Neither necessary

nor sufficient written by Ruud van Asseldonk

published 6 October, 2015

A few days ago I stumbled upon a blog post that raised the following question:

“A language without garbage collection, in 2015?”

The language referred to is Rust, but that is hardly relevant here. I wrote a reply on Reddit, but I thought I’d take the time to elaborate a bit more in the form of a blog post. Quoting Bjarne Stroustrup the point can be summarised succinctly:

“Garbage collection is neither necessary nor sufficient.”

A language without garbage collection, in 2015. Why? Because garbage collection does not solve the deeper underlying problem: resource management. Performance differences aside (there are many different metrics for performance and even more myths surrounding those — I don’t want to go down that rabbit hole here), a garbage collector only manages memory. This works well for memory, because there generally is more memory available than what is needed, and applications do not care about the actual addresses they use. The address space is uniform. An array does not care if it is stored at 0x3a28213a or 0x6339392c . If something that is no longer alive was stored at 0x3a28213a , the array is happy with being stored at 0x6339392c if the garbage collector has not yet discovered that 0x3a28213a is free.

The story is different when there is contention for a resource. From the lock protecting a critical section to the socket serving a website — you cannot afford to leave such resources lingering around until a GC comes along to decide what is still being used. In most languages sporting a GC , resource management is still utterly manual. You still need to close() your OutputStream in Java. You still need to Release() , Close() or Dispose() of your Semaphore in C#. Even this Haskell example straight from Real World Haskell is nothing more than fancy syntax for C’s file handles:

main = do inh <- openFile "input.txt" ReadMode outh <- openFile "output.txt" WriteMode mainloop inh outh hClose inh hClose outh

The problem with disposable objects or handles, is that they decouple resource lifetime from object lifetime. This allows for programming errors such as writing to a closed socket or not releasing a lock.

There exist constructs that can help in many cases. Python has with , C# has using , and Haskell has bracket . These constructs bind resource lifetime to scope. A scope-based solution is often sufficient, but in some cases a resource has to outlive the current scope. A using block for instance, is of no use for disposable member variables.

Of course, sometimes manual control of resoures is required. A device driver does care about actual addresses, and manual acquire and release calls might be clearer than scope-based locking for a thread-safe cache protected by a read-write lock. In the specialised cases where there is a need for manual resource management, neither garbage collection nor automatic resource mangement can help. But little code benefits from manual memory management, and I would argue that this is the case for other resourses as well.

So yes, I want a language without garbage collection. Because I want a language that can do resource management.