logo_ipparis.png     TelecomParis_endossem_IPP_RVB_100pix.png Telecom Paris
Dep. Informatique & Réseaux

Dessalles_2018.png J-L. DessallesHome page

July 2021
5
CANLP.png

Cognitive Approach to Natural Language Processing (SD213)

                                other AI courses
5

Procedural semantics

Many sentences make sense out of context, or within a reasonable "average" context.

This is why machine translation seems so efficient.

La vanille est une épice constituée par le fruit de certaines orchidées lianescentes tropicales

→    Vanilla is a spice made from the fruit of certain tropical vine orchids.

However, on many occasions, interpretation requires genuine intelligence, which presupposes the ability to interpret in context.

  • Desk.png"Can you get the little red book on the right stack on my desk?"
    →    which book?
However, on many occasions, interpretation requires genuine intelligence.

    
  • Galaxy.jpgMolecule.jpg"A small galaxy" vs. "A small molecule"definitely
    not the intersection with the set of small objects

  • Trees.pngKeys.png"I lost my keys to the right of the tree."
    how far away from the tree? 1cm, 1km?
However, on many occasions, interpretation requires genuine intelligence.

    
  • Footsteps.jpg"He felt so lonely in this desert that he used to walk backwards sometimes to see footsteps in front of him."
    (after Hortence Vlou)
    whose footsteps are we talking about?
Procedural semantics computes meaning in context.

One crude way to achieve meaning construction is to translate phrases into Prolog predicates,
and then to execute the resulting Prolog program using a knowledge base.
Background knowledge
book('Kritik der reinen Vernunft’, yellow, left)
book('Des intelligences TRES artificielles’, white, right)
book('On the origin of species’, beige, right)
book('La serpe d\’or’, white, left)
book('The geometry of meaning’, white, right)
book('Metaphors we live by’, red, right)

For example:
"the little red book on the right stack on my desk"




?- book(T, red, right).
    T = 'Metaphors we live by'.
This Prolog-based solution is great, as it achieves interpretation in context.

However, it only works if we have convenient predicates that match words.
The Prolog-based solution won’t work when

  • the evaluation of predicates isn’t purely logical, but requires analog computation
    (e.g. "small", or "to the right of the tree")
  • undertanding requires visualizing the scene
    ("walk backwards to see footsteps")
  • interpretation depends on semantic constraints
    (binding, aspect...)
The present lab deals with the Prolog-based interpretation.

    
Echecs.jpg We will be able to interpret     
"the pawn to the right of the white knight"
as meaning:
pawn(black,(2,5))
(the black pawn at location (2,5))
In subsequent lectures and labs,
we will consider

  • the contrast operation and
  • the rules of Aspect.


Introduction to semantics









The goal of the present lab work is to explore various ways to introduce semantic calculus on top of syntactic processing. In particular, we will illustrate the idea that semantic interpretation involves more than structural translation and requires some context-dependent calculus, according to what is called procedural semantics. For instance, we would like that a sentence like:

the child talks with the nice girl about the room

be interpreted, in an appropriate context, as:


talk(John, Ann, my_room)

In what follows, we will consider the use of semantic features, and then the possibility of interfacing syntax with reasoning and perception.

Features structures

The grammar used here is built around DCG rules. If you are not yet familiar with DCG rules, go back to the lab work on basic syntactic parsing before going any further. We will make use of feature structures as arguments of phrases. But we need more. We will build a semantic representation that can be interpreted as a prolog predicate. For instance, "Mary thinks of John" will be interpreted as think(Mary, John). To do so, we introdue a second slot in phrases, beside features, to host the predicate structure.

Lexical entries will be DCG rules like the following one, which is an amplification of v --> [believes]:


v([gloss:believe,num:sing,pers:3,subj:dp(_), cpl:[cp(that)]], believe(_,_), v(believe)) --> [believes].

Note the three extra arguments in green, red and blue. These three extra arguments also appear in rules.
Here is the rule vp --> v, dp:


vp(FV,PVP,TVP) --> v(FV,PV,TV), dp(FDP,PDP,TDP),
    { % transitive verb, eg. 'like'
        att(FV, cpl, [dp(FDP)]),
        link(_, PV,PDP,PVP),
        TVP = vp(TV,TDP) }.

