Have you actually used a tape machine? I've used 7040's & DECtape (pdp-8)'s. Calculate the moment of inertia of a large tape reel some time. (e.g., take a look at the film reels used in IMAX theaters.) http://en.wikipedia.org/wiki/IMAX Actually, combinators are very roughly stack machines & can be quite practical. I've written about this ("Lively Linear Lisp"). The original Emacs was a set of TECO macros for the excellent TECO system at MIT (much, much better than DEC's TECO). TECO was essentially a 2-stack implementation, where the "cursor" was simultaneously the top of the left stack and the top of the right stack. I'm still working on trying to perfect a really elegant 2-stack set of combinators. At 02:12 AM 8/3/2012, Adam P. Goucher wrote:
Or a matter of practicality? I don't see how one could easily build a physical machine to directly run lambda calculus, whereas a Turing machine is very mechanical in its definition.