# News for UCLA Computer Science 131, winter 2019

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Here are news items that affect the core assignments and class material.

2019-02-24
• The last test case in Homework 5 has been corrected as follows, where deleted text is marked like this and inserted text is marked like this:
```(expr-compare '((lambda (a) (eq? a ((λ (a b) (let ((a b) (b a)) (a b)))
a (lambda (a) a))))
(λlambda (b a) (b a)))
'((λ (a) (eqv? a ((lambda (b a) (let ((a b) (b a)) (a b)))
a (λ (b) a))))
(lambda (a b) (a b))))
⇒ ((λ (a)
((if % eq? eqv?)
a
((λ (a!b b!a) (let ((a (if % b!a a!b)) (b (if % a!b b!a))) (a b)))
a (λ (a!b) (if % a!b a)))))
(lambda (b!a a!b) (b!a a!b)))
```
(Thanks to Micah Kim for spotting the problems.)
2019-02-05
• Homework 3 is available and is due a week from today.
2019-01-25
• Homework 2's test6 has been corrected by replacing Node (Term, [Node (Num, [Leaf "2"])]) with Node (Expr, [Node (Term, [Node (Num, [Leaf "2"])])]). (Thanks to Micah Kim.)
2019-01-22
• Homework 2 now has three more little test cases test5, test6 and test7.
2019-01-19
• A typo Nuam has been corrected to Num in Homework 2.
2019-01-18
• As a result of today's discussions, Homework 2 has been corrected as follows.
1. In the sample session just before the user types '#use "hw2sample.ml";;', the type that make_parser eventually returns has been corrected from ('a, 'b) parse_tree to ('a, 'b) parse_tree option.
2. For correctness some text was inserted in the following: "This test should check that parse_tree_leaves is in some sense the inverse function of make_parser gram, in that when make_parser gram frag returns Some tree, then parse_tree_leaves tree equals frag."
3. For clarity some text was inserted and some deleted in the following: "When applied to an acceptor accept and a fragment frag, the matcher must return the first acceptable match of a prefix of frag, by trying the grammar rules in order; try the grammar rules in order and return the result of calling accept on the suffix corresponding to the first acceptable matching prefix of frag; this is not necessarily the shortest nor the longest acceptable match."
• Homework 2 is available.
2019-01-08
• Homework 1 is available.
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