OpenEuphoria

1. variable declaration reset

• Posted by petelomax Apr 15, 2022
• 793 views

This just surprised me:

sequence s = {1,2,2,3,4,4,5} 
for i=1 to length(s) do 
    integer curr = s[i], prev 
    if i>1 and curr=prev then 
        ?{i,s[i]} 
    end if 
    prev = curr 
end for 

On desktop/Phix you get {3,2} and {6,4}, but under pwa/p2js you get precisely nuttin.
JavaScript actually resets prev at the start of every iteration, whereas desktop/Phix leaves it be.
One fix is to simply move the declaration of prev outside of|above the loop, and then they match.
Another way to fix it is to replace "let" in the JavaScript with "var", which triggers legacy hoisting...
[However a blanket use of "var" would fail badly in say "if cond then atom a ... else atom a" cases]

I'm intrigued to know how (various versions of) Euphoria handle this,
and whether to do the same on desktop/Phix and risk breaking legacy code,
or not do that, but somehow (and that's the question) issue a warning/error.

I've also created a rosettacode task to see what that yields.

3. Re: variable declaration reset

• Posted by euphoric (admin) Apr 16, 2022
• 725 views

sequence s = {1,2,2,3,4,4,5} 
for i=1 to length(s) do 
    integer curr = s[i], prev 
    if i>1 and curr=prev then 
        ?{i,s[i]} 
    end if 
    prev = curr 
end for 

That's the behavior I expect, no matter what language I'm using. A variable defined is a variable reset.

5. Re: variable declaration reset

• Posted by _tom (admin) Apr 17, 2022
• 692 views

Phix is working as expected.

JavaScript is working in a plausible, internally consistent, ?

• instantiate a type and give it a name integer prev

• • prior to this, any code that used the name ''prev'' crashes Phix
  • just after this moment: ? object(prev) is False.

• assign a value prev := s[i]

• • after this moment: ? object(prev) is True.
  • since it is a loop, prev is now in-scope before the declaration, next time the loop is performed ... this makes sense, but maybe not obviously

• observations
  • prev is out-scope after ''end for''

• • it makes no sense (in a compiled program) for ''integer prev'' to be duplicated ''i'' number of times that is a waste of energy
  • the variable is created exactly once in the loop

• • in a traditional interpreter (one-line-at-time design) it is not surprising that ''integer prev'' is actually duplicated for every loop iteration which causes a "reset"... very inefficient

• • I would expect ''integer prev'' to be ignored in a loop after the first encounter

• • once the loop goes past ''one iteration'', then prev is in-scope from the top of the loop ? object(prev) just before the declaration returns True

This seems to be more idiomatic for OE/Phix:

for i=2 to length(s) do 
	if s[i-1] == s[i] then 
		? { s[i-1], s[i] } 
	end if 
 
	---- optional 
	integer curr,prev 
	{curr,prev} := { s[i-1], s[i] } 
end for 

It would be interesting if a statement existed that would "undo" an assignment and "reset" a variable to an assigned state so ? object(prev) does return False

be well
_tom