Created
July 26, 2017 19:33
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Free Monads : https://www.youtube.com/watch?v=ZasXwtTRkio (slides : https://speakerdeck.com/marakana/dead-simple-dependency-injection-in-scala)
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import java.sql.{Connection, DriverManager} | |
import scala.io.StdIn._ | |
object FreeMonads { | |
def getUserPwd(id: String)(c: Connection): String = { | |
"" | |
} | |
def setUserPwd(id: String, pwd: String)(c: Connection): Unit = { | |
val stmt = c.prepareStatement("update users set pwd = ? where id = ?") | |
stmt.setString(1, pwd) | |
stmt.setString(2, id) | |
stmt.executeUpdate | |
stmt.close() | |
} | |
case class DB[A](g: Connection => A) { | |
def apply(c: Connection): A = g(c) | |
def map[B](f: A => B): DB[B] = | |
DB(c => f(g(c))) | |
def flatMap[B](f: A => DB[B]): DB[B] = | |
DB(c => f(g(c))(c)) | |
} | |
def pureDB[A](a: A): DB[A] = DB(_ => a) | |
implicit def db[A](f: Connection => A): DB[A] = DB(f) | |
def changePwd(userid: String, oldPwd: String, newPwd: String): DB[Boolean] = | |
for {pwd <- getUserPwd(userid) | |
eq <- | |
if (pwd == oldPwd) for {_ <- setUserPwd(userid, newPwd)} yield true | |
else pureDB(false) | |
} yield eq | |
abstract class ConnProvider { | |
def apply[A](f: DB[A]): A | |
} | |
def mkProvider(driver: String, url: String) = | |
new ConnProvider { | |
def apply[A](f: DB[A]): A = { | |
Class.forName(driver) | |
val conn = DriverManager.getConnection(url) | |
try { | |
f(conn) | |
} finally { | |
conn.close() | |
} | |
} | |
} | |
lazy val sqliteTestDB: ConnProvider = mkProvider("org.sqlite.JDBC", "jdbc:sqlite::memory:") | |
lazy val mysqlProdDB: ConnProvider = mkProvider("org.gjt.mm.mysql.Driver", "jdbc:mysql://prod:3306/?user=one&password=two") | |
def myProgram(userid: String)(r: ConnProvider): Unit = { | |
println("Enter old password") | |
val oldPwd = readLine() | |
println("Enter new password") | |
val newPwd = readLine() | |
r(changePwd(userid, oldPwd, newPwd)) | |
} | |
def runInTest[A](f: ConnProvider => A): A = f(sqliteTestDB) | |
def runInProduction[A](f: ConnProvider => A): A = f(mysqlProdDB) | |
def main(args: Array[String]): Unit = runInTest(myProgram(args(0))) | |
case class Reader[C, A](g: C => A) { | |
def apply(c: C): A = g(c) | |
def map[B](f: A => B): Reader[C, B] = | |
Reader(c => f(g(c))) | |
def flatMap[B](f: A => Reader[C, B]): Reader[C, B] = | |
Reader(c => f(g(c))(c)) | |
} | |
def pureReader[C, A](a: A): Reader[C, A] = Reader(_ => a) | |
implicit def funToReader[C, A](read: C => A): Reader[C, A] = Reader(read) | |
trait KeyValueStore { | |
def put(key: String, value: String): Unit | |
def get(key: String): String | |
def delete(key: String): Unit | |
} | |
def modify(k: String, f: String => String): Reader[KeyValueStore, Unit] = | |
for { | |
v <- _.get(k) | |
_ <- _.put(k, f(v)) | |
} yield () | |
} |
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see also: https://gist.github.com/debasishg/4606509