Why is the order of evaluation of expressions used for concatenation undefined in Awk?

Question

In GNU Awk User's Guide, I went through the section 6.2.2 String Concatenation and found interesting insights:

Because string concatenation does not have an explicit operator, it is often necessary to ensure that it happens at the right time by using parentheses to enclose the items to concatenate.

Then, I was quite surprised to read the following:

Parentheses should be used around concatenation in all but the most common contexts, such as on the righthand side of ‘=’. Be careful about the kinds of expressions used in string concatenation. In particular, the order of evaluation of expressions used for concatenation is undefined in the awk language. Consider this example:

BEGIN {
   a = "don't"
   print (a " " (a = "panic"))
}

It is not defined whether the second assignment to a happens before or after the value of a is retrieved for producing the concatenated value. The result could be either ‘don't panic’, or ‘panic panic’.

In particular, in my GNU Awk 5.0.0 it performs like this, doing the replacement before printing the value:

$ gawk 'BEGIN {a = "dont"; print (a " " (a = "panic"))}'
dont panic

However, I wonder: why isn't the order of evaluation of expressions defined? What are the benefits of having "undefined" outputs that may vary depending on the version of Awk you are running?

Answer 1

This particular example is about expressions with side-effects. Traditionally, in C and awk syntax (closely inspired by C), assignments are allowed inside expressions. How those expressions are then evaluated is up to the implementation.

Leaving something unspecified would make sure that people don't use potentially confusing or ambiguous language constructs. But that assumes they are aware of the lack of specification.