package pgocaml_ppx

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PPX extension for PGOCaml


Dune Dependency






PGOCaml provides an interface to PostgreSQL databases for OCaml applications. This PPX syntax extension enables one to directly embed SQL statements inside the OCaml code. The extension uses the 'describe' feature of PostgreSQL to obtain type information about the database. This allows PGOCaml to check at compile-time if the program is indeed consistent with the database structure.

Published: 21 Jul 2020


PG'OCaml is a set of OCaml bindings for the PostgreSQL database.

Please note that this is not the first or only PGSQL bindings for OCaml. Here are the others which you may want to consider:

  • postgresql-ocaml PostgreSQL-OCaml by Markus Mottl

  • ocamlodbc ODBC bindings by Max Guesdon which can be used to access PostgreSQL

  • ocamldbi a Perl-like DBI layer by the present author

PG'OCAML is different than the above bindings:

  • It ISN'T just a wrapper around the C libpq library. Instead it's a pure OCaml library which talks the frontend/backend protocol directly with the database.

  • It has a PPX (macro) layer which lets you write SQL statements directly in your code, TYPE SAFE at compile time, with TYPE INFERENCE into the SQL, and using the full PostgreSQL SQL grammar (sub-selects, PG-specific SQL, etc.). But the flip side of this is that you need to have access to the database at compile time, so the type checking magic can be done; also if you change the database schema, you will need to recompile your code to check it is still correctly typed.

  • (A minor point) - It requires PostgreSQL >= 7.4. The default interface (PGOCaml) provided is synchronous. But it also supports any asynchronous interface that implements the PGOCaml_generic.THREAD signature.

  • It doesn't work with other databases, nor will it ever work with other databases.


PG'OCaml uses environment variables (or in-code parameters, which are [ill advised] ( to connect to your database both at compile-time and at runtime.

Variable Default Additional information
PGHOST If this starts with a / or is unspecified, PG'OCaml assumes you're specifying a Unix domain socket.
PGPORT 5432 This is also the default PostgreSQL port.
PGUSER The username of the current user, or postgres if that can't be found.
PGPASSWORD empty string
PGPROFILING no profiling Indicates the file to write profiling information to. If it doesn't exist, don't profile
COMMENT_SRC_LOC no If set to yes, 1, or on, PG'OCaml will append a comment to each query indicating where it appears in the OCaml source code. This can be useful for logging.
PGCUSTOM_CONVERTERS_CONFIG nothing Points to a file containing custom type conversions

Using the PPX

The PPX aims to be more or less a carbon copy of the former extension.

let () =
  let dbh = PGOCaml.connect () in
  let insert name salary =
    [%pgsql dbh "insert into employees (name, salary) VALUES ($name, $salary)"]
  ignore(insert "Chris" 1_000.0);
  let get name =
    [%pgsql dbh "select salary from employees where name = $name"]
  let () = [%pgsql dbh
        name TEXT PRIMARY KEY,
        salary FLOAT)"]
  let name = "Chris" in
  let salary = get name
    |> List.hd
    |> function
        | Some(x) -> x
        | None -> raise(Failure "The database is probably broken.")
  Printf.printf "%s's salary is %.02f\n" name salary;

The PPX allows you to specify that queries returning results should be returned as objects, rather than tuples.

let%lwt res =
  [%pgsql.object dbh "SELECT * FROM employees"]
  (fun row ->
    Printf.printf "%s makes $%f\n" row#name row#salary)

The PPX now also supports ${...} expansions.

(* where [e] is a row returned by a [pgsql.object] query *)
let%lwt incr_sal e =
  [%pgsql dbh "UPDATE employees SET salary = ${e#salary +. 1.0}"]

You may wish to print all the fields of an object for debugging purposes.

let%lwt rows = [%pgsql.object dbh "show" "SELECT * FROM employees"] in
  (fun row -> print_endline row#show)

The above code will not work if one of the selected fields is named show. The PPX allows one to explicitly name the pretty-printer method as follows:

let%lwt rows = [%pgsql.object dbh "show=pp" "SELECT * FROM employees"] in
  (fun row -> print_endline row#pp)

It's important to note that the show directive causes values to be printed in the same format used by the Postgres API, so things like Calendar.t values and custom converters (see below) may not work as expected.

Custom Type Conversions

Custom serializers and deserializers may be provided in a configuration file specified by PGCUSTOM_CONVERTERS_CONFIG (see above). An example configuration file follows:

( ( ( Or
      ( (Rule (typnam userid)) ; userid is a fully abstract type
        (Rule (colnam userid))
    ( (serialize Userid.to_string)
      (deserialize Userid.from_string)
  ( ( Or
      ( (Rule (typnam cash_money)) ; for strings beginning with a $ and possibly needing to be trimmed
        (And ; there exists a column elsewhere also named salary, but it has a different type
          ( (Rule (typnam float))
            (Rule (colnam salary))
    ( (serialize "fun x -> String.(sub x 1 (length x - 1)) |> String.trim")
      (deserialize "fun x -> \"$\" ^ x")

In case you're working on a large project, and don't want to write many convoluted rules to play nicely with your existing database structure, you can selectively enable custom serialization for individual queries:

let rows =
    "SELECT * FROM customtable"]

PG'OCaml's PPX is not given type information by the compiler, so it will sometimes have trouble figuring the correct types for arguments. While one may find a parameter's serializer based in its name, this is not ideal, particularly when using ${...} parameters. In cases like this, you may use a syntax similar to OCaml's native type constraint:

let () = [%pgsql dbh "INSERT INTO users VALUES ${u : userid}"] in

Please note that the ${u : userid} above will NOT be compiled into an OCaml type constraint. Its only effect is to supply userid as the typnam to the serializer resolver.

PG'OCaml (C) Copyright 2005-2009 Merjis Ltd, Richard W.M. Jones ( and other authors (see CONTRIBUTORS.txt for details).

This software is distributed under the GNU LGPL with OCaml linking exception. Please see the file COPYING.LIB for full license.

For an example, please see tests

Dev Dependencies


Used by (1)

  1. ocsigen-start >= "2.9.1"




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