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c85571f Mar 28, 2017
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@franciscodr @juanpedromoreno
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* scala-exercises - exercises-doobie
* Copyright (C) 2015-2016 47 Degrees, LLC. <>
package doobie
import doobie.DoobieUtils.CountryTable._
import doobie.imports._
import org.scalaexercises.definitions.Section
import org.scalatest._
import scalaz.Scalaz._
import scalaz._
/** ==Introduction==
* doobie is a monadic API that provides a number of data types that all work the same way
* but describe computations in different contexts.
* In the doobie high level API the most common types we will deal with have the form
* ConnectionIO[A], specifying computations that take place in a context where a
* java.sql.Connection is available, ultimately producing a value of type A.
* doobie programs are values. You can compose small programs to build larger programs. Once you
* have constructed a program you wish to run, you interpret it into an effectful target monad of
* your choice (Task or IO for example) and drop it into your main application wherever you like.
* ==First programs==
* {{{
* import doobie.imports._
* import scalaz._, Scalaz._
* import scalaz.concurrent.Task
* }}}
* So let’s start with a ConnectionIO program that simply returns a constant.
* {{{
* val program = 42.point[ConnectionIO]
* program: ConnectionIO[Int] = Return(42)
* }}}
* This is a perfectly respectable doobie program, but we can’t run it as-is; we need a Connection
* first. There are several ways to do this, but here let’s use a Transactor.
* {{{
* val xa = DriverManagerTransactor[Task](
* driver = "org.postgresql.Driver",
* url = "jdbc:postgresql:world",
* user = "postgres",
* pass = ""
* )
* }}}
* A Transactor is simply a structure that knows how to connect to a database, hand out
* connections, and clean them up; and with this knowledge it can transform ConnectionIO ~> Task,
* which gives us something we can run. Specifically it gives us a Task that, when run, will
* connect to the database and run our program in a single transaction.
* The DriverManagerTransactor simply delegates to the java.sql.DriverManager to allocate
* connections, which is fine for development but inefficient for production use.
* @param name connecting_to_database
object ConnectingToDatabaseSection extends FlatSpec with Matchers with Section {
* Right, so let’s do this.
def constantValue(res0: Int) =
42.point[ConnectionIO].transact(xa).run should be(res0)
/** We have computed a constant. It’s not very interesting because we never ask the database to
* perform any work, but it’s a first step
* We are gonna connect to a database to compute a constant.
* Let’s use the sql string interpolator to construct a query that asks the database to compute
* a constant. The meaning of this program is “run the query, interpret the resultset as
* a stream of Int values, and yield its one and only element.”
def constantValueFromDatabase(res0: Int) =
sql"select 42".query[Int].unique.transact(xa).run should be(res0)
/** What if we want to do more than one thing in a transaction? Easy! ConnectionIO is a monad,
* so we can use a for comprehension to compose two smaller programs into one larger program.
def combineTwoPrograms(res0: (Int, Int)) = {
val largerProgram = for {
a <- sql"select 42".query[Int].unique
b <- sql"select power(5, 2)".query[Int].unique
} yield (a, b)
largerProgram.transact(xa).run should be(res0)
/** The astute among you will note that we don’t actually need a monad to do this; an applicative
* functor is all we need here. So we could also write the above program as:
def combineTwoProgramsWithApplicative(res0: Int) = {
val oneProgram = sql"select 42".query[Int].unique
val anotherProgram = sql"select power(5, 2)".query[Int].unique
(oneProgram |@| anotherProgram) { _ + _ }.transact(xa).run should be(res0)