The above rule says that a verb phrase may consist of a verb followed by a determiner phrase: likes the boy.
The three extra arguments represent structures:
The part within {brackets} is Prolog code.
- att is used to get the value of a feature (here cpl = complement) in the feature structure.
- link is used to perform semantic link between syntactically connected phrases. When two phrases are connected, as in the daughter of the child, then the corresponding predicates daughter(X, Y) and child(Z) must share a variable. This is what link achieves. Its first argument is used to control the place of the shared argument in the first predicate.
For instance, link(2, daughter(X,Y), child(Z)) will impose Y = Z.

Note the presence of the determiner phrase dp, which linguists recommend to use instead of np (np is now used to represent undetermined noun phrases, like nice girl, whereas a nice girl becomes a dp).

The files used during this lab work are in tp_sem.zip: Install these four files in a directory. Execute sem_main.pl and type the predicate go.


?- consult(sem_main).
true.    
?- go.
sentence > the girl likes the boy

[gloss:like,num:sing,pers:3,subj:dp([gloss:girl,num:sing]),cpl:[dp([gloss:boy,num:sing])]]
      s
       |__dp
       |  |__det : the
       |  |__np
       |     |__n : girl
       |__vp
          |__v : like
          |__dp
             |__det : the
             |__np
                |__n : boy    

Our grammar accepts the two following sentences: In order to discriminate between these two sentences, one may wish to introduce a "semantic agreement" between the subject and the verb.
To do this, introduce a new attribute, anim, which may take two values: animate and inanimate. You may change the lexical entries of sleep, child and room, and the rule s --> dp, vp. to check for semantic agreement, so that the child sleeps be still accepted, while *the room sleeps become incorrect. Since other words in the lexicon may not have the attribute anim in their structure, the predicate att returns a variable when the attribute is absent. As a consequence, the game sleeps will still be accepted.
Semantic Agreement (0)
Copy the new definitions of child and sleep here.

    

Semantic Agreement (1)
Copy the new rule s --> dp, vp..

    

Note that it would be better if sleep assigned the attribute value animate to its subject, but this would require recursive attributes (see advanced lab work).
Semantic Agreement (2)
What kind of attributes do you think could be used to discard nonsense sentences like:
  • *the cat talks with the room about the cat
  • *the room is in Mary
What can you say about your solution?

    

Interfacing NL with Prolog

DreamBigGirl.png Let’s say that in our context, The nice girl refers to Ann. To get this interpretation, some inference steps might be needed between words and meaning. Prolog seeems ideally suited to bridge the gap between natural language expression and meaning. The idea consists in representing the meaning of parsed sentences as Prolog programs that may be executed. For instance, we expect the interpretation of the boy dreams of the nice girl to be boy(X), girl(Y), nice(Y), dream(X, Y) which, once executed, might end up as dream(john, ann) if the programme is able to understand that the nice girl stands for ann.

As it stands, the program provides a minimal semantic interpretation. the boy dreams of the nice girl returns dream(_G9853,_G9854). To get a better result, comment the catch-all line that comes first in the definition of link in sem_main.pl. This will allow the clause coming next to it to be executed and so the semantic linking to operate. Reload the program and run it. You should get the following outputs in turn:


dream(girl,boy)
dream(boy,girl)

(note that you may disable the tree display by commenting a line in dcg_parse)

This is better, but it is not yet the kind of interpretation we want.
To fix this problem, we have to restrict argument sharing during semantic linking.
Look into sem_grammar.pl for calls to link.
Instantiate in each case the first argument of link to indicate which argument of the first predicate should be shared.
(1 means that the first argument is shared, 2 that the second argument is shared, and so on).
If you do it correctly, the sentence the boy dreams of the nice girl will now receive only one interpretation.     
Semantic Linking
Copy the new rules s --> dp, vp. and vp --> v, dp. here.

    

Ditransitive Verbs
Add a rule in sem_grammar.pl for verbs that take two prepositional complements.
For instance, Ann talks with Pat about Mary should be successfully analyzed.
Copy the rule below.

    

Procedural semantics

To go one step further in the interpretation, the program should understand that the nice girl means Ann. To do this, we have to say to Prolog that Ann is a girl and that she is nice.

Open sem_world.pl and replace the existing lines by this knowledge:


girl('Ann').
boy('John').
nice('Ann').

Now we execute our interpreter again.


?- go.
Sentence > the boy dreams of the nice girl
[gloss:dream,num:sing,pers:3,subj:dp([gloss:boy,num:sing]),cpl:[pp(of)]]
--> dream(John,Ann)
this sentence makes sense

Now, the program has understood who we were talking about by saying the nice girl. Not bad!

