package sexp_decode

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A library to decode S-expression into structured data


Dune Dependency






A library of monadic combinators that help translating S-expressions (as provided by the Csexp library) into structured data

Published: 19 Jul 2022



Sexp_decode is a library of monadic combinators for decoding S-expressions (as defined in the Csexp library) into structured data.

Important remark: the current API is well suited to the complete parsing of S-expressions. Partial parsing (e.g., parsing only some fields of an S-expression encoded record) is doable, but remains convoluted with the current API.


The purpose of the library is to help the translation of S-expressions into structured data.

For example, you may want to transform an address book encoded as an S-expression into structured data, that is easier to process.

Let's assume your address book looks like the following:

open Sexp_decode

let address_book : sexp =
        Atom "entry";
        List [ Atom "name"; Atom "John Doe" ];
        List [ Atom "country"; Atom "New Zealand" ];
        Atom "entry";
        List [ Atom "name"; Atom "Mary Poppins" ];
        List [ Atom "email"; Atom "" ];
        Atom "entry";
        List [ Atom "name"; Atom "Groot" ];
        List [ Atom "country"; Atom "Groot" ];

A representation as an OCaml value that is probably easier to work with, is by using the following entry type:

type entry =
  { name : string; country : string option; email : string option }

type address_book = entry list

It is easy to define decoders that produce values of types entry and address_book:

let entry_decoder : entry decoder =
field "entry"
@@ let* name = field "name" atom in
   let* country = maybe @@ field "country" atom in
   let+ email = maybe @@ field "email" atom in
   { name; country; email }

let address_book_decoder : address_book decoder = list entry_decoder

Then, you can execute the run function, that has type 'a decoder -> sexp -> 'a option. It produces the following result on our address_book example:

let result = run address_book_decoder address_book
(* result =
      [{name = "John Doe"; country = Some "New Zealand"; email = None};
       {name = "Mary Poppins"; country = None;
        email = Some ""};
       {name = "Groot"; country = Some "Groot"; email = None}]

In addition to the field, maybe, atom and list decoders, the Sexp_decode library provides combinators to build compound decoders from basic ones, and compose them together. In particular, decoders for variants and records are provided.

For example, with the fields combinator, you could define entry_decoder as follows:

let entry_decoder_alt : entry decoder =
  field "entry"
  @@ fields
       ~default:{ name = ""; country = None; email = None }
         ("name", atom >>| fun name entry -> { entry with name });
         ("country", atom >>| fun country entry -> { entry with country = Some country });
         ("email", atom >>| fun email entry -> { entry with email = Some email });

With this alternative decoder for entries, the fields "name" "country" and "email" might occur in any order, and any number of times.

How to install the library

Sexp_decode is available on opam and can be installed with the following command:

opam install sexp_decode

Alternatively, you can install the development version as follows:

git clone
cd sexp_decode
opam pin add .

Author: Benoît Montagu

Copyright Inria 2022

Dependencies (4)

  1. ocaml >= "4.08"
  2. ppx_inline_test >= "v0.14.1"
  3. csexp >= "1.5.1"
  4. dune >= "2.9"

Dev Dependencies (1)

  1. odoc with-doc

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