X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?a=blobdiff_plain;f=exercises%2Fassignment2_answers.mdwn;h=3eaaa1566c4791f3116b7c2fcb5bb32e8b6d74a6;hb=0da178367cda4feaf08df9a101b7d36eedee9f58;hp=e131311c7d2913bab89028656380da66c8642162;hpb=ccb65934b58c91f5bf06f6172e6877050d05d214;p=lambda.git
diff --git a/exercises/assignment2_answers.mdwn b/exercises/assignment2_answers.mdwn
deleted file mode 100644
index e131311c..00000000
--- a/exercises/assignment2_answers.mdwn
+++ /dev/null
@@ -1,205 +0,0 @@
-Syntax
-------
-
-Insert all the implicit `( )`s and λs into the following abbreviated expressions.
-
-1. `x x (x x x) x`
- (((x x) ((x x) x)) x)
-2. `v w (\x y. v x)`
- ((v w) (\x (\y (v x))))
-3. `(\x y. x) u v`
- (((\x (\y x)) u) v)
-4. `w (\x y z. x z (y z)) u v`
- (((w (\x (\y (\z ((x z) (y z)))))) u) v)
-
-Mark all occurrences of `(x y)` in the following terms:
-
-5. (\x y. x y) x y
-6. (\x y. x y) (x y)
-7. \x y. x y (x y)
-
-Reduction
----------
-
-Find "normal forms" for the following---that is, reduce them until no more reductions are possible. As mentioned in the notes, we'll write λx as `\x`. If we ever say "reduce" without qualifications, we mean just "beta-reduce" (as opposed to "(beta + eta)-reduce").
-
-8. `(\x \y. y x) z` ~~> `\y. y z`
-9. `(\x (x x)) z` ~~> `z z`
-10. `(\x (\x x)) z` ~~> `\x x`
-11. `(\x (\z x)) z` ~~> `\y z`, be sure to change `\z` to a different variable so as not to "capture" `z`
-12. `(\x (x (\y y))) (\z (z z))` ~~> `\y y`
-13. `(\x (x x)) (\x (x x))` umm..., reductions will forever be possible, they just don't "do" much
-14. `(\x (x x x)) (\x (x x x))` that's just mean
-
-
-
-Booleans
---------
-
-For these questions, and the ones on triples below, we're setting them up so as to encourage you to experiment with Racket and to formulate your answer in Scheme/Racket syntax. But you can answer in Lambda Calculus syntax if you prefer.
-
-Recall our definitions of true and false.
-
-> **true** is defined to be `\t f. t`
-> **false** is defined to be `\t f. f`
-
-In Racket, these functions can be defined like this:
-
- (define true (lambda (t) (lambda (f) t)))
- (define false (lambda (t) (lambda (f) f)))
-
-(Note that they are different from Racket's *primitive* boolean values `#t` and `#f`.)
-
-
-15. Define a `neg` operator that negates `true` and `false`.
-
- Expected behavior:
-
- (((neg true) 10) 20)
-
- evaluates to `20`, and
-
- (((neg false) 10) 20)
-
- evaluates to `10`.
-
- (define neg (lambda (p) ((p false) true)))
-
-16. Define an `or` operator.
-
- (define or (lambda (p) (lambda (q) ((p p) q))))
-
- or:
-
- (define or (lambda (p) (lambda (q) ((p true) q))))
-
-
-17. Define an `xor` operator. If you haven't seen this term before, here's a truth table:
-
- true xor true == false
- true xor false == true
- false xor true == true
- false xor false == false
-
-
-
- (define xor (lambda (p) (lambda (q) ((p (neg q)) q))))
-
-
-Triples
--------
-
-Recall our definitions of ordered triples.
