XGitUrl: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=exercises%2Fassignment3.mdwn;h=7121faee69e9b388627e33f971b4f0d2b8ee299d;hp=95450217af616d66cbef151d142bae4efef359fb;hb=a68ef218fe101b9d4e7aec684d93bc8391b971a9;hpb=ee79965dbd07b39cd3ab7d489ba87b8cc5e2b68b
diff git a/exercises/assignment3.mdwn b/exercises/assignment3.mdwn
index 95450217..7121faee 100644
 a/exercises/assignment3.mdwn
+++ b/exercises/assignment3.mdwn
@@ 1,30 +1,35 @@
** *Work In Progress* **
+## Lists and List Comprehensions
## Comprehensions
+1. In Kapulet, what does `[ [x, 2*x]  x from [1, 2, 3] ]` evaluate to?
1. In Kapulet, what does `[ [x, 2*x]  x from [1, 2, 3] ]` evaluate to?
+2. What does `[ 10*x + y  y from [4], x from [1, 2, 3] ]` evalaute to?
2. What does `[ 10*x + y  y from [4], x from [1, 2, 3] ]` evalaute to?
+3. Using either Kapulet's or Haskell's list comprehension syntax, write an expression that transforms `[3, 1, 0, 2]` into `[3, 3, 3, 1, 2, 2]`. [[Here is a hintassignment3 hint1]], if you need it.
3. Using either Kapulet's or Haskell's list comprehension syntax, write an expression that transforms `[3, 1, 0, 2]` into `[3, 3, 3, 1, 2, 2]`. [[Here is a hintassignment3 hint1]], if you need it.
+4. Last week you defined `head` in terms of `fold_right`. Your solution should be straightforwardly translatable into one that uses our proposed rightfold encoding of lists in the Lambda Calculus. Now define `empty?` in the Lambda Calculus. (It should require only small modifications to your solution for `head`.)
+5. If we encode lists in terms of their *left*folds, instead, `[a, b, c]` would be encoded as `\f z. f (f (f z a) b) c`. The empty list `[]` would still be encoded as `\f z. z`. What should `cons` be, for this encoding?
## Lists
+6. Continuing to encode lists in terms of their leftfolds, what should `last` be, where `last [a, b, c]` should result in `c`. Let `last []` result in whatever `err` is bound to.
4. Last week you defined `head` in the Lambda Calculus, using our proposed encoding for lists. Now define `empty?` (It should require only small modifications to your solution for `head`.)
+7. Continuing to encode lists in terms of their leftfolds, how should we write `head`? This is challenging. [[Here is a solutionassignment3 hint2]], if you need help.
5. If we encode lists in terms of their *left*folds, instead, `[a, b, c]` would be encoded as `\f z. f (f (f z a) b) c`. The empty list `[]` would still be encoded as `\f z. z`. What should `cons` be, for this encoding?
+8. Suppose you have two lists of integers, `left` and `right`. You want to determine whether those lists are equal, that is, whether they have all the same members in the same order. How would you implement such a list comparison? You can write it in Scheme or Kapulet using `letrec`, or if you want more of a challenge, in the Lambda Calculus using your preferred encoding for lists. If you write it in Scheme, don't rely on applying the builtin comparison operator `equal?` to the lists themselves. (Nor on the operator `eqv?`, which might not do what you expect.) You can however rely on the comparison operator `=` which accepts only number arguments. If you write it in the Lambda Calculus, you can use your implementation of `leq`, requested below, to write an equality operator for Churchencoded numbers. [[Here is a hintassignment3 hint3]], if you need it.
6. Continuing to encode lists in terms of their leftfolds, what should `last` be, where `last [a, b, c]` should result in `c`. Let `last []` result in whatever `err` is bound to.

