Generarl programming question.

Terry Reedy tjreedy at udel.edu
Sat Apr 11 20:47:51 CEST 2015


On 4/11/2015 12:23 PM, Thomas 'PointedEars' Lahn wrote:
> Chris Angelico wrote:
>
>> The 'x' inside each function is completely separate, no matter how
>> many times they get called. They're usually stored on something called
>> a "call stack" - you put another sheet of paper on top of the stack
>> every time you call a function, local variables are all written on
>> that paper, and when you return from a function, you discard the top
>> sheet and see what's underneath.
>
> Thank you for that description; I shall use it from now on when teaching
> laymen about the call stack.

What Chris is describing is one local namespace (sheet of paper) per 
function *call*.  In early Fortran (at least the first version I used), 
there was one local namespace (sheet) per *function*.  The call stack 
was a stack of (pointers to) functions.  While a function object was in 
use (after a call, before the return), it could not be called again.  In 
other words, recursion, direct or indirect, was not allowed.  I believe 
the same was (is?) true of some versions of BASIC.

It has been proposed that Python use a hybrid model.  Function objects 
would have space for local variables for the first call, but there would 
also be a mechanism to allocate additional 'sheets' for recursive calls. 
  The idea is that most functions are not called recursively, so the 
overhead of allocating and freeing the per-call space is usually not 
needed.  I do not believe that anyone has implemented the idea to test 
feasibility and the actual speedup in relation to the additional complexity.

-- 
Terry Jan Reedy




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