I'm just curious about why return ends the function.
Why do we not write
function Foo (){
BAR = calculate();
give back BAR;
//do sth later
log(BAR);
end;
}
Why do we need to do this?
function Foo (){
BAR = calculate();
log(BAR);
return BAR;
}
Is this to prevent multiple usage of a give back/return value in a function?
The idea of a function stems from mathematics, e.g. x = f(y). Once you have computed f(y) for a specific value of y, you can simply substitute that value in that equation for the same result, e.g. x = 42. So the notion of a function having one result or one return value is quite strong. Further, such mathematical functions are pure, meaning they have no side effects. In the above formula it doesn’t make a difference whether you write f(y) or its computed result 42, the function doesn’t do anything else and hence won’t change the result. Being able to make these assumptions makes it much easier to reason about formulas and programs.
return in programming also has practical implementation implications, as most languages typically pop the stack upon returning, based on the assumption/restriction that it’s not needed any further.
Many languages do allow a function to “spit out” a value yet continue, which is usually implemented as generators and the yield keyword. However, the generator won’t typically simply continue running in the background, it needs to be explicitly invoked again to yield its next value. A transfer of control is necessary; either the generator runs, or its caller does, they can’t both run simultaneously.
If you did want to run two pieces of code simultaneously, be that a generator or a function’s “after return block”, you need to decide on a mode of multitasking like threading, or cooperative multitasking (async execution) or something else, which brings with it all the fun difficulties of managing shared resource access and the like. While it’s not unthinkable to write a language which would handle that implicitly and elegantly, elegant implicit multitasking which manages all these difficulties automagically simply does not fit into most C-like languages. Which is likely one of many reasons leading to a simple stack-popping, function-terminating return statement.