Tuesday, December 4, 2007

Zen and Groovy's Expando

In the past, I had trouble remembering the concept behind Groovy's Expando class. I knew it wasn't difficult but it was elusive. Perhaps I was distracted by the groovy-ness of the name.

In a recent training class with Jeff Brown, I was struck by a metaphor.

Readers of CtJ know the old refrain of dynamic-typing:

If it walks like a duck and quacks like a duck, I would call it a duck.

In other words, if "it" can provide the desired behaviour (e.g. quacking), then it can pass for a duck, in so far as a dynamic language cares. Cool beans.

Well, in Groovy, the Expando class is an empty "it"! It is a container for various behaviours, to which we can add stuff. Check out this code sample:


// tests the incoming, purported "duck" to see
// if "it" really quacks
def duckTester = { duck ->
duck.quack()
}

////////////////////////// Main

// create "it"... it cannot quack -- yet
def it = new Expando()

// add quacking
it.quack = { println "quack!" }

// call test: "if it quacks like a duck"
duckTester.call(it)
See? We create "it", then add quacking behaviour, and then test it. Easy. This prints
quack!
to the console.

Question: does anyone know the origin of the Expando name?

I've searched but not found it.

ps. A less glamorous way of thinking about it is that Expando is a map of methods and attributes. I'm sticking with ducks and it.

pps. Expando's are especially useful for mocking objects in unit-tests.

5 comments:

  1. You can do a similar thing in Beanshell, with if I remember correctly, a This object, which serves as your context.

    I found that this was incredibly useful for writing DSLs in BeanShell as you could write scripts that work on a well-known (to the script) set of attributes that are dynamically added to your current context. Presumably, you could leverage Expando in the same way.

    For example, we created a set of beanshell functions that were basically a DSL for testing JDBC, so we had a function createConnection(...) that took some url/user/pw info, created a connection and stuffed it into a Connection object in context (no Connection is returned). Then other functions that executed a query (using Connection, and stuffing results into a ResultSet in context), walked forward and backward, etc.

    This made testing JDBC scenarios for our custom JDBC driver really easy to write and understand.

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  2. Expando is full of schizzle, isn't it? ;)

    This doesn't really have much to do with Expando but your code is calling a closure using the .call() method. If you are invoking a closure from groovy, you can simplify that by treating the closure as a method.

    // invoke the call method...
    duckTester.call(it)

    // invoke the closure...
    duckTester(it)

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  3. Thanks for the comments, gents...

    @Alex. I've thought about a 'Rosetta Stone' diagram that maps out certain concepts across languages. I didn't know about Beanshell but certainly Javascript (and probably Python, Ruby) have similar behaviour. That's cool re: DSLs.

    @Jeff. Thanks... Groovy never ceases to amaze me in the sense that there is always one more thing one can do to make it tighter.

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  4. For my part one and all must browse on this.

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  5. Ok, years late to say... but Expando == Expendable bean

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