[junke]

Context:

Special Forms in Lisp [0] by Kent Pitman (1980) is about FEXRs vs. MACRO:

> It is widely held among members of the MIT Lisp community that FEXPR, NLAMBDA, and related concepts could be omitted from the Lisp language with no loss of generality and little loss of expressive power, and that doing so would make a general improvement in the quality and reliability of program-manipulating programs.

> There are those who advocate the use of FEXPR's, in the interpreter for implementing control structure because they interface better with certain kinds of debugging packages such as TRACE and single-stepping packages. Many of these people, however, will admit that calls to FEXPR's used as control structure are bound to confuse compilers and macro packages, and that it is probably a good idea, given that FEXPR's do exist, to require compatible MACRO definitions be provided by the user in any environment in which FEXPR's will be used. This would mean that a person could create a FEXPR named IF, provided he also created an IF MACRO which described its behavior; the FEXPR definition could shadow [12] the MACRO definition in the interpreter, but programs other than the interpreter could appeal to the MACRO definition for a description of the FEXPR's functionality.

[0] http://www.nhplace.com/kent/Papers/Special-Forms.html

[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.