On Mar 7, 5:43 pm, James Reeves <weavejes...@googlemail.com> wrote:
> The nearest equivalent to Haskell type classes in Clojure are
> multimethods, but as Clojure is a dynamically typed language, the
> compiler does not enforce the 'interface' defined by the multimethods.
>
> In Clojure, I can't think of any way you could enforce such a contract
> at compile time. Clojure is not statically typed, and does not place
> any restriction on the data you can pass to a function. You could
> write a predicate to check for invalid data at runtime, of course, but
> this would have an obvious performance impact (which may or may not be
> worth the trouble).
>
> You could just always state the interface in the documentation, and
> leave it up to the programmer to implement it correctly.
That's all true, barring one point (nothing prevents a dynamically-
typed language from enforcing a declared type restriction; certainly,
it can be done in Common Lisp) but beside the point.
With Java interfaces, we have a handy means of representing a
contract, in the sense of a set of defined operations. That contract
is a runtime data structure that we can examine to see what operations
are provided by objects that implement the interface. Implementing it
produces an object that looks to the Java and Clojure type systems
like an object whose type includes the specified interface so that,
for example, Clojure built-ins do the right thing with proxy objects
so long as they are built on appropriate interfaces and implement the
necessary methods. It's quite easy to ensure that the necessary
methods are implemented by your proxies, because you can examine the
proxy object once it's constructed to see what methods it thinks it
should be implementing, according to the interfaces it's built on.
That's all good. It's quite handy when I want to create proxy objects
that are to behave like IRefs, or like IMetas, or whatever behavior it
is that I want them to have.
Now suppose I want to create some new objects that have all of those
advantages, but the interfaces that I want them to conform to don't
yet exist. How do I supply them? As far as I know, I have to write
Java code. I'd rather write Clojure code.
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