[kazinator]

But, of course, if you just write the macro IF, it's pointless to then write a FEXPR. This is because the interpreter can use the IF macro just fine. Nobody is going to write every operator twice in a large code base, first as a FEXPR operator and then a macro operator. It's extra development work, plus extra work to validate that the two behave the same way and are maintained in sync.

Kent is writing very theoretically there and being very generous to the idea.

Single stepping through macro-expanded code is perfectly possible. There is no debugging disadvantage between stepping through a macro-expanded control flow operator, versus one which is interpreted. In both cases, the single-stepping interpreter can know the source code location where the argument expressions came from and jump the cursor there, providing visual stepping.

Not to mention that compiled code can be stepped through a source code view; countless programmers have been doing this in C for decades, and similar languages. Given that we can write an if statement in C, compile it and step through it in gdb, the position that we benefit from an FEXPR to do the same thing in a Lisp interpreter is rather untenable.