PPX extension for PGOCaml

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

  • 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


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. | |
| PGDATABASE | falls back on PGUSER | |
| 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

12 Jun 2021
>= "0.16.0"
= version
>= "4.07"
>= "1.10"
Reverse Dependencies
>= "2.9.1"