The reasons are:
- I want to locate all implmenetations of the get_fcn() handler. By
consistently using this macro, you can just grep for the macro and
find them all.
- all implementations should follow the same style. This macro
enforces the same names for arguments and avoids copy&paste.
- if we are going to add or change an argument, it becomes easier.
That's because we can easily identify all implementation and can
change arguments in one place.
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