In case you are still reading this: the book is about ancient LISP (LISP 1.0, 1.5, PDP-6 LISP), which was dynamically scoped. The Y combinator requires lexical scoping, though.
Hmm, that's a good point. The general-case solution they adopted for not having lexical scoping at the time was FUNARG, but of course that's just as much of a kludge as LABEL (which was, yes, in 1959 Lisp, before FUNARG). There are cheats like this, of course:
Perhaps surprisingly, this does work in dynamically-scoped Lisp-2s like Elisp (which, unlike Lisp 1.5, doesn't have FUNARG), even for λ-expressions:
(funcall
(quasiquote-y '(lambda (f n)
(if (< n 2) 1
(* n (funcall f f (- n 1))))))
4)
Defining a version of QUASIQUOTE-Y that works for the following subtly different LAMBDA form has so far escaped me but seems like it ought to be feasible:
(lambda (f n)
(if (< n 2) 1
(* n (funcall f (- n 1)))))
Now, of course, in modern Lisps, we consider it a barbaric practice to treat a list beginning with the symbol LAMBDA as a function as such, as opposed to a piece of code which could be EVALed to produce a function. But this ancient abomination remains honored in the dark precincts of Elisp, and of course was entirely supported in Lisp 1.5 (though doing the equivalent of QUASIQUOTE would have been considerably more awkward; FUNARG was the favored design error of the day) and McCarthy 1959.
ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-199.pdf ("The function of FUNCTION") relates the 1970 perspective on this problem.
Oh, so he is. I wonder if he'll explain I misunderstood him or correct the error.
In case you are still reading this: the book is about ancient LISP (LISP 1.0, 1.5, PDP-6 LISP), which was dynamically scoped. The Y combinator requires lexical scoping, though.
Hmm, that's a good point. The general-case solution they adopted for not having lexical scoping at the time was FUNARG, but of course that's just as much of a kludge as LABEL (which was, yes, in 1959 Lisp, before FUNARG). There are cheats like this, of course:
Perhaps surprisingly, this does work in dynamically-scoped Lisp-2s like Elisp (which, unlike Lisp 1.5, doesn't have FUNARG), even for λ-expressions:
Defining a version of QUASIQUOTE-Y that works for the following subtly different LAMBDA form has so far escaped me but seems like it ought to be feasible:
Now, of course, in modern Lisps, we consider it a barbaric practice to treat a list beginning with the symbol LAMBDA as a function as such, as opposed to a piece of code which could be EVALed to produce a function. But this ancient abomination remains honored in the dark precincts of Elisp, and of course was entirely supported in Lisp 1.5 (though doing the equivalent of QUASIQUOTE would have been considerably more awkward; FUNARG was the favored design error of the day) and McCarthy 1959.
ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-199.pdf ("The function of FUNCTION") relates the 1970 perspective on this problem.
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