Riddle 2 Answer

This is somewhat esoteric, and I wouldn’t be surprised if very few people had any idea what I was even asking. I discovered this largely by chance, while fiddling around with function bytecode, though I think it could be deduced from observation without that.

So, the answer:

:: is usually taught as a sui generis operator, called “global amend”, which has the specific behavior (when used as a verb inside a function) of setting a global variable (instead of the local one that : would set in the same place). No connection is typically drawn between it and any other operator (other than :).

However, I’m pretty sure this is inaccurate. While obviously I don’t know for certain, I strongly suspect that there is no code anywhere in the q binary saying that :: is defined as “global amend”. Rather, it is a specific case of the dyadic “f:” pattern, where f is some dyadic function—e.g. dyadic +:, -:, *:, etc.

These all have the same behavior—x f:y is defined as x:x f y.

Additionally, when used inside functions on variables that have not been identified by the compiler as locals, they modify (and if necessary, create), global variables.

q){a:1;a+:1;a}[]
2
q)a
'a
q){a+:1}[]
q)a
1
q)

It follows that if f is :, then the operation involved is assignment, and so x gets y assigned to it, as a global variable if not identified as a local variable.

In fact, this can be seen in the same way:

q){a:1;a::2;a}[]
2
q)a
'a
q){a::2}[]
q)a
2
q)

Thus arises “global amend”.

If anything, the “create view” sense of :: must be the special case, as ordinarily, dyadic f: verbs behave identically inside and outside functions.