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Objects, functions, properties, and methods

Whether you’re trying to interpret SCI code in its source form, or compile it into bytecode, there are some inferences to make. Consider the following statements:

(foo1 bar:)
(foo2 bar: 42)
(foo3 69)
(foo4)

You can have object references, kernel calls, and local function calls, and those object references can be local instances or pointers which in turn can be stored in global variables, local variables, variables temporary to the current function or method, or properties of the current method’s object. How would you determine what each foo is?

First, you can see if there is a second item in the expression. If that item ends in :, like in the first two cases, you know that’s a selector so the identifier at the start must be an object reference of some sort. If it’s not, like in the third case, or if there is no second item at all, it must be a function or kernel call since anything else would be an error.

For the first two examples, we now know that foo must be an object. Having looked through the whole script before, we already have a list of all the parameters, temporary variables, local variables, and those from script 0, which are global. If either of those contains an item by that name, we know it’s a pointer to dereference. If it’s a local or imported object’s name, we’d be able to find that as well and can continue on. If it doesn’t appear in any of these six lists, the source code is in error.

For the other two examples, we know it must be a function or kernel call. There are three lists to check this time, being the local functions, imported functions, and kernels. Other than that, things are the same as before.

That leaves the matter of selectors. They can refer to either properties or methods, which are… actually rather trivial to tell apart considering the objects have two dictionaries, one for each type. Objects may have superclass chains reaching all the way to the Base Object and inherit properties and methods from those superclasses, but you might consider folding those superclasses’ dictionaries into the object instance’s so there’s only two to scan through.

Let’s say bar is a property. The first example would then mean “take the foo object and return its bar property’s value. Likewise the second would mean “set it to this expression.” If it’s a method, you’re given a pointer to that method’s code (which may be unique to that instance, having overwritten whatever the superclass chain started with) and you can pass it each non-selector argument in turn, until the next selector. I wrote about that before.

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