f ($a) + g ($a)
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• fetch the value of $a
• call f() with $a as argument, remember the return value
• fetch the value of $a
• call g() with $a as argument, remember the return value
• add the return values of f() and g().

Last but not least, if the order of evaluation is defined, people will start writing code that depends on this. This will lead to fragile code, that will break if you swap the arguments of an otherwise symmetric operator.

Finally, I don't see how lazy evaluation and a defined order of evaluation combine to a happy programming language. Perl6 will have a defined order of evaluation, and have lazy evaluation. I wonder how Larry is going to pull off that one.

Abigail

I don't think the problem is with having laziness and defined order of evaluation. It's side-effects per-se that create the problem. Which is why laziness is usualy only present in purely functional (free of any side-effects) languages.

The defined O-of-E could actually make the problem slightly smaller. Anyway IMHO the only thing Lary can do is to write a warning in the docs. "Don't mix laziness and side effects. If you do, don't come to us crying ;-)"

Jenda
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
   -- Rick Osborne

Edit by castaway: Closed small tag in signature

Say, you have a defined order of evaluation, and you define the order of evaluation for comma in list context from left to right. You also have lazy evaluation.

Now, in the following expression, which one will be evaluated first, g() or h():

    sub f ($$) {$_ [1] && $_ [0]}
    f (g (), h ())
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The biggest problem I have with defining the order of evaluation is that people will start to write code that depends on it. It will mean that the code is fragile, and. specially in combination with lazy evaluation, hard to understand. If the order of evaluation is undefined, the code has to be correct, no matter in which order it's evaluated. That makes code easier to understand (because any order I choose is ok).

Abigail