Not defining order of evaluation leaves more possibilities open for optimalizations. Consider for instance the expression:

    f ($a) + g ($a)
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If you define an order of evaluation, it would probably be:

• 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().

Notice the twice fetching of $a. If the order of evaluation isn't defined, you can first fetch the value of $a twice. Even if you do fetch the value twice (instead of fetching it once and remembering the results), doing it before calling f() and g() could be a win due to an increased chance of cache hits. It will also give you the opportunity to evaluate sub expressions in parallel.

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

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