ocaml-sqlexpr is a simple library and syntax extension for type-safe, convenient execution of SQL statements, currently compatible with Sqlite3.

The latest version can be found at https://github.com/mfp/ocaml-sqlexpr

ocaml-sqlexpr features:

• automated prepared statement caching, parameter binding, data extraction, error checking (including automatic statement reset to avoid BUSY/LOCKED errors in subsequent queries), statement finalization on database close, etc.
• higher order functions like iter, fold, transaction
• support for different concurrency models: everything is functorized over a THREAD monad, so you can for instance do concurrent treatmments with Lwt
• support for SQL statement syntax checks and some extra semantic checking (column names, etc.)

ocaml-sqlexpr is used as follows:

module Sqlexpr = Sqlexpr_sqlite.Make(Sqlexpr_concurrency.Id)
module S = Sqlexpr

let () =
  let db = S.open_db "foo.db" in
  S.iter db
    (fun (n, p) -> Printf.printf "User %S, password %S\n" n p)
    sqlc"SELECT @s{login}, @s{password} FROM users";
  List.iter
    (fun (n, p) -> S.execute db sqlc"INSERT INTO users VALUES(%s, %s)" n p)
    [
     "coder24", "badpass";
     "tokyo3", "12345"
    ]

See also the example file example.ml.

Dependencies

• cppo
• csv
• lwt (>= 2.2.0)
• lwt.syntax
• lwt.unix
• ppx_tools
• re
• sqlite3
• threads
• unix

Optional Dependencies

• camlp4
• estring

The optional dependencies allow building of the Camlp4 syntax extension.

Camlp4 syntax extension

ocaml-sqlexpr includes a syntax extension to build type-safe SQL statements and expressions:

• sql"..." denotes a SQL statement or expression
• sqlc"..." denotes a SQL statement or expression that is to be cached
• sql_check"sqlite" returns a tuple of functions to initialize, check the validity of the SQL statements or expressions and check against an auto-initialized temporary database.
• sqlinit"..." is equivalent to sql"...", but the statement will be added to the list of statements to be executed in the automatically generated initialization function

sql_check"sqlite" is used as follows:

let auto_init_db, check_db, auto_check_db = sql_check"sqlite"

which creates 3 functions

val auto_init_db : Sqlite3.db -> Format.formatter -> bool
val check_db : Sqlite3.db -> Format.formatter -> bool
val auto_check_db : Format.formatter -> bool

each of them returns false on error, and writes the error messages to the provided formatter.

PPX syntax extension

In addition to the camlp4-based syntax extension, ocaml-sqlexpr includes a syntax extension using extension points (ppx). The conversion from camlp4 to ppx is as follows:

• [%sql "..."] corresponds to sql"..."
• [%sqlc "..."] corresponds to sqlc"..."
• [%sqlcheck "sqlite"] corresponds to sql_check"sqlite"
• [%sqlinit "..."] corresponds to sqlinit"..."

SQL statement/expression syntax

Literals marked with sql or sqlc are similar to Printf's format strings and their precise types depend on their contents. They accept input parameters (similarly to Printf) and, in the case of SQL expressions, their execution will yield a tuple whose type is determined by the output parameters.

Input parameters are denoted with %X where X is one of:

Input parameter	OCaml type
%d	int
%l	Int32.t
%L	Int64.t
%s	string
%S	string (handled as BLOB by SQLite)
%f	float
%b	bool
%a	('a -> string) (resulting string handled as BLOB by SQLite)

A literal % is denoted with %%.

A parameter is made nullable (turning the OCaml type into a _ option) by appending a ?, e.g. %d?.

Output parameters are denoted with @X{SQL expression} where X is one of:

Output parameter	OCaml type
@d	int
@l	Int32.t
@L	Int64.t
@s	string
@S	string (handled as BLOB by SQLite)
@f	float
@b	bool

A literal @ is denoted with @@ As in the case of input parameters, output parameters can be made nullable by appending a ?.

A sql"..." or sqlc"..." literal is of type _ statement if it has no output parameters, and of type _ expression if it has at least one.

Examples

sql"SELECT @s{name} FROM users"                   is an expression
sql"SELECT @s{name} FROM users WHERE id = %d"     is an expression
sql"SELECT @s{name}, @s{email} FROM users"        is an expression
sql"DELETE FROM users WHERE id = %d"              is a statement

Statements are executed with execute or insert (which returns the id of the new row); expressions are “selected” with a function from the select* family or a higher order function like iter or fold.

Examples

module Sqlexpr = Sqlexpr_sqlite.Make(Sqlexpr_concurrency.Id)
module S = Sqlexpr

let insert_user_stmt =
    sqlc"INSERT INTO users(login, password, email) VALUES(%s, %s, %s?)"

let insert_user db ~login ?email ~password =
  S.execute db insert_user_stmt login password email

(* insert user and return ID; we use partial application here *)
let new_user_id db = S.insert db insert_user_stmt

let get_password db =
  S.select_one db sqlc"SELECT @s{password} FROM users WHERE login = %s"

let get_email db =
  S.select_one db sqlc"SELECT @s?{email} FROM users WHERE login = %s"

let iter_users db f =
  S.iter db f sqlc"SELECT @L{id}, @s{login}, @s{password}, @s?{email}
                     FROM users"

Test and Sample Build Instructions

Example Camlp4 Code:

ocamlfind ocamlc -package sqlexpr,pa_sqlexpr -syntax camlp4o -linkpkg -thread -o sqlexpr_camlp4 tests/syntax/example.ml

Example PPX Code

ocamlfind ocamlc -package sqlexpr.ppx -linkpkg -thread -o sqlexpr_ppx tests/ppx/example.ml

or

dune build tests/ppx/example.exe

Camlp4 based tests:

ocamlfind ocamlc -package sqlexpr,pa_sqlexpr,lwt.syntax,oUnit -syntax camlp4o -linkpkg -thread -o sqlexpr_camlp4_test tests/syntax/t_sqlexpr.ml

PPX based tests:

ocamlfind ocamlc -package sqlexpr.ppx,lwt_ppx,oUnit -ppxopt lwt_ppx,-no-debug -linkpkg -thread -o sqlexpr_ppx_test tests/ppx/t_sqlexpr.ml

or

dune runtest ./tests/ppx