Re: [web-devel] Database.Persistent

So a quick review: we have a few general approaches here: 1) Create a few different datatypes, or a single datatype with multiple constructors. 2) Somehow annotate the fields to indicate that data may or may not be present. 3) Fill in some dummy values for fields not loaded. I think option (1) is by far the most type safe. It's also the most involved and unintuitive. Greg's point about multiple constructors being ugly is very true. And two datatypes will be hard to manage. Option (2) will work, but it's significantly less type safe. There's nothing at the type level distinguishing "get" and "getPartial". And it will have a strongly negative impact on all code branches using "get". Option (3) is not as type-safe as option (1). In it's scariest form, it's going to allow undefined to float around programs wreaking havoc. In its more benign form, it will simply allow the user to provide dummy values. The downside versus option (1) is that there's nothing at the type level distinguishing get from getPartial. The downside versus option (2) is that there is nothing at the value level distinguishing get from getPartial. But the advantage of (3) is that it's much simpler to implement and will have no effect on regular code at all. I think it's also much easier to understand how to use it to someone. Anyone see something I'm missing? Michael On Fri, May 6, 2011 at 7:49 AM, Aristid Breitkreuz wrote:

How about generating a version of the records lifted to Maybe?

Am 06.05.2011 04:03 schrieb "Max Cantor" :

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Personally, I vote against this. Putting in undefineds maintains type-safety. However, in my mind the primary purpose of static type-safety is the avoidance of runtime errors. This only encourages that. Perhaps a better approach is to have some a TH function to generate "subselects" which would be records that only contain the requested data. Its a bit more constraining, but I fear that the alternative is tantamount to curing the disease by killing the patient.

furthermore, I worry that allowing undefined's here will start a slippery, runtime-error-laden slope which we're better off avoiding..

max

On May 5, 2011, at 9:55 PM, Michael Snoyman wrote:

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That's interesting: I'd never considered the idea of inserting undefined into fields you want excluded... That could work. The reason I've avoided putting in this (often requested) feature is that I could think of no way to do so and keep type safety. That might be an approach.

We may consider requiring the user to supply the value instead, thereby avoiding the library inserting undefined.

Michael

On Thu, May 5, 2011 at 4:32 PM, Greg Weber wrote:

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An alternative to laziness would be selecting a subset of fields. There is no support for this directly in persistent, but it might be possible to add it and the future and have the value of an unselected field be something like undefined. At the moment you can select sub fields by dropping down to lower-level sql methods (ask Michael about these methods if you are interested). I think there is a technique for building your persistent data structure back up from the return of raw sql, which again you might be able to do by inserting dummy error fields. Greg Weber

On Thu, May 5, 2011 at 4:18 AM, Michael Snoyman wrote:

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On Thu, May 5, 2011 at 1:28 PM, Felipe Almeida Lessa wrote:

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On Thu, May 5, 2011 at 2:20 AM, Jeremy Hughes wrote: > Is Database.Persistent lazy wrt reading fields? I need to iterate > over > entities containing both small and large fields. I do not need to use > the large fields in this instance, and so would rather they were not > read from the database.

IIRC, they are read strictly. I guess you should put them on a different entity.

That's correct. In fact, Persistent avoids any form of lazy I/O to ensure that database connections are returned to the pool as soon as possible (amongst other reasons).

Michael

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