Libary for automatic recovery of syntactic elements. LL parsing for case classes made simple.
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ARSE Library

Libary for automatic recovery of syntactic elements. LL parsing for case classes made simple.

Author: Gidon Ernst

Feedback is welcome! I'd appreciate to hear whether anyone found this library useful.

Motivation & Overview

Scala's case classes are a fine way to represent syntax trees. For example, consider a simple Lisp-like language for expressions with identifiers, numbers and function application:

trait Expr
case class Id(name: String) extends Expr
case class App(args: List[Expr]) extends Expr

This definition corresponds (almost) directly to the following attributed grammar:

expr := id | app
id   := string        { Id(_)  }
app  := "(" expr* ")" { App(_) }

The aim of this library is to minimize the fuss to create a parser for such a language. The main idea is that, given a parser for the arguments of a case class constructor, the parser for the case class is instantiated automatically. With arse you can specify this as follows:

val name = S("[a-zA-Z]+")
val expr: Parser[Expr] = P(app | id)
val id = Id(name)
val app = App("(" ~ (expr *) ~ ")")

This example demonstrates a few features of the library:

  • Primitive combinators are scanners S("regex") and literals "lit"
  • Sequential composition p ~ q, which is strict in q if p succeeds
  • Ordered choice p | q, which backtracks if p fails and tries q
  • Recursive parsers through P(p), which evaluates p lazily
  • Implicit lifting of functions to parsers, where f(p1 ~ ... ~ pn) parses p1 ~ ... ~ pn and applies f(a1, ..., an) to the results a1, ..., an

The complete example including a test case can be seen here.



Compile & Install

sbt compile
sbt package
sbt publishLocal # if you want to use it in other projects locally


Parsers p: Parser[A] parse a piece of text and return a result a: A.

Using parsers

  • define implicit val w: Whitespace = ... (interleave literals with whitespace w)
  • p.parse(in) parse p on in: String or fail with an error
  • p.parse(in, cm=false) try to parse p and fail with backtrack() (do not commit, internally used for backtracking, see also bk)
  • p.parseAt(in, pos, cm) parse in at position pos

Constructing parsers

  • S(regex) matches and returns part of the input against regex
  • int, double, char, string: predefined scanners for numbers, character literals 'x' and strings "..." (support escaping \' resp \")
  • val n: Parser[T] = P(p) declares a recursive parser with name n that defers evaluation of p to allow forward declaration of p; works in class/object scope when p refers to fields, but not when p refers to local variables defined later
  • ret(a) succeeds without consuming input and returns a
  • p ~ q parses p and returns a pair (a,b) of the results
  • p ~> q and p <~ q are variants of sequential composition that discard the right resp. left result
  • p | q tries p and if it fails parses q
  • "lit" ~ p matches a literal "lit", parses p and returns it's result, similarly p ~ "lit"
  • p $ parses p but causes an error if there is remaining input after p
  • p ?, p *, p +: optional and sequence parsers, returning Option[_] resp. List[_]
  • p ~* q and p ~+ q: parse p ~ q ~ ... ~ q ~ p and return the results of p as List[_]; the latter requires at least one p result
  • p.filter(f), p.filterNot(f), filter/map a result by a predicate/function f
  • p.reduceLeft(f), p.reduceRight(f): parse p+ and reduce the result with f
  • p.foldLeft(z)(f), p.foldRight(z)(f), parse z ~ p+ resp. p+ ~ z and fold the result with f


Beware that this library takes a nuanced view on backtracking: Ordered choice p | q permits p to fail and tries q only if p has 1) not consumed input and 2) not produced a result (which includes ret(a)). Similarly, p* returns an error when p can be parsed partially.

This leads to somewhat reasonable automatic error reporting and it is usually the behavior that one needs. However, it means that you cannot backtrack over arbitrary complex pieces of syntax, which can sometimes be a nuisance. For example, (int ~ "a") | (int ~ "b") fails on the input "0b" because it commits to the first choice after successfully parsing "0" with int.

Mixfix parsing

  • M(p, op, ap, s) parses mixfix expressions

    • p is used to parse inner expressions
    • op: parse an operator such as "+"
    • ap: apply subexpressions as arguments to an operator previously returned by op
    • s: Syntax: a database of mixfix operators

trait Syntax is parametrized by

def prefix_ops: Map[Op, Int]
def postfix_ops: Map[Op, Int]
def infix_ops: Map[Op, (Assoc, Int)]

which classify an operator returned by op into prefix, postfix, and infix and return the respective precedence. If you want a dynamic set of mixfix operators, just implement these as vars in your Syntax object, no need to re-create the M(...) parser everytime you change the set of operators.

Note that it is possible to overload operators in the different categories, the parsing algorithm can discern e.g. between unary and binary - (minus). Infix takes precedence over postfix, i.e., parsing repetition * will precede binary multiplication only if the postfix priority of * is high enough to prohibit a right argument (at the given source location).

See also:


The library is probably slow and would suffer from backtracking on ambiguous grammars.

The combinator library (no longer) shipped with Scala is much more feature complete and supports left recursion by packrat parsing.

The fastparse library for Scala provides a similar interface, but has many additional (cool!) features and it is designed for speed.

The parseback handles all context free grammars through parsing with derivatives. Give this library a try if you need a more flexible grammar or run into limitations of arses naive backtracking.

If you need guaranteed linear runtime complexity, try a LALR parser generator instead, such as beaver.


  • line and position tracking
  • default implementation of syntax trees that store positioning information
  • mixfix pretty printer