Saturday, 13 June 2009

Groovy and Language Design

I was a little surprised to read Elliotte Rusty Harold’s recent comments on Groovy, calling it the ‘Edsel of programming languages’.  It’s an amusing comment, but pejorative and challenging enough to motivate a response. 

The past few years have been interesting and fruitful from a programming language perspective: Ruby, Groovy, Scala, F#, and the evolution of C# are those which spring to mind.

Many of these developments have been about finding a way to push functional-programming idioms (lambda expressions, currying) or dynamic language features (type inference, closures) into well-established imperative languages like C# and Java. I think the results have been mixed: imperative languages are essentially assignment-based, not expression-based, so you end up with a hybrid which, from a language syntax point of view, is neither one thing nor another and arguably suffers as a result.  But it’s difficult to argue that these efforts don’t deliver real positive utility.

An example of this is how C# has evolved support for lambda expressions, expression trees and implicit typing. First (in v1) we had simple delegates, then (in v2) we were able to drop the delegate syntax and declare anonymous methods inline, and finally (in v3) we can use a compact lambda syntax which looks a bit like ML. But because all this has been added to what remains fundamentally a statically-typed, compiled language, you still have to put up with all the nasty explicit declaration of types (and type parameters) on the l.h.s. of every assignment, just to keep the compiler happy.  So the final step was to introduce the var keyword and bring implicit typing into the language. Not dynamic typing, just implicit typing: we get to trim away the ugly declarations but keep the compile-time type safety. 

The confluence of these developments enabled things like LINQ: blending a query sublanguage into the primary language syntax. The best exposition of this I have read is by Ian Griffiths, in his excellent post on expression trees: see the example code just over half-way down, under the section ‘Expression Trees, LINQ, Deferred Queries, and Databases’. It’s all so superficially appealing – just look at the motivating use-case in Ian’s post. But still there’s a small, nagging voice in my head saying this isn’t quite right...

It feels as if we’re trying to pretend there’s no boundary (or intermediary) between the program text and the resource we’re querying. This is similar to the way we tried to pretend that remote procedure calls were really local calls in the old (and now largely discredited) distributed programming models such as CORBA and DCOM. Recall the fallacies of distributed computing and the tenets of SOA: these are essentially warnings about the dangers of implicit or flawed assumptions.  Although the tenets have been challenged (and were threatened with retirement) most of what they said is timelessly valid.  In the context of LINQ, I think my nagging voice is pointing to the boundary-crossing in a LINQ query and muttering “private implementation technique, not primitive construct”.

What’s this got to do with my starting point for this piece, which was Harold’s ‘Edsel’ jibe?  I suppose it’s this: whatever we may feel about the virtue of purity in language design, in the end productivity and usability are the bigger drivers, and with skill it is possible to combine language features without creating a monster.

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