Squeryl is a great Scala database API. On its website, it is describe like this: "A Scala ORM and DSL for talking with Databases with minimum verbosity and maximum type safety".
Preparing an introduction to Squeryl for a Swedish computer magazine, I sent a number of questions to Maxime Lévesque, the man behind Squeryl. The answers were so interesting, that I asked his permission to post them here:
Could you describe yourself in a few words?
I'm a dad, a programmer, a hobbyist bass player and percussionist.
I'm the kind of programmer who prefers to write libraries and frameworks to writing applications. If I was in the construction industry I'd probably be making bricks, mortar and nails rather than houses.
Do you develop Squeryl as part of your work, or is it a hobby?
Squeryl started as a hobby, only later did I start using it in a commercial project.
What are the most important features of Squeryl? Why should you use it?
The main reason to use Squeryl in an application, in my opinion, is to have the data access code validated by the compiler. I've seen many projects where the database schema stops evolving after a lot of code has been written against it. Ugly workarounds are sometimes chosen because there isn't enough time to investigate the repercussions of a schema change or conduct all the testing required.
Strongly typed languages are good for "deterministic refactoring". A data access layer needs to be refactorable, as any part of a system does. Perhaps to an even greater extent, because in a sense, bad design decisions get persisted with the data.
A developer needs all the help he can get from tools such as compilers and IDEs. Hard work and discipline don't scale. Why rely on it when you can have automated validation?
Reusability is another big one. Squeryl queries are composable, reusable pieces of code. A query that encodes a particular piece of application logic needs only be written once, and reused anywhere it is needed. I'm a big believer in the DRY principle (Don't Repeat Yourself).
Low verbosity would be another strength. I dislike APIs or frameworks that require you to write more than you should.
What's the story behind Squeryl?
In 2005 I wrote an ORM for dotNet. I was in need of one at the time and I couldn't find a decent one that exploited generics and annotations, so I wrote my own. By the time I considered publishing it, LINQ came out, and instantly obsoleted my ORM (and all other ORMs except HaskelDB in my opinion).
A few years later I started to write a query DSL in Java, and at every step, I got bitten by language limitations. Every time I worked around them, the solution became a bit more ugly and verbose. I then discovered Scala, and started experimenting with writing a statically typed query DSL. I was amazed by the expressivity of the language.
The fact that it was possible to write Squeryl as a library (i.e., without a compiler plug-in) speaks a lot about the potency of the language. The first two attempts were abandoned when they reached a critical level of inelegance. They were Squeryl's pre-history.
Squeryl is in fact my third attempt at a Scala ORM. When I became confident that a fourth rewrite wouldn't be necessary, I published it on GitHub.
If Squeryl didn't exist, what would you use?
If Squeryl didn't exist, I'd have a look at ScalaQuery or Circumflex. I only have a superficial knowledge of them, but I would surely try them out before going to any of the Java based ORMs.
If you are to demo Squeryl (e.g., to a Java programmer), do you have a favourite example?
Here's a one liner that says a lot :
val avgHeight: Option[Float] = from(people)(p => compute(avg(p.heightInCentimeters)))
Apart from the shortness of the code, we can see a few implicit conversions at work. The compiler "knows" that the sum query can translate into a 32 bit floating point value, but it also "knows" that it is an Option, because the avg aggregate function is not guaranteed to return something (the table can be empty). In fact it won't compile if you try to refer to it as a (non Option) Float.
Where has Squeryl turned up? Who uses it?
I haven't made any survey, it's on my todo list, but I've exchanged emails with developers that are building systems with Squeryl in fields ranging from finance to bioinformatics.
I read something about Lift...?
Ross Mellgren from the Lift team has written an integration module that is part of Lift 2.1 (release candidate).
What's on the roadmap?
High on my priority list is free text search (backed by Lucene). Longer term I'd like to add things like support for sharding and extending the DSL to exploit the geospatial capabilities of databases like Postgres, Oracle and H2.
Is it of any importance that Squeryl was written i Scala? Or was this merely a coincidence?
Without Scala there wouldn't be a way to have strongly typed queries on the JVM without having verbosity that reaches a caricatural level. Not only wouldn't there be Squeryl, but there wouldn't be anything like it.
When Java came out I was impressed with all the features it had built in: serialization, RMI, garbage collection, portability. It was in its time a game changing technology. Today I have the same impression of Scala: the level of static validation that it gives you, all this with minimal verbosity. If I could say just one thing to qualify it, I'd have to say: game changing.
So the answer is yes, Scala made Squeryl possible. I expect a lot of interesting Scala DSLs will get written in many domains in the coming years. I have a few other DSLs I'd like to write myself.
Any particular advice for someone beginning with Squeryl?
I would just copy an example from the Squeryl site, and modify it gradually so that it becomes your own schema. And most importantly, don't hesitate to ask questions in the discussion groups. I'm often impressed by the quality of the answers given by the community.
Thanks a lot for the great answers!
Thanks a lot for the great answers!