So in terms of my example: We have the general form: ( (lambda (x) e) V) ~ e with all [free] x replaced by V
And for V <-- 2 :: ( (lambda (x) (* x y) 2) ~ (* 2 y) ? But for V <-- z^2 :: error and NOT ~ (* (* z z) y) ? Thanks On 11/02/2014, at 3:59 PM, Matthias Felleisen <matth...@ccs.neu.edu> wrote: > > > Ouch, I forgot to explain V. NO, you canNOT replace x with arbitrary > expressions. You may use ONLY VALUES (V for Value). In this context a value > is one of: #t/#f, number, or a lambda expression. -- Matthias > > > > > > On Feb 11, 2014, at 9:50 AM, Matthew Johnson <mcoog...@gmail.com> wrote: > >> The general form: ( (lambda (x) e) V) ~ e with all [free] x replaced by V >> >> In concrete terms means: ( (lambda (x) (* x y) 2) ~ (* 2 y) ? >> >> I figure best to be sure now. >> >> Thanks and best regards >> >> matt johnson >> >> >> On 11/02/2014, at 3:35 PM, Matthias Felleisen <matth...@ccs.neu.edu> wrote: >> >>> >>> On Feb 11, 2014, at 12:48 AM, Matthew Johnson <mcoog...@gmail.com> wrote: >>> >>>> I am still a little unclear. Is this a logical result, or a result of the >>>> evaluator's rules? Or are they the same thing? >>> >>> >>> They are the same thing. To calculate with the Racket that you see in the >>> book/derivation, use this rule: >>> >>> ( (lambda (x) e) V ) ~ e with all [free] occurrences of x replaced by V >>> >>> [This is the rule you learn in pre-algebra, with the 'free' needed here >>> because >>> variables can occur at more places than in pre-algebra.] Using this rule, >>> plus >>> simple car/cdr/cons/+1 rules, you can confirm every step in the derivation. >>> As it >>> turns out, the compiler implements this rule totally faithfully BUT instead >>> of >>> copying actual code it "copies" references to code by moving things from >>> registers >>> to registers. Because the two are equivalent, a programmer can use the above >>> rule to think about code and the compiler writer (one among 10,000s of >>> language >>> users) is the only one who has to keep references, registers, and >>> replication >>> in mind (kind of). >>> >>> -- Matthias > ____________________ Racket Users list: http://lists.racket-lang.org/users