-
-> the triple **(**a**, **b**, **c**)** is defined to be `\f. f a b c`
-
-To extract the first element of a triple `t`, you write:
-
- t (\fst snd trd. fst)
-
-Here are some definitions in Racket:
-
- (define make-triple (lambda (fst) (lambda (snd) (lambda (trd) (lambda (f) (((f fst) snd) trd))))))
- (define fst_of_three (lambda (fst) (lambda (snd) (lambda (trd) fst))))
- (define snd_of_three (lambda (fst) (lambda (snd) (lambda (trd) snd))))
-
-Now we can write:
-
- (define t (((make-triple 10) 20) 30))
- (t fst_of_three) ; will evaluate to 10
- (t snd_of_three) ; will evaluate to 20
-
-If you're puzzled by having the triple to the left and the function that
-operates on it come second, think about why it's being done this way: the triple
-is a package that takes a function for operating on its elements *as an
-argument*, and returns *the result of* operating on its elements with that
-function. In other words, the triple is a higher-order function.
-
-
-18. Define the `swap12` function that permutes the elements of a triple. Expected behavior:
-
- (define t (((make-triple 10) 20) 30))
- ((t swap12) fst_of_three) ; evaluates to 20
- ((t swap12) snd_of_three) ; evaluates to 10
-
- Write out the definition of `swap12` in Racket.
-
- (define swap12 (lambda (x) (lambda (y) (lambda (z)
- (lambda (f) (((f y) x) z))))))
-
-
-19. Define a `dup3` function that duplicates its argument to form a triple
-whose elements are the same. Expected behavior:
-
- ((dup3 10) fst_of_three) ; evaluates to 10
- ((dup3 10) snd_of_three) ; evaluates to 10
-
-
-
- (define dup3 (lambda (x)
- (lambda (f) (((f x) x) x))))
-
-20. Define a `dup27` function that makes
-twenty-seven copies of its argument (and stores them in a data structure of
-your choice).
-
- OK, then we will store them in a triply-nested triple:
-
- (define dup27 (lambda (x) (dup3 (dup3 (dup3 x)))))
-
-Folds and Lists
----------------
-
-21. Using Kapulet syntax, define `fold_left`.
-
- # fold_left (f, z) [a, b, c] == f (f (f z a) b) c
- letrec
- fold_left (f, z) xs = case xs of
- [] then z;
- x' & xs' then fold_left (f, f (z, x')) xs'
- end
- in fold_left
-
-
-22. Using Kapulet syntax, define `filter` (problem 7 in last week's homework) in terms of `fold_right` and other primitive syntax like `lambda`, `&`, and `[]`. Don't use `letrec`! All the `letrec`-ing that happens should come from the one inside the definition of `fold_right`.
-
- let
- filter (p, xs) = fold_right ((lambda (y, ys). if p y then y & ys else ys), []) xs
- in filter
-
-23. Using Kapulet syntax, define `&&` in terms of `fold_right`. (To avoid trickiness about the infix syntax, just call it `append`.) As with problem 22 (the previous problem), don't use `letrec`!
-
- let
- xs && ys = fold_right ((&), ys) xs
- # or append (xs, ys) = ...
- in (&&)
-
-24. Using Kapulet syntax, define `head` in terms of `fold_right`. When applied to a non-empty list, it should give us the first element of that list. When applied to an empty list, let's say it should give us `'err`. As with problem 22, don't use `letrec`!
-
- let
- head xs = fold_right ((lambda (y, _). y), 'err) xs
- in head
-
-25. We mentioned in the Encoding notes that `fold_left (flipped_cons, []) xs` would give us the elements of `xs` but in the reverse order. So that's how we can express `reverse` in terms of `fold_left`. How would you express `reverse` in terms of `fold_right`? As with problem 22, don't use `letrec`!
-
- See the [[hint|assignment2 hint]].
-
-
-Numbers
--------
-
-26. Given that we've agreed to Church's encoding of the numbers:
-
- 0 ≡ \f z. z
- 1 ≡ \f z. f z
- 2 ≡ \f z. f (f z)
- 3 ≡ \f z. f (f (f z))
- ...
-
- How would you express the `succ` function in the Lambda Calculus?
-
- let succ = \n. \f z. f (n f z) in ...
-
- Compare the definition of `cons`, which has an additional element:
-
- let cons = \d ds. \f z. f d (ds f z) in ...