Re: [Haskell-cafe] [ANN] relational-record - relational-algebraic query building DSL

On Wed, Dec 17, 2014 at 07:04:12PM +0200, Riivo Kolka wrote:

On 14 December 2014 at 18:52, Manuel Gómez wrote:

...

Congratulations on the release! It’s great to see more and more interesting abstractions for relational databases in the Haskell ecosystem. [...] I'm very sorry, but I had a strong gut reaction to Manuels sentiment. Really?! What's so great about it? Isn't this rather a sign that something is askew? For such an important, ubiquitous yet mundane task as accessing database there shouldn't be so many ad hoc half-baked (sorry - with some limitations) solutions. That's unfair, I know.

Hello Riivo, It's not clear to me from your message whether you've looked at Opaleye, a relation query embedded domain specific language for Haskell http://hackage.haskell.org/package/opaleye I wrote it, so all due caveats about my opinion apply, but I do not consider Opaleye either ad hoc or half-baked. Opaleye has a solid theoretical foundation and thorough attention has been given to its semantics.

I look at those examples and think, this is not the Haskell I'd like to write and despite type safety and composability, which are great features, it's not even the query language I'd like to write. Ie it's not better than SQL, yet.

If you feel this way about writing code using Opaleye perhaps you could be more specific so I can get a clearer understanding of the issues involved. [...]

If Haskell hasn't got tuples as in relations (named, not ordered), type system such that tuple types' arity is not fixed (if that's the correct way to put it) and types for sets of such tuples aka relation type, then how can Haskell support databases and relational algebra NATURALLY? On the other hand, if Haskell had those, it would be quite a relational language, right? And a GREAT one, no?

I should mention at this point that I do not consider "relational algebra" to be a paragon to which to aspire. There is a lot of value in that body of work, but in the light of developments in programming language semantics over the last forty years relational algebra really appears to need an overhaul. (Technical note: One thing I noticed when writing Opaleye is that restriction, projection and inner join are different sorts of operation than aggregation and outer join, the former being what are called "algebraic operations", the latter not. I think the only way this observation can be made is if you encode relational algebra in a typed setting.) Opaleye is an implementation of relational algebra to about the same extent that Haskell is an implementation of the untyped lambda calculus, and I'm very happy with that difference between them. Tom