The Absurdity Of Stacks

:: lisp, programming

Very often people regard the stack as a scarce, expensive resource, while the heap is plentiful and very cheap. This is absurd: the stack is memory, the heap is also memory. Deforming programs so they are ‘iterative’ in order that they do not run out of the stack we imagine to be so costly is ridiculous: if you have a program which is inherently recursive, let it be recursive.

In a previous article my friend Zyni wrote some variations on a list-flattening function1, some of which were ‘recursive’ and some of which ‘iterative’. Of course, the ones which claim to be iterative are, in fact, recursive: any procedure which traverses a recursively-defined data structure such as a tree of conses is necessarily recursive. The ‘iterative’ versions just use an explicitly-maintained stack rather than the implicit stack provided by the language. That makes sense only if stack space is very small compared to the heap and must therefore be conserved. And, well, for many systems that’s true. But it is small only because we have administratively decided it should be small: the stack is just memory. If there is plenty of memory for the heap, there is plenty for the stack.

There are, or may be, arguments for why stacks needed to be small on ancient machines. The history is fascinating, but it is not relevant to today’s systems, other than tiny embedded ones. The persistent view of modern machines as giant PDP–11s has been a blight for well over two decades now: it needs to stop.

The argument that the stack should be small often seems to be that, if it’s not, people will write programs which run away. That’s spurious: if such a program is, in fact, iterative, then good compilers will eliminate the tail calls and it will not use stack: a small limit on the stack will not help. If it’s really recursive then why should it run out of storage before its conversion to a program which manages the stack explicitly does? Of course that’s exactly what compilers which do CPS conversion already do: programs written using compilers which do that won’t have these weird stack limits in the first place. But it should not be necessary to rely on a CPS-converting compiler, or to write in continuation-passing style manually to avoid stack usage: it should be used for other reasons, because the stack is not, in fact, expensive.

Still less should people feel the need to write programs which explicitly manage a stack except in extraordinary cases.

There need to be some limits on stack size, just as there need to be some limits on heap size, but making the limit on stack size far smaller than the limit on heap size simply encourages people to believe things which aren’t true, and to live in fear of recursive programs.

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