Re: [Haskell-cafe] Subject: A universal data store interface

Persistent is a database abstraction layer with no dependencies on Yesod. Those that need a persistence layer have the same needs to interface with the database in a type-safe way, regardless of whether their program presents a web interface. Have you tried using Persistent? We have never heard a report from a user that the Persistent DSL schema syntax is confusing. These complaints always seem to be from someone that hasn't actually tried it but is adverse to quasi-quoting. The DSL is basically the exact same as Haskell record syntax. I am not sure who the mythical users are that can figure out Haskell but can't understand dead-simple DSL's. That being said, I would like to have a Template Haskell interface instead of just a QQ interface. The reason why we haven't bothered with doing that ourselves is because the record name-spacing issue makes the current QQ interface much more convenient. I am actively working to solve the namespace issue. This all brings up a great point though: as part of the GSoC we should create a Template Haskell interface (similar to acid-state as you mention). Given the structure of things that Michael has already pointed out, and that we are already using Template Haskell with the DSL, this should only be a few day's work. On Mon, Feb 13, 2012 at 11:40 AM, Tom Murphy wrote:

It seems that all tutorials and resources for Persistent use Template Haskell along with several Yesod specifics.

But, I could be wrong, or new tutorials could be written.

Tom

On 2/13/12, Michael Snoyman wrote:

...

Actually, Persistent is fully usable without any special syntax, DSLs, or Template Haskell. In fact, Persistent itself has no template-haskell dependencies, specifically so that it can be built on ghc-iphone. Additionally, the Persistent DSL syntax is completely separate from any other Yesod DSL syntaxes that exist, so it's not like you have to learn five new things to get the automatic code generation.

On Mon, Feb 13, 2012 at 9:30 PM, Tom Murphy wrote:

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    With respect, I don't think that Persistent is a natural choice as the go-to tool for Haskell users, simply because it requires knowledge of a lot of Yesod-EDSL syntax.     The set of users with persistent data needs seems a very different set than that of those who are familiar with Yesod, and I think the syntax is quite confusing without fuller understanding of Yesod.

    The syntax of acid-state (not familiar with this one), and swapper (https://github.com/roman-smrz/swapper/blob/master/test/) seem to have a much more linear learning curve for an intermediate Haskell user.

amindfv / Tom

On 2/13/12, Greg Weber wrote:

...

Hi Sergiu,

Thanks you for your interest in that proposal. I rushed it off a year ago. Since then we have made a lot of improvements to Persistent and the library forms a basic building block for most Yesod users and other Haskellers. Persistent offers a level of type-safety and convenience not available elsewhere (except perhaps for libraries like acid-state that are limited to in-memory storage). That being said, there are still a lot of improvements that could be made. With the effort of a GSoC volunteer we could probably get it to the point of being the go-to data storage library for Haskellers, at least those planning on using the subset of backends (likely SQL) with great support. This proposal is vague and we would need to work with you to narrow things down a bit.

I am biased, but I believe the Yesod project is one of the most compelling in the Haskell ecosystem. There are a lot of different ways a GSoC project could help make things even better besides improving the associated Persistent library, and we would really like to mentor at least one GSoC student. I would open more tickets for this in the system, but I am not sure how helpful it will be. It seems that we need to reach out to more students like yourself, but I am not sure how to do that unless I see messages like these first.

Greg Weber

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