zbroyar / pgocaml

PG'OCaml provides an interface to PostgreSQL databases for OCaml applications. It uses Camlp4 to extend the OCaml syntax, enabling one to directly embed SQL statements inside the OCaml code.

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 camlp4 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.

Usage

PG'OCaml uses environment variables (or in-code parameters, which are [ill advised] (https://hackernoon.com/how-to-use-environment-variables-keep-your-secret-keys-safe-secure-8b1a7877d69c)) 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.

Using the PPX

In addition to the camlp4 syntax extension, there is also a PPX available for more recent versions of OCaml. The PPX aims to be more or less a carbon copy of the camlp4 extension.

let () =
  let dbh = PGOCaml.connect () in
  let insert name salary =
    [%pgsql dbh "insert into employees (name, salary) VALUES ($name, $salary)"]
  in
  ignore(insert "Chris" 1_000.0);
  let get name =
    [%pgsql dbh "select salary from employees where name = $name"]
  in
  let () = [%pgsql dbh
      "execute"
      "CREATE TEMP TABLE IF NOT EXISTS employees (
        name TEXT PRIMARY KEY,
        salary FLOAT)"]
  in
  let name = "Chris" in
  let salary = get name
    |> List.hd
    |> function
        | Some(x) -> x
        | None -> raise(Failure "The database is probably broken.")
  in
  Printf.printf "%s's salary is %.02f\n" name salary;
  PGOCaml.close(dbh)

The PPX allows you to specify that queries returning results should be returned as objects, rather than tuples. This is not currently supported in the Camlp4 version (which is being deprecated).

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

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}"]

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PG'OCaml (C) Copyright 2005-2009 Merjis Ltd, Richard W.M. Jones ([email protected]) 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.

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For an example, please see tests