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Ideas / Macro-system design
« Last post by bas on January 30, 2019, 09:17:13 AM »Since C2 wants to avoid the pitfalls of the C textual-replacement based macro system (and C2 doesn't have includes), a replacement macro system is needed.
Macros have a various, valid use-cases. I'll focus on one case here and try to find a nice syntax + semantics.
What I want is to be able to put a loop over a data structure in a macro, so that changing the underlying data structure maybe doesn't need any 'api' change
for users. So something like
(in some function)
So:
The definition could be roughly something like:
For C macro's all these questions are easy, since it's just replacing text. But that gives rise to a lot of issues, so a new system
is definitely needed. But I think a non-textual system will have a lot more restrictions (which can be good), so when designing a
new system, the goal should not be to find a replacement for every possibility in C macros. This won't work. My plan is to
start creating a set of use-cases and build on top of that. The foreach example is just one of those use-cases.
Macros have a various, valid use-cases. I'll focus on one case here and try to find a nice syntax + semantics.
What I want is to be able to put a loop over a data structure in a macro, so that changing the underlying data structure maybe doesn't need any 'api' change
for users. So something like
(in some function)
Code: [Select]
// there is a list.List* mylist that points to the list (let's say a linked-list)
list.foreach(mylist, i) { // i is the name of the Element*
// use Element* i here
if (i.value == 10) break;
}
So:
- list.foreach is the macro
- When changing the data structure from a linked list to a vector, nothing much has to change in the fragment.
- The break statement should break out of the macro loop even if the macro itself has multiple, nested loops.
The definition could be roughly something like:
Code: [Select]
public macro foreach(List* thelist, iname) {
Element* iname = thelist.first;
while (iname != nil) {
// macro body should be here, How to specify?
iname = iname.next;
}
}
So this brings up a lot of questions:- how to let a user specify the name of the iterator?
- Do we required types for macro arguments?
- How can we specify the place for the macro body?
- Do we allow using variables from the calling scope (and how to specify)
- How can we implement the break in user code
- Do we allow nested macro calling?
For C macro's all these questions are easy, since it's just replacing text. But that gives rise to a lot of issues, so a new system
is definitely needed. But I think a non-textual system will have a lot more restrictions (which can be good), so when designing a
new system, the goal should not be to find a replacement for every possibility in C macros. This won't work. My plan is to
start creating a set of use-cases and build on top of that. The foreach example is just one of those use-cases.