Sorry, my previous mail was sent from a flashmail, was a bad idea ! I hope
this one will be more readable...
As other users (see comp.lang.rexx/2005-11/msg00065.html), I use sometimes
a function iif which on the basis of a condition selects between two
values :
iif:
use strict arg condition, iftrue, iffalse
if condition then
interpret "return" iftrue
else
interpret "return" iffalse
I see two drawbacks with this function :
The caller must surround the 2nd and 3rd argument by quotes, to pass a
string to the function. Example : say iif(s == .nil, 's', 's~id')
The whole scope of the caller must be visible, to let the evaluation have
access to all the variables of the caller. So I can't implement this
function as a procedure or a routine or a method. And I can't put it in
my library that I requires from my scripts.
I had a look at the function RexxActivation::run (putting a breakpoint
here is very instructive).
I see that each instruction is evaluated by : nextInst->execute(this,
localStack)
It seems that each instruction is in charge of the evaluation of its
arguments, using : result = this->expression->evaluate(context, stack)
Could we imagine that such a behavior is possible at the level of user
rexx code ? The not-evaluated-expression passed to the rexx code should
bring the context and stack with her, to let the user write
expression->evaluate()
A not-evaluated-expression is not of type String. Should be of a new type
(Closure ?) which supports the method evaluate().
Example :
iif: procedure
use unevaluated arg condition, iftrue, iffalse
if condition->evaluate() then
return iftrue->evaluate()
else
return iffalse->evaluate()
Here " unevaluated " is a keyword to indicate the deferred evaluation of
the arguments.
I can use a procedure or routine or method since the evaluation of the
expression is done in the caller's context.
The caller calls the function without surrounding the expressions in
quotes :
say iif(s == .nil, s, s~id)
And there is no error like Object "The NIL object" does not understand
message "ID" when s is nil.
Jean Louis
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