LexiconInterface.png Note that lexical entries are now duplicated, as a word such as ‘girl’ now appears both in the grammar and in the ‘world’ definition. We may think of phonetic forms such as girl or nice as being merely associated both with concepts in the "world" and with syntactic categories in the grammar.

Uncomment the "execute" line in dcg_parse to trigger the interpretion of the final output predicate. Add the line:
dream(_,_) :- writeln('\n***** Great! *****\n').
to the world and try again the sentence "the boy dreams of the nice girl".
Procedural Semantics
Augment the world so that the child talks with the nice girl about the room may be interpreted as:

talk(John, Ann, my_room)
talk(Pat, Ann, my_room)

Insert the new clauses below.

    

Procedural Semantics (2)
Introduce a new lexical entry daughter in the grammar and augment the world so that the child talks with the daughter of the nice girl about the room may be interpreted as:

talk(John, Lisa, my_room)

Insert the lexical entry and the new clauses below.

    

Suggestion

You may note that as it is implemented, the system is underconstrained. The semantic linking doesn’t know which argument to choose when linking talk('John',Y) with daughter('Ann', 'Lisa').
The problem comes from the fact that Prolog’s execution of daughter('Ann', X) produces an instantiated predicate daughter('Ann', 'Lisa') instead of merely 'Lisa'.
You may change the definition of ‘link’ to make it closer to a ‘merge’, that is, the output of daughter('Ann', X) should be ‘Lisa’ rather than an instantiated predicate.

Semantic contraints

The current program accepts nonsense sentences like the room talks with the child about Mary. One solution to avoid this would be to introduce semantic features such as ‘animate’ to constrain matching. This solution is wrong for two reasons: The solution adopted by procedural semantics is to transfer the ‘animate’ constraint to the world.
World Constraints
Define animate by adding Prolog clauses to the world saying that an entity is animate if it is a boy or a girl.
Then add a predicate talk to the world that checks whether the two first arguments comply with the constraint of being animate.

Insert the new clauses below. Does the program now make a difference between:
- the girl talks with the child about Mary
- the room talks with the child about Mary ?

    

It seems that we merely replaced static syntactic features by static clauses in a Prolog program. The point of procedural semantics is to separate what belongs to the grammar (e.g. agreement in number) from what belongs to semantics. The latter depends on the knowledge you have (e.g. that Ann is nice while Mary is not) or even on perception, as when you ask for the book which is on top of the right stack. It is normal to consider that this kind of processing should be performed by programs, possibly sophisticated ones, rather than by the grammar.

Interfacing NL with perception

Echecs.jpg Since we established a connection between NL and Prolog, we can deal with spatial relations, up to a point. Load the Chess.pl program by adding the following line to sem_world.pl:


:- consult('Chess.pl').

This program contains the location of chess pieces on the chessboard pictured on the right. Normally, your program should be able to interpret the following phrases:


?- go(dp).
Phrase > the white knight
...> knight(white,(1,5))
|:
...> knight(white,(4,5))


?- go.
Sentence > the white knight is white
...> white(white)
|:
...> white(white)

but it should refuse the follwing sentence:


?- go.
Sentence > the black knight is white
false.

Now add a definition for "right" in your grammar:


n([gloss:right, num:sing],right(_,_),n(right)) --> [right].

and then look into Chess.pl and correct the implementation of the predicate right. You should get this kind of result:


?-go(pp).
Phrase > to the right of the white knight
...> right((2,5),(1,5))
|:
...> right((5,5),(4,5))


?-go(dp).
Phrase > the pawn to the right of the white knight
...> pawn(black,(2,5))
Chess
Paste the modified clause of right.

    

A word of conclusion

Procedural semantics can solve many problems that feature semantics is unable to achieve.
We must remain aware of a strong limitation, however. Syntax-semantics interfaces like the one illustrated in this lab work rely on the fact that words can be matched to predicates. For instance, the lexical entry believes is matched to the predicate belive. This presupposes that part of the interpretation relies on a translation that uses a mental dictionary. The translation would merely associate words to predicates. This solution is better than nothing, but it is utterly simplistic from a cognitive point of view. A more realistic solution would consist in building predicates ‘on the fly’. This will be explained during the lectures.

Webography


    


Line.jpg

Back to main page