Comments on Wondering around: Google App Engine Data Store Api is definitely bet...

I think the design is correct. get is used for loo...

2009-06-22T14:17:13.815-07:00

I think the design is correct. get is used for looking up a known key. if the key doesnt exist, that is an exceptional situation. If you dont know if the key exist or not, you should write a query.

Think of it as Map.apply in scala (which also throws an exception for unknown keys), and suddenly its starting to look quite good.

Lacking a proper Option type in java, I very much prefer the explicit EntityNotFoundException over the dreaded NullpointerException.

I wonder if it throwing an exception is anything t...

2009-06-22T13:31:06.884-07:00

I wonder if it throwing an exception is anything to do with python dictionaries? In python, if you attempt to access a non-existant key in a dictionary it throws an exception.

Very nice! Thanks Daniel.

2009-06-22T13:03:57.083-07:00

Very nice!
Thanks Daniel.

def exceptionToRight[T](f : => T): Either[T,jav...

2009-06-22T11:43:47.408-07:00

def exceptionToRight[T](f : => T): Either[T,java.lang.RuntimeException] =

  try {

    Left(f)

  } catch {

    case ex => Right(ex)

  }

exceptionToRight(ds.get(queryKey)) match {

  case Right(_) => { logger.info("did not got it: " + queryKey.toString)

    ...

  }

  case Left(entity: Entity) if (null != entity) => {
logger.info("got it: " + entity.toString)

    ...

  }

}

Doesn't seem much worse.

I think this is a very "ActiveRecord" wa...

2009-06-21T12:37:16.725-07:00

I think this is a very "ActiveRecord" way of doing things, where if you do a Model.find(nonexistent_key) it'll throw a RecordNotFound exception. The reason that paradigm seems to work is specifically because the Rails framework provides some syntatically pleasant ways of handling exceptions for an entire group of methods or for globally handling an exception.

Yeah, I can see why this sucks for GAE. I'm surprised there isn't an option to throw an exception or return null on an invalid key. Next version, I guess.