1. LING/C SC/PSYC 438/538
Lecture 24
Sandiway Fong

2. Today's Topics Homework 11 Review
(Reminder: Homework 12 due tomorrow night)
Agreement example
Dealing with left recursion by grammar transformation

3. Homework 11 Review
Question 1: write a Prolog CFG for the following sentences:
John ate (sensibly) (intransitive eat)
I fish (intransitive fish)
I ate fish (transitive eat)
Bill ate rice
Harry ate roast beef
Note: you can use lowercase names… (or quotes, e.g. 'John')
Note: use Penn Treebank tagset for words (see inside the cover of your textbook)
nnp(prp(i)) --> [i].
nnp(nnp(john)) --> [john].
vbd(vbd(ate)) --> [ate].

4. Homework 11 Review
Question 2: expand your grammar to handle these sentences:
I ate fish, and Bill ate rice
*I ate fish, Bill ate rice
I ate fish, Bill ate rice, and Harry ate roast beef
Note: the comma can be a quoted terminal, e.g. [',']
comma(comma(',')) --> [','].
','(','(',')) --> [','].
Note: be careful of left recursion on S
List: (i) S; (ii) S, and S; (iii) S, S, and S
(Stanford Parser)

5. Subject Verb Agreement
We need feature percolation:
Form Ending Comment
eat uninflected not 3rd person singular
eats -s 3rd person singular
ate -ed past
eaten -en past participle
eating -ing gerund
Subject and VP come together at this rule
Person
Number
Ending
POS tags
eats
*eat
Person
Number
Ending

6. Subject Verb Agreement
Implementation: using POS tags
Constraint table:
% table of Person Number Tag possible combinations
table(3,pl,vb).
table(3,pl,vbd).
table(3,sg,vbz).
table(3,sg,vbd).
Person, Number
from Subject NP
POS tag
from verb

7. Left recursion and Prolog
Left recursive grammars:
we know from an earlier lecture that left recursive rules are a no-no given Prolog’s left-to-right depth- first computation rule… s a
...
Example:
s --> a, [!].
a --> ba, [a].
a --> a, [a].
ba --> b, [a].
b --> [b].
?- s([b,a,!],[]).
ERROR: Out of local stack

8. Preposition Phrase (PP) Attachment
The preferred syntactic analysis is a left recursive parse
Examples:
John saw the boy with a telescope
(structural ambiguity: automatically handled by Prolog)
withinstrument
withpossessive

9. Preposition Phrase (PP) Attachment
The preferred syntactic analysis is a left recursive parse
Can “stack” the PPs:
John saw the boy with a limp with Mary with a telescope
ambiguity: withpossessive , withaccompaniment, withinstrument

10. Preposition Phrase Attachment
Linguistically:
PP (recursively) adjoins to NP or VP
np(np(NP,PP)) --> np(NP), pp(PP).
vp(vp(VP,PP)) --> vp(VP), pp(PP).
Left recursion gives Prolog problems
Derivation (top-down, left-to-right): vp
vp pp
vp pp pp
vp pp pp pp
vp pp pp pp pp infinite loop…
Note: other extra arguments
not shown here …

11. Transformation Apply the general transformation: to NP and VP rules:
np(np(DT,NN)) --> dt(DT,Number), nn(NN,Number).
np(np(NP,PP)) --> np(NP), pp(PP).
vp(vp(VBD,NP)) --> vbd(VBD), np(NP).
vp(vp(VP,PP)) --> vp(VP), pp(PP).
Note:
w is a fresh
non-terminal
that takes 2
arguments
x(X) --> [z], w(X,x(z)).
x(x(z)) --> [z].
w(W,X) --> [y], w(W,x(X,y)).
w(x(X,y),X) --> [y].
x(x(X,y)) --> x(X), [y].
x(x(z)) --> [z].
[z]
[y] x x
x is the recursive nonterminal
[z]
[y] x x
Let's write some code!