4 Erratum corrected 11PM Sun 3 Oct: the following line
6 let tb = (make_list t12 (make_list t3 empty)) in
10 let tb = (make_list t12 t3) in
12 This has been corrected below, and in the preloaded evaluator for
13 working on assignment 3, available here: [[assignment 3 evaluator]].
17 Once again, the lambda evaluator will make working through this
18 assignment much faster and more secure.
20 #Writing recursive functions on version 1 style lists#
22 Recall that version 1 style lists are constructed like this (see
23 [[lists and numbers]]):
27 let false = \x y. y in
28 let and = \l r. l (r true false) false in
30 let make_pair = \f s g. g f s in
32 let get_snd = false in
33 let empty = make_pair true junk in
34 let isempty = \x. x get_fst in
35 let make_list = \h t. make_pair false (make_pair h t) in
36 let head = \l. isempty l err (l get_snd get_fst) in
37 let tail = \l. isempty l err (l get_snd get_snd) in
39 ; a list of numbers to experiment on
40 let mylist = make_list 1 (make_list 2 (make_list 3 empty)) in
43 let iszero = \n. n (\x. false) true in
44 let succ = \n s z. s (n s z) in
45 let add = \l r. l succ r in
46 let mul = \m n s. m (n s) in
47 let pred = (\shift n. n shift (make\_pair 0 0) get\_snd) (\p. p (\x y. make\_pair (succ x) x)) in
48 let leq = \m n. iszero(n pred m) in
49 let eq = \m n. and (leq m n)(leq n m) in
51 ; a fixed-point combinator for defining recursive functions
52 let Y = \f. (\h. f (h h)) (\h. f (h h)) in
53 let length = Y (\length l. isempty l 0 (succ (length (tail l)))) in
54 let fold = Y (\f l g z. isempty l z (g (head l)(f (tail l) g z))) in
59 Then `length mylist` evaluates to 3.
61 1. What does `head (tail (tail mylist))` evaluate to?
63 2. Using the `length` function as a model, and using the predecessor
64 function, write a function that computes factorials. (Recall that n!,
65 the factorial of n, is n times the factorial of n-1.)
67 Warning: it takes a long time for my browser to compute factorials larger than 4!
69 3. (Easy) Write a function `equal_length` that returns true just in case
70 two lists have the same length. That is,
72 equal_length mylist (make_list junk (make_list junk (make_list junk empty))) ~~> true
74 equal_length mylist (make_list junk (make_list junk empty))) ~~> false
77 4. (Still easy) Now write the same function, but don't use the length
80 5. In assignment 2, we discovered that version 3-type lists (the ones
82 work like Church numerals) made it much easier to define operations
83 like `map` and `filter`. But now that we have recursion in our
85 reasonable map and filter functions for version 1 lists are within our
86 reach. Give definitions for `map` and a `filter` for verson 1 type
89 #Computing with trees#
91 Linguists analyze natural language expressions into trees.
93 We'll need trees in future weeks, and tree structures provide good
94 opportunities for learning how to write recursive functions.
95 Making use of the resources we have at the moment, we can approximate
96 trees as follows: instead of words, we'll use Church numerals.
97 Then a tree will be a (version 1 type) list in which each element is
98 itself a tree. For simplicity, we'll adopt the convention that
99 a tree of length 1 must contain a number as its only element.
101 Then we have the following representations:
110 [[1];[2];[3]] [[[1];[2]];[3]] [[1];[[2];[3]]]
113 Limitations of this scheme include the following: there is no easy way
114 to label a constituent with a syntactic category (S or NP or VP,
115 etc.), and there is no way to represent a tree in which a mother has a
118 When processing a tree, you can test for whether the tree contains
119 only a numeral (in which case the tree is leaf node) by testing for
120 whether the length of the list is less than or equal to 1. This will
121 be your base case for your recursive functions that operate on these
125 <LI>Write a function that sums the values at the leaves in a tree.
129 let t1 = (make_list 1 empty) in
130 let t2 = (make_list 2 empty) in
131 let t3 = (make_list 3 empty) in
132 let t12 = (make_list t1 (make_list t2 empty)) in
133 let t23 = (make_list t2 (make_list t3 empty)) in
134 let ta = (make_list t1 t23) in
135 let tb = (make_list t12 (make_list t3 empty)) in
136 let tc = (make_list t1 (make_list t23 empty)) in
148 <LI>Write a function that counts the number of leaves.