I conjecture the line is not so easy to draw.

If you are creating Lisp because you want to create Lisp, like creating Lisp, want to show off creating Lisp, that obviously is not what the Law is about.

Furthermore, if you create Lisp because you know the Law, know it is inevitable, and want to avoid the caveats and missed bars by doing so explicitly, well then that also is not what the Law is about.

But if you are going about your business, focused on your independent goal, realize you need Lisp like lists, and then 250 lines of code later realize you have created a solid Lisp unintentionally? Well, congrats on falling into the trap but not getting hurt!

—

Personally, I have created both Lisp and Forth multiple times for suitable projects where I wanted some flexible runtime scripting. I am not familiar with the standard version libraries of either and don’t need them.

Minimal foundation implementations are extremely easy to create, and eliminate any dependencies or sources of mystery.

Anyone know of any other well designed mininal languages?

yes but not recursively!

The corollary to Greenspun’s rule is that any sufficiently complicated Common Lisp program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Prolog.

In a struct! Pseudo code for extracting the types from subclasses and calling a multimethod on them:

  template <typename Result, typename Head, typename ...Tail>
  struct Foo : Visitor {
    Result res;
    UntypedList lst; // This does not exist!
    Tail... tail;    // This is not possible!
    Foo(UntypedList l; Head hd, Tail... tl) : lst(l), tail(tl) {
      hd->accept(*this);
    }
    void visit(float *f) {
      res = Foo<Result, Tail...>(append(lst, f), tail)::res; }
    }
  };
 
  // Base case that calls the multimethod omitted.

There are two issues here: You can't store the parameter pack in Foo() which is later required by the continuation in visit(). And you would have to use tuples and tuple_cat() instead of UntypedList.

Why can the compiler not infer the types in such an UntypedList? Why is there no car/cdr for tuples?

Greenspun's tenth rule, corollary: Any sufficiently precise definition of "complex system" contains an ad hoc, informal, bug-ridden, slow specification of half of Common Lisp.

No, I think it's pretty fair. One could argue about these, but except for "slow" these are more quality qualifiers, rather than quantity. So you either agree it's "bug-ridden" or not (i.e. the number and seriousness of bugs in it is negligible by whatever standards). And I think even "slow" can be discussed in the same manner, the actual speed is quantitative, of course, but in the end one either argues that this speed is "slow" or isn't. So, given some rhetoric skills of your opponent it's at least possible for that statement to be proven false, if he convinces you the implementation actually isn't slow nor bug-ridden, or at least if there's no Lisp-implementation indeed.

But what is "sufficiently" complicated? Now it's a silly statement that just doesn't pass Popper criterion: even if nobody dares to deny your program is complicated, one can always say it isn't sufficiently complicated, hence the fact it doesn't contain Lisp-interpreter disproves nothing. A man of culture wouldn't use such definitions.