mark richardson wrote :
Hi,
I have been trying to understand something unusual and I wonder if
anyone can shed any light on this.
If I feed this program
declare
{Browse {Plus 5 5}}
fun {Plus A B} A+B end
by buffer, the program blocks on the Browse procedure (I assume
because 'Plus' has not yet been determined?)
If however, I now feed a new line
fun {Plus A B} A+B+A end
I get an error saying that Plus has already been defined (which
initially seems sensible)
Tell: <P/3 Plus> = <P/3 Plus>
but, and here is the confusing thing, the browser now displays the
result from the second function????
There are a couple of contradictions I don't understand here:-
1 . Browse can't display the result of {Plus} because it hasn't been
determined yet - {Plus} has been determined enough to tell me I've
made an error when I try to redefine it.
2. If redefining {Plus} is an error why does Browse show the result
from the new function?
If anyone can help I'd be most appreciative.
Regards
Mark
Hello,
Let's consider the Core syntax (kernel language) translation of your
experiment.
The first part translates to:
declare Plus in
local UnnestApply1 UnnestApply2 UnnestApply3 in
UnnestApply2 = 5
UnnestApply3 = 5
{Plus UnnestApply2 UnnestApply3 UnnestApply1} %1
{Browse UnnestApply1}
proc {Plus A B Result1}
Result1 = A + B
end
end
And the second to:
proc {Plus A B Result1}
local UnnestApply1 in
UnnestApply1 = A + B
Result1 = UnnestApply1 + A
end
end
Now, when you feed the first part, it blocks on the line marked %1
since Plus is unbound. But note that the variable Plus is declared.
When you feed the second part, it binds the variable Plus to the second
procedure. This allows the first feed to continue execution, browsing
the result 15. Only then is the definition of the second procedure
executed, which of course fail since the variable is already bound (to
the second procedure).
Yves
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