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About Cake Pattern
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About Cake Pattern.md

About Cake Pattern

Below are the sources I took notes and hope to learn about Cake Pattern from.

Videos:

Dead-Simple Dependency Injection

Dead-Simple Dependency Injection

  • setUserPwd - a Scala method that updates some data in a database
  • Version 1 - everything's hardcoded
  • Everything = SQL driver + database URL
  • It's self-contained, reusable, fine for little toy tests
  • It's not how you wanna write your actual applications
  • Version 2 - global factory
  • Provides database connections
  • What about closing a database connection?! Who's in charge? It's not clear whose responsibility it is to close a connection.
  • Bad idea
    • Hidden dependency on a global object
    • Requires magic initialization step = you can't call this method before that object
  • Inversion of control = pretentious way of saying "taking an argument"
    • That reminds me the other presentation about functional programming...
  • Connection taken from ...FIXME
  • Version 3 - Inversion of Control
  • taking an argument = injecting c which is a database connection
  • I don't care where this connection comes from - just accept that I have one and can use it.
  • I don't care how it's discarded
  • I just assume that the connection is open
  • You may object and say "What if the thing that knows how to obtain and a connection is way up here "
  • In the call graph it needs the connection. Do I need to pass the connection through? It can get pretty easily cumbersome.
  • Version 4 - Currying
  • Instead of passing in the connection right away, simply partially apply
  • Instead of returning Unit, I'm going to return a function that accepts a connection at convenient time
  • The only difference is that I'm delaying the actual passing-in a connection
  • Design patten - Connection Reader
  • Capture this idea in a data type - just for convenience (as usual, isn't it?)
  • case class DB[A](g: Connection => A)
  • A wrapper around the type from Connection to A
  • DB[A] = a database action = Given a Connection produce some value of type A
  • apply to have a convenient call
  • Lift existing functions using map
    • map[B](f: A => B): DB[B] ~ (A => B) => (DB[A] => DB[B])
    • Map (or promote) an existing function A => B to DB[A] => DB[B]
    • Implementation of map is just a function composition
  • Combine two actions
    • I have an action that produces DB[A] and you call flatMap with a function that
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