package earley

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Parsing library based on Earley Algorithm


Dune Dependency






  • OCaml (at least 4.07.0)

  • Dune (at least 2.7)

  • stdlib-shims (at least 0.1)

  • GNU Make

Installation procedure

You can either pin the repository using Opam with

opam pin add earley

or clone an install manually with

git clone
cd ocaml-earley
sudo make install

Note: the make command will compile pa_ocaml from an ascii OCaml files in directory boot (generated by pa_ocaml before distribution). It the does a second compilation pass using the newly generated pa_ocaml binary as the preprocessor directly.

Getting started writing parser with Earley

The Earley library (lib folder), which provides a set of parser combinators, is not intended to be used directly (although it can be). Indeed, calls to these combinators can be automatically generated from a BNF-like syntax, accessed through a very natural syntax extension for the OCaml language (documented bellow).

Compilation example (single file)

Assuming a project with a single file, and no other dependency than Earley, a binary could be produced with the following command.

ocamlfind ocamlopt -pp pa_ocaml -package earley -linkpkg -o my_project

Merlin support

For Merlin to (roughly) recognise our syntax extension, you can add the following line to the .merlin file of your project.

FLG -pp pa_ocaml

Syntax extension

Parser expression

A parser expression, corresponding to a value of type 'a Earley.grammar (where 'a is the type of object the parser produces), can be constructed using:

| ...

Note: the syntax is similar to match ... with .... The first | can be omitted, and parentheses are often required around parser expressions. In particular, parser is a keyword.

Parser declaration at the top level

Declaration of a simple parser:

let parser p1 = (* Here, "parser" is a keyword, like "rec" in "let rec". *)
  | RULE11
  | RULE12
  | ...
  | RULE1N

Note: this syntax is equivalent to:

let p1 =
  | RULE11
  | RULE12
  | ...
  | RULE1N

Parsers can be defined mutually recursive with other parsers (and functions):

let parser p1 =
  | RULE11
  | RULE12
  | ...
  | RULE1N

and f x1 ... xk =

and parser p2 =
  | RULE21
  | ...
  | RULE2M

and f x y z ... =


and parser pK arg =
  | RULEK1
  | ...

Note: parsers can take arguments (see pK).

Parsing rule

A parsing rule is formed of a sequence of atoms, followed by an action.

x1:ATOM1 x2:ATOM2 ... xN:ATOMN -> ACTION

Here, the parsed input corresponding to ATOM1 is put in variable x1, which is bound in ACTION, and similarly for other atoms. If no labels is given in front of an atom, then the parsed input is not recorded.

If no action is given, the value of the atoms are gathered in a tuple. As a consequence, the following three rules are equivalent.

x1:ATOM1 x2:ATOM2 ... xN:ATOMN -> (x1, ..., xN)
x1:ATOM1 x2:ATOM2 ... xN:ATOMN

Note: in fact, in the last rule, only the atoms which value is really meaningful are added to the couple (e.g., atoms corresponding to regexps, but not atoms that only accept a single input string). It is possible to prevent a meaningful atom from being added to the couple using the syntax _:ATOM.


Flexible support for positions is provided. It is enabled using a line of the following form, involving a locate function. It should be in scope and have type Earley.input -> int -> Earley.input -> int -> t, where t is a type of your choice for representing a position.

#define LOCATE locate

Once positions have been enabled, position variables of the form _loc_x are automatically created for each atom label x. They are available in the corresponding action. Note that a _loc variable is also provided. It corresponds to the full rule.

Parsing atoms

We provide atoms of the following form:

ANY         (* Accepts any character, and returns it. *)
EOF         (* Accepts only the end of file. *)
EMPTY       (* Does not accept anything. *)
'a'         (* Accepts only the character 'a'. *)
CHR('a')    (* Same as above. *)
"ab"        (* Accepts only the string "ab". *)
STR("ab")   (* Same as above. *)
''[ab]+''   (* Accepts the input matching the given regexp, and returns it. *)
RE("[ab]+") (* Same as above, but needs escaping as with usual Str regexp. *)

More complex atoms can be built from OCaml expressions or other atoms:

(expr)      (* Any OCaml expression corresponding to a parser. *)
ATOM?       (* Optionally accept an ATOM, returns option type. *)
ATOM?[v]    (* Optionally accept an ATOM, returns v as default value. *)
ATOM*       (* Accepts a list of ATOM, returns a list. *)
ATOM+       (* Accepts a non-empty list of ATOM, returns a list. *)

Finally, a parser can be used as an atom with the following syntax:

{ RULE1 | RULE2 | ... | RULEN }

Scanner-less parsing

Earley parsers do not require a lexer. Non-significant parts of the input (blank characters, comments, ...) are ignored using a blank function, which is called between every atom. The top level blank function is provided when calling a parsing function (see Earley.parse_file for example).

Note: the current blank function can be changed at any time (see the Earley.change_layout function).

Note: blanks can be temporarily disabled between two atoms with the syntax ATOM1 - ATOM2.


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