swap by name

[Wayne Throop]

> gwyn at brl-smoke.UUCP (Doug Gwyn (VLD/VMB) <gwyn>)
> It may be amusing and/or instructive to contemplate the fact that
> there is no way to write a function that exchanges the contents of
> two variables in a language where parameters are passed "by name".

Amusing, instructive.... and wrong.

I am assuming that you mean "Given a language which passes arguments to
functions with call-by-name semantics, it is impossible write an
'exchange argument pair' function, no matter what other facilities the
language provides".  This is clearly incorrect.  In fact, the C language
is a counterexample (assuming that the C language includes the
preprocessor).

Consider: macros act much like functions with by-name arguments.  The
trick is to utter the swap while mentioning the arguments only once.
Thus, one needs a bind-by-reference operation, which in C is nicely
provided by the '&' operator (with some restrictions, granted).

My conclusion: a swap function is easy to implement even with
pass-by-name functions, *provided* there is a bind-by-reference operator
in the language.

--
It is a lovely language, but it takes a very long time to say anything
in it, because we do not say anything in it, unless it is worth taking
a long time to say, and to listen to.
                                                ---     J. R. R. Tolkien
-- 
Wayne Throop      <the-known-world>!mcnc!rti-sel!dg_rtp!throopw