hello-caqti-lwt

Hello, caqti-lwt!

Now, we will talk to our database again with caqti, but in an asynchronous fashion via its lwt module.

Remember those monads?

Whereas before we could "extract" the data out of the Option type (which can be seen like a wrapper type), like so:

flowchart RL
    Option --> Data

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Now, we will have to do the same, but we will have to "cross" 2 contexts to successfully extract our data, like so:

flowchart RL
    Lwt --> Result
    Result --> Data

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To make things easier to write and read, we will again use a "let operator", like so:

let ( let* ) = Lwt_result.bind

This will allow us to successfully "cross" the 2 "monad" contexts, in one line of code (or short-circuit to a failure state).

cd ./03-hello-caqti-lwt
PGHOST=localhost PGDATABASE=caqti_study PGPORT=5433 dune exec ./bin/main.exe
PGHOST=localhost PGDATABASE=caqti_study PGPORT=5433 dune runtest --watch

Test via the REPL

NOTE: utop can resolve promises automagically. Rather than using the resolve_ok_exn helper, try to evaluate only the promise at the prompt.

$ PGHOST=localhost PGDATABASE=caqti_study PGPORT=5433 dune utop

# open Repo;;
# let conn = Init.connect_exn ();;
val conn : Caqti_lwt.connection = <module>
# let promise = Exec.add conn 1 2;;
val promise : (int, [> Caqti_error.call_or_retrieve ]) result Lwt.t = <abstr>

# open Repo;;
# let conn = Init.connect_exn ();;
val conn : Caqti_lwt.connection = <module>
# let promise_add = Exec.add conn 1 2;;
val promise_add : (int, [> Caqti_error.call_or_retrieve ]) result Lwt.t =
  <abstr>
# Repo.Exec.resolve_ok_exn promise_add;;
- : int = 3
# let promise_mul = Exec.mul conn 3 4;;
val promise_mul : (int, [> Caqti_error.call_or_retrieve ]) result Lwt.t =
  <abstr>
# Repo.Exec.resolve_ok_exn promise_mul;;
- : int = 12