Re: [web-devel] (Yesod) PersistEntity Key types

On Sun, Apr 3, 2011 at 5:44 PM, Rick Richardson wrote:

On Sun, Apr 3, 2011 at 10:35 AM, Michael Snoyman wrote:

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As a short term solution, I think you're right. However, I think we might want to consider a slightly more sophisticated approach... I say consider specifically, because I'm really not certain that what I'm saying is a good idea. I think there's basically three problems with the approach of using a PersistValue inside a PersistKey: 1) It will probably hurt performance, since we'll need to do more checking. 2) The code becomes a bit more fragile. By adding this sum type to the mix, we're adding necessity for a number of checks that can fail. 3) There's nothing stopping you from inserting a value into one database (say, SQLite), getting a key, and then looking up in MongoDB. (Not that this is a flaw in the current approach as well.)

I agree on the first two points. I think the principle behind 3 is actually a feature.

Can you elaborate here? I see it as a definite flaw in the type safety we otherwise have.

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So here's the idea: each database backend will have an associated type for its key datatype. Then, instead of having:     data Key entity = Int64 we'll have     data Key entity backend = BackendKey backend (ignoring all the newtype wrappers). I think this should solve both the issue you raise about MongoDB, and the three points I mention above. However, I'm still concerned that it might lead to difficult-to-follow code. There's really only one way to find out, but I just wanted to bounce the idea around before diving in.

I am going to negate my previous email. After thinking on this a bit. I think that setting they key type at the PersistEntity is the correct approach. This would be the only way to handle keys as strings or composite keys (if we want to support either) If we want to restrict all Yesod databases to a single pre-defined key type per database, then I will implement your solution.  If we want to be flexible (for legacy database support) then I think we should go with a parameterized type solution, and I have no idea how to make it efficient.

I think the parameterized type solution *will* be efficient. What I was worried about was having a sum type where we need to perform conditionals every time we want to use it. I actually just tried out the parameterized approach, and so far the code required very minor modifications to compile. Of course, getting the tests to pass (which will involve fixing up the TH code) may be a bit more involved. If you (or Greg) have other opinions on this, I'd be happy to hear them. I think you two are the only people who have worked on a non-SQL backend (besides my pittance attempt at Redis), so you'll likely have very good insight that I need to hear. Michael