Wednesday 27 May 2015

Post-Patterns Patterns Part 2 - Partial at my house

So in my sausagefactory library, my first attempt at adding an extension mechanism was very Java-esque.
trait FieldConverter {
  def convert(a: A, b: B):Any 
The companion object for the CaseClassConverter supplied a default FieldConverter if you didn't need one:
object CaseClassConverter {

  def apply(converter: => FieldConverter = defaultFieldConverter)

  class DefaultFieldConverter extends FieldConverter {
     def convert(a: A, b: B):Any = {
Ye gads look at the boilerplate! All to wrap a simple function! And yet there is huge scope to still get it wrong, because if you do actually supply a custom FieldConverter, it actually ends up being you who handles all conversions from then on, even though you really don't care about most of them. So you need an if for everything to work properly:
class MyFieldConverter extends FieldConverter {

  def convert(a: A, b: B):Any = {
   if (specialCase) {
      ... // do special conversion
   } else {
      // Do the normal conversion

That sucks.

So, following a nice little nugget I found in Effective Scala, I refactored the whole extension mechanism into using PartialFunctions, like this:
Make FieldConverter a type alias
  type FieldConverter = PartialFunction[(Type, Any), Any]
Chain up a user's custom FieldConverter with the default one
  val exhaustiveConverter:FieldConverter = userConverter orElse defaultConverter
Scala will check whether the userConverter is defined at a given input - if not, it'll fall through to the defaultConverter - perfect.
Now custom converters are simple case blocks
  val alwaysMakeJavaLongsIntoInts: FieldConverter = {
    case (t: Type, v: Any) if (isInt(t) && isJLong(v.getClass)) => {
A userConverter only has to worry about converting one type of thing, and doesn't know (or care) about downstream converters. A simplified Chain of Responsibility.

Monday 4 May 2015

Strongly-Typed Time. Part 2: Design

Following on from my lightbulb moment, I tried to sketch out what I wanted from a strongly-typed system for representing timezoned instants in time.

The Look

Ever since Martin Odersky gave us Generics in Java 5, we've become comfortable with reading parameterized types like Set<String> ("Set of String") for container classes. Unsurprisingly, with the move to Odersky's Scala has come further use of parameterization; for example Try[Double] and Future[User]. Essentially, I wanted a type that looked like this. Hence:
  val pierreTime: TimeInZone[Paris]

  val johnTime: TimeInZone[Melbourne]

  val canonicalTime: TimeInZone[UTC]


I want wrong code to look wrong but more than that, I want the compiler to consider it wrong too:
  val pierreTime: TimeInZone[Paris]

  def doSomethingInMelbourne(mTime: TimeInZone[Melbourne]) ...

  // later...
[error]  type mismatch;
[error]  found   : TimeInZone[Paris]
[error]  required: TimeInZone[Melbourne]

Functional Familiarity

I'm only a tiny way down the path to true functional-programming enlightenment. Hell, I've only just started looking at Scalaz, mainly thanks to eed3si9n's excellent tutorials.

But the following patterns seem pretty sensible to me:

map from one timezone to another

The instant-in-time is unchanged, but the type changes, and the local time changes too:
  val pierreTime: TimeInZone[Paris] 
  // Local: 2015-04-29T09:15:49.739+02:00
  // Millis (UTC): 1430291749739

  val johnTime: TimeInZone[Melbourne] =[Melbourne]  
  // Local: 2015-04-29T17:15:49.739+10:00
  // Millis (UTC): 1430291749739

transform the time inside the container

I can make adjustments* to the DateTime contained within the TimeInZone[T] using any Joda-Time method that returns another DateTime:
  val pierreTime: TimeInZone[Paris]
  // Local: 2015-04-29T09:15:49.739+02:00 

  val pierreWakeUpTime: TimeInZone[Paris] = pierreTime.transform(_.withTime(7,0,0,0))
  // Local: 2015-04-29T07:00:00.000+02:00 

  val pierreLunchTime: TimeInZone[Paris] = pierreTime.transform(_.plusHours(4))
  // Local: 2015-04-29T13:15:49.739+02:00 

(*) Everything is immutable (including within Joda-Time) so "adjustments" naturally result in a new object being returned. Do we have a word yet for "A modified copy of an immutable thing"?

Companion-object for construction

I should be able to get a TimeInZone[T] via its companion object for every conceivable scenario:
  // "Now" in whatever TZ my JVM is running in: 
  val myLocalTime: TimeInZone[TimeZone] =
  // -> TimeInZone[Melbourne] UTC: '2015-04-28T12:37:00.000Z'

  // "Now" in the given TZ: 
  val myParisTime: TimeInZone[TimeZone] ="Europe/Paris")
  // -> TimeInZone[Paris] UTC: '2015-04-28T12:37:23.000Z'

  // "Now" in UTC: 
  val myUTCTime: TimeInZone[UTC] = TimeInZone.nowUTC
  // -> TimeInZone[UTC] UTC: '2015-04-28T12:37:28.000Z'

  // If I pass millis, UTC is implied: 
  val myUTCTime: TimeInZone[UTC] = TimeInZone.fromUTCMillis(1430703466430)
  // -> TimeInZone[UTC] UTC: '2015-05-04T01:37:46.430Z

  // Reflective methods where the desired [TimeZone] affects the result:

  // Give me "now" on the West Coast:
  val paloAlto = TimeInZone[PST]
  // -> TimeInZone[PST] UTC: '2015-05-04T01:37:56.430Z

  // Give me "then" on the West Coast:
  val paloAltoLastWeek = TimeInZone[PST](new DateTime().minusDays(7))
  // -> TimeInZone[PST] UTC: '2015-04-28T01:37:59.430Z