"Fixing" Groovy?

In response to my last article on Groovy's lack of static typing James Strachan, the original Groovy guy, tweeted "groovy should fix this really IMHO." Well, perhaps. But "fixing" it would substantially change Groovy's semantics. Most importantly it would prevent Groovy from executing some programs that work just fine right now.

An Example

Here's a simple example of a program that works now but wouldn't under a static typing regime. It's a simple variation on the program from the previous article.

interface Foo {}
interface Bar {}
class Baz implements Foo, Bar {}

Foo test(Bar x) {return x}

test(new Baz())
println("Everything's Groovy")

Compile it, run it, and feel groovy.

~/test$ groovyc test.groovy
~/test$ groovy test
Everything's Groovy

If I transliterate into Scala

trait Foo
trait Bar
class Baz extends Foo with Bar

def test(x : Bar) : Foo = x

test(new Baz)
println("Everything's Scala?")

And try to compile

~/test$ scalac -Xscript test test.scala
!!!
discarding <script preamble>
(fragment of test.scala):5: error: type mismatch;
 found   : this.Bar
 required: this.Foo
def test(x : Bar) : Foo = x
                           ^
one error found

Scala rejects a program that Groovy is happy with.

Conclusion

That's the nature of the beast. Static typing rejects some good programs that might otherwise work. And as my last article showed, dynamic typing allows some bad programs that can't possibly work. That's a pretty fundamental difference and there's no way around it.

If the Groovy community wants to make its type annotations work in a more statically typed manner that's ok. This article just shows that such a change wouldn't be 100% compatible with today's Groovy.