Re: [stack] Re: Parameters: ordered versus named

pml060912 scripsit:

> That looks like something I came across in P.J.Brown's "Writing 
> Interactive Compilers and Interpreters", although I forget if he 
> stated it explicitly or just implied it. In essence, he suggested 
> handling parameters and local variables by a "don't care" method, in 
> which they were really global variables like any other, only their 
> values were stacked/reinitialised and then restored on call/return. 

That's dynamic scoping (not to be confused with dynamic *typing*) and
it is a Really Really Bad Idea, because reasoning about the behavior
of programs becomes insanely difficult.  You wind up not being able to
know where the value of a variable is set unless you know the exact
path by which it was called, and no procedure can guarantee that the
values of its parameters are safe from tampering unless it never calls
any other procedure.  What's more, that particular *implementation*
of dynamic scoping (known as "shallow binding") only works in a single
thread of control; if there are multiple threads, it goes horribly wrong
in all sorts of ways.

Lexical scoping, in which variables only exist within the particular
portion of the program that declares them, is a far superior regime.
There are occasional uses for dynamic scoping here and there, and
some languages support it as an alternative to lexical scoping for
restricted use (Perl and Common Lisp, notably; also because of backward
compatibility).  But it's hard to believe that any modern work on
interpreters would recommend it.

-- 
"Well, I'm back."  --Sam        John Cowan <[email protected]>