X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=exercises%2Fassignment3.mdwn;h=5bee7a43484f71cbf42f3016132caafac4489920;hp=a6d317303b72c6b04fecf444794b15117e7b5b3b;hb=cff9857e52ac59c4905efeb80fcf8a91c2fd1ed8;hpb=b55563aa31950b6d5c27f1baf90338d1339ff5ee diff --git a/exercises/assignment3.mdwn b/exercises/assignment3.mdwn index a6d31730..5bee7a43 100644 --- a/exercises/assignment3.mdwn +++ b/exercises/assignment3.mdwn @@ -1,25 +1,22 @@ -** *Work In Progress* ** - ## Lists and List Comprehensions -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? +1. In Kapulet, what does `[ [x, 2*x] | x from [1, 2, 3] ]` evaluate 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 hint|assignment3 hint1]], if you need it. +2. What does `[ 10*x + y | y from [4], x from [1, 2, 3] ]` evalaute 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`.) +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 hint|assignment3 hint1]], if you need it. -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? +4. Last week you defined `head` in terms of `fold_right`. Your solution should be straightforwardly translatable into one that uses our proposed right-fold 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`.) -6. Continuing to encode lists in terms of their left-folds, what should `last` be, where `last [a, b, c]` should result in `c`. Let `last []` result in whatever `err` is bound to. +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? -7. Continuing to encode lists in terms of their left-folds, how should we write `head`? This is challenging. [[Here is a solution|assignment3 hint2]], if you need help. +6. Continuing to encode lists in terms of their left-folds, what should `last` be, where `last [a, b, c]` should result in `c`. Let `last []` result in whatever `err` is bound to. -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 built-in 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 Church-encoded numbers. [[Here is a hint|assignment3 hint3]], if you need it. +7. Continuing to encode lists in terms of their left-folds, how should we write `head`? This is challenging. [[Here is a solution|assignment3 hint2]], if you need help. -(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 the strategy discussed this week for defining `tail`. Purely extra credit.) +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 built-in 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 Church-encoded numbers. [[Here is a hint|assignment3 hint3]], if you need it. + (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 @@ -28,6 +25,10 @@ 10. In last week's homework, you gave a Lambda Calculus definition of `succ` for Church-encoded 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/lambda-calc.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 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.) + + + 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)`.