1. LING/C SC/PSYC 438/538
Lecture 26
Sandiway Fong

2. Administrivia
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5. 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
rule for nonterminal a
immediately calls a

6. 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

7. 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

8. 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)
sentential forms: vp
vp pp
vp pp pp
vp pp pp pp
vp pp pp pp pp infinite loop…
Note: other extra arguments
not shown here …

9. 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).
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].
Note:
w is a fresh
non-terminal
that takes 2
arguments
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 a grammar!