Question

I writing a DSL that executes as a Script; it has various classes for various bits of syntax. e.g., for the “foo” keyword that takes a closure, I have a FooSyntax class and evaluate the closure “with” an instance of that syntax. This works fine, e.g.

bar = thing {} // make a thing 
baz = foo {
    mykeyword bar
}

passes the thing called bar to an invocation of FooSyntax#mykeyword.

I’m trying to add some better error messages for when there is an unknown variable reference. This manifests as a MissingPropertyException so my current approach is to add a propertyMissing method to FooSyntax. This works indeed for variables that are missing.

Unfortunately, it breaks the example above: bar becomes a missing property instead of falling through to the Binding. Why does adding a propertyMissing cause the Binding not to be consulted? (Does this have to do with the Closure‘s resolve strategy?) How can I fix this?

You can play with this with a sample script at https://gist.github.com/1237768

Answer 1

I shall delegate my answer to the gist on which I commented. Basically, you should not be using the with() method to execute the closure against the FooSyntax delegate. For future reference, the standard approach is:

def foo(Closure cl) {
    def f = new FooSyntax()
    def c = cl.clone()
    c.delegate = f
    c.call()
}

You can fine tune the behaviour by changing the resolution strategy on the closure like so:

c.resolveStrategy = Closure.DELEGATE_FIRST

but in this case you want the default Closure.OWNER_FIRST to ensure the binding is queried first.