1. LING 581: Advanced Computational Linguistics
Lecture Notes
April 19th

2. Semantic Grammars Use slides from course
LING 324 – Introduction to Semantics
Simon Frasier University, Prof. F.J. Pelletier
we’re computational linguists, so we’re going to implement the slides

3. Syntax
fjpSlides4.pdf
Slide 3

4. Syntax We already know how to build Prolog grammars
See executive summary
prolog.org/pldoc/doc_for?object=section(2,'4.12',swi('/doc/Manual/DCG.html'))

5. Syntax
Step 1: let’s build the simplest possible Prolog grammar for this
fjpSlides4.pdf
Slide 4

6. Simplest possible grammar
psg1.pl
Excluding (2b) from previous
slide for the time being …
Examples (3), (4) and (5) from two slides back

7. Syntax Step 2: let’s add the parse tree component to our grammar …
Recall: grammar rules can have extra arguments:
Parse tree
Implement agreement etc.

8. Syntax Note: on handling left recursion in Prolog grammar rules
techniques:
use a bottom-up parser
rewrite grammar (left recursive -> right recursive – last semester)
or use lookahead (today’s lecture)
lookahead is a dummy nonterminal that does not contribute to the parse, it is a “guard” that prevents rule from firing unless appropriate
lookahead succeeds if it can find a conjunction in the input and marks it (so it can’t find it twice)

9. Grammar: version 2
psg2.pl

10. Grammar: version 2

11. Grammar: version 2
Examples (3), (4) and (5) again from slide 9

12. Grammar: version 2
Examples (6) and (7) from slide 9
Just syntax, but we'll need to produce a semantic parse (instead)

13. Semantics
We want to obtain a semantic parse for our sentences that we can “run” (i.e. evaluate) against the Prolog database (i.e. situation or possible world).
So the semantic parse output should be valid Prolog code (that we can directly call)
Prolog language support we'll need:
call(Goal) Goal is any query we type at the command line
member(X,List)
setof(V,Goal,Set) Set is implemented as a list

14. setof/3 See Prolog built-in:
prolog.org/pldoc/doc_for?object=section(2,'4.29',swi('/doc/Manual/allsolutions.html'))
Prolog built-in:

15. setof/3
Example:

16. member/2
See

17. Semantics
fjpSlides4.pdf
Slide 7

18. Semantics
fjpSlides4.pdf
Slide 8

19. Semantics
fjpSlides4.pdf
Slide 9

20. Semantics
fjpSlides4.pdf
Slide 10

21. Semantics: Implementation
Desired implementation:
The extra argument returns a Prolog query X that can be evaluated against the current database (situation)
Note: we are bypassing the (explicit) construction of the syntax tree
Imagine if the Penn Treebank was labeled using a semantic representation

22. Semantics: Implementation
Let’s write the semantic grammar to handle “Jack is hungry”
first, let’s introduce a bit of notation (lambda calculus)
λ = function
λx.x+1 denotes a function that takes an argument x and computes value x+1
(a period separates the argument from the function body)
(λx.x+1)(5) means apply 5 to the lambda function
substitute 5 in place of x and evaluate
answer = 6

23. Semantics: Implementation
Syntax:
setof(X,hungry(X),S), member(jack,S)
jack
setof(X,hungry(X),S)

24. Semantics: Implementation
Semantic grammar:

25. Semantics: Implementation
Semantic grammar:
Two possible worlds

26. Semantics: Implementation
More examples of computation:
fjpSlides4.pdf
Slide 10

27. Semantics: Implementation
More examples of computation:

28. Semantics: Implementation
More examples of computation:

29. Semantics
fjpSlides4.pdf
Slide 11

30. Semantics: Implementation
Scope of negation: wide or narrow
narrow
wide

31. Grammar: revised
sg2.pl