Re: [web-devel] Database.Persistent

6 May 2011


      It looks to me like a combination of options 2 and 3 below.

On Fri, May 6, 2011 at 1:07 PM, Aristid Breitkreuz
 wrote:
...
What about
data Foo = X { a :: Int }
data FooLifted = XLifted { aLifted :: Maybe Int }
As I indicated earlier. Sorry for not being more eloquent on my phone.
Aristid
Am 06.05.2011 11:48 schrieb "Michael Snoyman" :
...
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" :
...
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:
...
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:
...
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:
>
> On Thu, May 5, 2011 at 1:28 PM, Felipe Almeida Lessa
>  wrote:
>> 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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