(call)

For it to work, grammars can be described with the tag combinator (or its symbolic equivalent @@):

val char: G[Char] = "char" @@ consume(...)
val alpha: G[Char]  = char.filter("expected: alphabetical")(_.isLetter).tag("alpha")

It interprets directly into a value, which is a printout of all tagged grammars. This is how it looks like for the example above

<alpha>       ::= <char>
<open paren>  ::= <char>
<comma>       ::= <char>
<args>        ::= (<call> List(<comma> <call>) | )
<close paren> ::= <char>
<call>        ::= NEL(<alpha>) <open paren> <args> <close paren>