Compile-time vs run-time actions

[Vic]

You are all right in the sense that I had not fully baked in my head the ideas before first writing them here. I’ve edited the post with important changes.

@yuyichao

No. There can be side effects.

Right, I had realized that only after Ist posting ; I’ve corrected now.

@StefanKarpinski

In Julia, there is no semantic distinction between compile time and run time—everything behaves as if it is done at run time. The implementation can do something at compile time if there’s no behavioral difference between doing it at compile time and doing it at run time (aside from timing).

Right that the only difference is in duration taken by first- vs subsequent- calls of a function.
I wanted to find out whether I can predict when the difference will be big, that’s why I asked about that “rule of thumb”.

As exemplified in a reply, a function like

function f()
...
# ANY number of assignments with complex arithmetic expressions
# but which can be modeled by a (acyclic) dependency graph
# and don't involve side-effects
...
return(last_variable) # asssume last_variable is bound to a number -type value
end

and one like

function g()
   return(100)
end

will take exactly the same amount of time of execution, at all subsequent calls of the function (in a given program or REPL session). While first call of f() may take much much longer than g().