[ANN] scopes: a little library for resource scopes

[Paul Stadig]

I have released version 0.1.0 of scopes, a little library for resource scopes. It can be used to establish a resource scope with the with-resource-scope macro. Within the dynamic extent of that macro one can register a resource with the scoped! function so that when the macrco block goes out of scope .close is called on the registered resource.

An object and function can also be registered with (scoped! x (fn [x] ...)) so that when the macro block goes out of scope the function will be called and given x as an argument.

There is also a shorthand function called scoped-thunk! that will register a thunk (a function taking no arguments) to be called when the macro block goes out of scope.

The scopes library also defines a function named closeable? that will return true if its argument implements java.lang.AutoCloseable.

Because scopes uses the java.lang.AutoCloseable interface and the exception supression mechanism, it is only compatible with Java 7.

Finally, there is also a scopes-magic artifact, and when it is on the classpath it will automatically pull in the scopes library and add with-resource-scope, scoped!, scoped-thunk!, and closeable? to clojure.core so that they are automatically available everywhere with out having to be imported. I'm not saying that this is necessarily a good idea, but there it is. :)

https://clojars.org/pjstadig/scopes
https://clojars.org/pjstadig/scopes-magic


Paul

[Cedric Greevey]

What does this do that (with-open ...) doesn't do? Generalize to other cleanup methods than (.close x)?

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[Paul Stadig]

It separates the resource management from the creation and lexical binding of resources. You can create a scope high up in the call stack, and within the dynamic extent of that scope you can create resources, return them from functions and freely use them; when the scope eventually exits, then the resources will be cleaned up.

This library does also generalize a bit to allow you to register an object and a clean up function, or even just a thunk to get called when the scope exits.

[Jozef Wagner]

Is it based on http://dev.clojure.org/display/design/Resource+Scopes ?

[Paul Stadig]

No, I just implemented a simple intuitive scope, which looks like it is about the same as the basic idea there. There are some interesting ideas/questions about how scopes work across threads and how consumers can pass the buck for resource clean up to an enclosing scope (like exception handling).

In my case I explicitly don't handle extending scopes to child threads (for now). If the bindings convey to a child thread and you try to add an object to the current scope, then you'll get an exception for trying to set! a conveyed binding.

I'm not sure about the idea for passing resource clean up to an enclosing scope. I'd have to consider the problem and potential solutions before I could form an opinion.

That's not to say that these things are bad ideas or that some version of them won't end up in the library. I just wanted a minimal implementation of a scoping concept. It can evolve from there.

Paul

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[Jozef Wagner]

In JVM, we can assume (or should implement it in a way) that objects representing resource release the resource upon object's finalize method. If you push resource into the scope, it is not GCed until the scope ends. How about pushing to scope weak refs to resources instead?

JW