> Here is another solution, due to Martin Bunder and F. Urbanek:
- > let pred = \n. \s z. n (\u v. v (u s)) (K z) I in
+ > let box = \a. \v. v a in
+ > let pred = \n. \s z. n (\b. box (b s)) (K z) I in
> ...
- > That can be hard to wrap your head around. If you trace it through, you'll see that:
+ > That can be hard to wrap your head around. If you trace this expression through, you'll see that:
- > * when `n == 0`, it reduces to `\s z. (\v. z) I`, or `\s z. z`
- > * when `n == 1`, it reduces to `\s z. (\v. v z) I`, or `\s z. z`
- > * when `n == 2`, it reduces to `\s z. (\v. v (s z)) I`, or `\s z. s z`
- > * when `n == 3`, it reduces to `\s z. (\v. v (s (s z))) I`, or `\s z. s (s z)`
+ > * when `n == 0`, it reduces to `\s z. (K z) I`, or `\s z. z`
+ > * when `n == 1`, it reduces to `\s z. (box z) I`, or `\s z. z`
+ > * when `n == 2`, it reduces to `\s z. (box (s z)) I`, or `\s z. s z`
+ > * when `n == 3`, it reduces to `\s z. (box (s (s z))) I`, or `\s z. s (s z)`
- > The technique used here is akin to that used in [[the hint for last week's `reverse`|assignment2_answers#cps-reverse]].
+ > `box a` is `\v. v a`; this stands to `pair a b` as one stands to two. Since boxes (like pairs) are really just functions, the technique used in this definition of `pred` is akin to that used in [[the hint for last week's `reverse`|assignment2_answers#cps-reverse]].
(Want a further challenge? Define `map2` in the Lambda Calculus, using our right-fold encoding for lists, where `map2 g [a, b, c] [d, e, f]` should evaluate to `[g a d, g b e, g c f]`. Doing this will require drawing on a number of different tools we've developed, including that same strategy for defining `tail`. Purely extra credit.)
> let leq? = \l r. zero? (sub l r) in
> ...
- > Here is another solution. Jim crafted this particular implementation, but like a great deal of the CS knowledge he's gained over the past eight years, Oleg Kiselyov pointed the way.
+ > Here is another solution. Jim crafted this particular implementation, but like a great deal of the CS knowledge he's gained over the past eight years, Oleg Kiselyov pointed the way. <!-- see "lambda-calc-opposites.txt" at http://okmij.org/ftp/Computation/lambda-calc.html#neg -->
> let leq? = (\base build consume. \l r. r consume (l build base) fst)
> ; where base is
25. For each of the above translations, how many `I`s are there? Give a rule for describing what each `I` corresponds to in the original lambda term.
+ This generalization depends on you omitting the translation rule:
+
+ 6. @a(Xa) = X if a is not in X
+
+ > With that shortcut rule omitted, then there turn out to be one `I` in the result corresponding to each occurrence of a bound variable in the original term.
+
Evaluation strategies in Combinatory Logic
------------------------------------------
<OL start=30>
<LI><code>(\x. x (\y. y x)) (v w) ~~> v w (\y. y (v w))</code>
<LI><code>(\x. x (\x. y x)) (v w) ~~> v w (\x. y x)</code>
-<LI><code>(\x. x (\y. y x)) (v x) ~~> v w (\y. y (v x))</code>
-<LI><code>(\x. x (\y. y x)) (v y) ~~> v w (\u. u (v y))</code>
+<LI><code>(\x. x (\y. y x)) (v x) ~~> v x (\y. y (v x))</code>
+<LI><code>(\x. x (\y. y x)) (v y) ~~> v y (\u. u (v y))</code>
<LI><code>(\x y. x y y) u v ~~> u v v</code>
<LI><code>(\x y. y x) (u v) z w ~~> z (u v) w</code>