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Date: Wed, 15 Jul 2015 22:32:09 +0900
From: Oleg <oleg@okmij.org>
To: martindemello@gmail.com
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Subject: Re: [Caml-list] looking for "real world" sqlite3 examples


> 3. is there a maintained library for generating sql queries in a typed manner?

> But mostly, I want to look at someone else's code and get an idea of
> how this is done in ocaml; pretty much all my database code to date
> has been written in dynamically typed languages and relied on code
> generation.

I'd like to point out
        http://logic.cs.tsukuba.ac.jp/~ken/quel/

which is a library for writing query in a typed, functional style and
generate efficient SQL (without nested SELECTs). The library makes SQL
composable, however odd it may seem. One may consider the library
similar to T-LINQ, described by Cheney et al at ICFP 2013 -- only we
use pure OCaml rather than F# and generate a SQL statement that can be
submitted to any database. (We actually used PostgreSQL for testing).
The main theoretical difference is that our normalization rules are
typed and type-preserving by construction -- and extensible, to
compensate for difference among databases.

Here is a simple example

(* Should be automatically generated, but currently isn't *)
module type SCHEMA = sig   
  type 'a repr

  val oid      : <oid:int; pid:int; qty:int> repr -> int repr
  val opid     : <oid:int; pid:int; qty:int> repr -> int repr
  val qty      : <oid:int; pid:int; qty:int> repr -> int repr

  val orders   : unit -> <oid:int; pid:int; qty:int> list
end
module type SYM_SCHEMA = sig
  include SymanticsL
  include SCHEMA  with type 'a repr := 'a repr
end

(* simple query *)

module Q1'(S:SYM_SCHEMA) = struct
  open S
  let table_orders = table ("orders", orders ())

  let res =
      foreach (fun () -> table_orders) @@ fun o ->
      where (oid o =% int 2) @@ fun () ->
      yield o

  let observe = observe
end

let module M = Q1'(GenSQL) in M.observe (fun () -> M.res)
(* "SELECT x.* FROM orders AS x WHERE true AND x.oid = 2" *)