7. Continuing to encode lists in terms of their leftfolds, how should we write `head`? This is challenging. [[Here is a solutionassignment3 hint2]], if you need help.
+ (The function you're trying to define here is like `eqlist?` in Chapter 5 of *The Little Schemer*, though you are only concerned with lists of numbers, whereas the function from *The Little Schemer* also works on lists containing symbolic atoms  and in the final version from that Chapter, also on lists that contain other, embedded lists.)
## Numbers
8. Recall our proposed encoding for the numbers, called "Church's encoding". As we explained last week, it's similar to our proposed encoding of lists in terms of their folds. In last week's homework, you defined `succ` for numbers so encoded. Can you now define `pred` in the Lambca Calculus? Let `pred 0` result in whatever `err` is bound to. This is challenging. For some time theorists weren't sure it could be done. (Here is [some interesting commentary](http://okmij.org/ftp/Computation/lambdacalc.html#predecessor).) However, in this week's notes we examined one strategy for defining `tail` for our chosen encodings of lists, and given the similarities we explained between lists and numbers, perhaps that will give you some guidance in defining `pred` for numbers.
+9. Recall our proposed encoding for the numbers, called "Church's encoding". As we explained last week, it's similar to our proposed encoding of lists in terms of their folds. Give a Lambda Calculus definition of `zero?` for numbers so encoded. (It should require only small modifications to your solution for `empty?`, above.)
+
+10. In last week's homework, you gave a Lambda Calculus definition of `succ` for Churchencoded numbers. Can you now define `pred`? Let `pred 0` result in whatever `err` is bound to. This is challenging. For some time theorists weren't sure it could be done. (Here is [some interesting commentary](http://okmij.org/ftp/Computation/lambdacalc.html#predecessor).) However, in this week's notes we examined one strategy for defining `tail` for our chosen encodings of lists, and given the similarities we explained between lists and numbers, perhaps that will give you some guidance in defining `pred` for numbers.
+
+ (Want a further challenge? Define `map2` in the Lambda Calculus, using our rightfold 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.)
+
9. Define `leq` for numbers (that is, ≤) in the Lambda Calculus. Here is the expected behavior,
+
+11. Define `leq` for numbers (that is, ≤) in the Lambda Calculus. Here is the expected behavior,
where `one` abbreviates `succ zero`, and `two` abbreviates `succ (succ zero)`.
leq zero zero ~~> true
@@ 44,46 +49,55 @@ where `one` abbreviates `succ zero`, and `two` abbreviates `succ (succ zero)`.
Reduce the following forms, if possible:
10. `Kxy`
11. `KKxy`
12. `KKKxy`
13. `SKKxy`
14. `SIII`
15. `SII(SII)`
+
+Kxy
+KKxy
+KKKxy
+SKKxy
+SIII
+SII(SII)
+
16. Give Combinatory Logic combinators (that is, expressed in terms of `S`, `K`, and `I`) that behave like our boolean functions. You'll need combinators for `true`, `false`, `neg`, `and`, `or`, and `xor`.
+18. Give Combinatory Logic combinators (that is, expressed in terms of `S`, `K`, and `I`) that behave like our boolean functions. Provide combinators for `true`, `false`, `neg`, `and`, `or`, and `xor`.
Using the mapping specified in this week's notes, translate the following lambda terms into combinatory logic:
17. `\x x`
18. `\x y. x`
19. `\x y. y`
20. `\x y. y x`
21. `\x. x x`
22. `\x y z. x (y z)`
+
+\x x
+\x y. x
+\x y. y
+\x y. y x
+\x. x x
+\x y z. x (y z)
+
23. 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.
+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
+
Evaluation strategies in Combinatory Logic

23. Find a term in CL that behaves like Omega does in the Lambda
Calculus. Call it Skomega.
+26. Find a term in CL that behaves like Omega does in the Lambda
+Calculus. Call it `Skomega`.
24. Are there evaluation strategies in CL corresponding to leftmost
reduction and rightmost reduction in the lambda calculus?
+27. Are there evaluation strategies in CL corresponding to leftmost
+reduction and rightmost reduction in the Lambda Calculus?
What counts as a redex in CL?
25. Consider the CL term K I Skomega. Does leftmost (alternatively,
rightmost) evaluation give results similar to the behavior of K I
Omega in the lambda calculus, or different? What features of the
lambda calculus and CL determine this answer?
+28. Consider the CL term `K I Skomega`. Does leftmost (alternatively,
+rightmost) evaluation give results similar to the behavior of `K I
+Omega` in the Lambda Calculus, or different? What features of the
+Lambda Calculus and CL determine this answer?
26. What should count as a thunk in CL? What is the equivalent
+29. What should count as a thunk in CL? What is the equivalent
constraint in CL to forbidding evaluation inside of a lambda abstract?
@@ 92,15 +106,15 @@ More Lambda Practice
Reduce to betanormal forms:

 `(\x. x (\y. y x)) (v w)`

 `(\x. x (\x. y x)) (v w)`

 `(\x. x (\y. y x)) (v x)`

 `(\x. x (\y. y x)) (v y)`
+
+(\x. x (\y. y x)) (v w)
+(\x. x (\x. y x)) (v w)
+(\x. x (\y. y x)) (v x)
+(\x. x (\y. y x)) (v y)
 `(\x y. x y y) u v`

 `(\x y. y x) (u v) z w`

 `(\x y. x) (\u u)`

 `(\x y z. x z (y z)) (\u v. u)`
+
(\x y. x y y) u v
+(\x y. y x) (u v) z w
+(\x y. x) (\u u)
+(\x y z. x z (y z)) (\u v. u)