Download Simple Transition Networks

Natural Language Understanding
• Understanding NL (infinite language) means determining
the meaning of the sentence with respect to contest in which
it is used.
• It requires an analysis of the sentence on several different
levels:
• Syntactic
• Semantic
• Pragmatic
• Discourse
Syntactic:
Syntax (grammar) of the sentence is checked.
Syntax is a tool for describing the structure of sentences in
the language.
Semantics: denotes the ‘literal’ meaning we ascribe to a
sentence.
Pragmatics: refers to intended meaning of a sentence. How
sentences are used in a different contexts and how context
affects the interpretation of the sentence.
Discourse: refers to conversation between two or more
individuals.
Basic parsing Techniques
Context free grammars
S

NP, VP
VP

verb, NP
NP

det, NP
NP

det, noun, NP
NP

det, adj*, NP
• One can have top-down or bottom-up parsers.
Simple Transition Networks
• More convenient for visualizing the grammar (CFG)
• It consists of nodes and labeled arcs.
• Network for NP.
det
NP
noun
NP1
pop
NP2
Adj
• Arcs are labeled by word category.
• Starting at a given node, we can traverse an arc if the
current word in the sentence is in the category on the arc.
• This network recognizes the same set of sentences as the
following CFG.
NP

det, NP1
NP1 
adj, NP1
NP1 
noun
• Simple Transition Network formalism is not powerful
enough to describe all languages that can be described by
CFG.
• Recursive grammar can’t be defined by Transition
Network.
Recursive Transition Networks (RTN)
• To get the descriptive power of CFG, you need a notion of
recursion in the network grammar.
• In RTN which is like a simple Transition Network except
that it allows arc labels that refer to other networks rather
than word categories.
• The network for simple English sentences can be
expressed as
NP
S
verb
S1
NP
S2
pop
S3
• Uppercase labels refers to networks.
• The arc from S to S1 can be followed only if the NP network
can be successfully traversed to a pop arc.
• RTN might allow to have an arc labeled with its own name.
• Any language generated by CFG can be generated by RTN
and vice verse. Thus they are equivalent in their generative
capacity.
Implementation of RTN in Prolog
 Vocabulary for RTN can be stored as a set of facts as:
word_type (“ an“, det ).
word_type (“ the“, det ).
word_type (“man“, noun ).
word_type (“apple “,noun ).
word_type (“ eats“, verb ).
 The top level clause for RTN is called run and is as
follows:
run :- set_state(s0), writeln(“Enter your sentence),
readln(Sent), analyze(Sent),
writeln(“Your sentence is syntactically
correct”), clear_state.
run :- writeln(“Your sentence is wrong s
yntactically”), clear_state.
set_state(S) :- assert(current_state(S)).
/* Initialize current state to s0 */
analyze (S) :- S = “.”, final_state(_).
analyze (S) :- get_state(NS), !, transition(NS, S, S1),
analyze (S1).
get_state(S) :- current_state(S).
/* transition states are listed as */
transition (s0, A, B) :- check_np(np, A, B),
set_state(s1).
transition(s1, A, B) :- get_token(A, W, B),
word_type(W, verb),
set_state(s2).
transition (s2, A, B) :- check_np(np, A, B),
set_state(s3).
transition (np, A, B) :- get_token(A, W, B),
word_type(W, det),
set_state(np1).
transition (np1, A, B) :- get_token(A, W, B),
word_type(W, noun),
set_state(np2).
transition (np1, A, B) :- get_token(A, W, B),
word_type(W, adj),
set_state(np1).
check_np(np2, A, B) :- A = B, !.
check_np(St, A, B) :- transition(St, A, C),
get_state(Ns),
check_np(Ns, C, B), !.
final_state(s3) :- current_state(s3).
/* get a token from the sentence */
get_token(A, W, B) :- ?
clear_state :- retract(current_state(_)), fail.
clear_state.
Query:
?- run
the man eats an apple.
Yes.
?- run
man eat apple.
Yes. (This is actually wrong but we have not taken care of
number agreement into account)
• These formalisms are limited in the following ways.
• They could only accept or reject a sentence rather
than producing a analysis of the structure of the
sentence.
• Augmenting RTN formalism which involves both
generalizing the network notation and introducing more
information about words in the structure, collecting
information and testing features while paring becomes
augmented transition network (ATN).
• Similar kind of augmentation & extension can be made to
CFG called Definite Clause Grammar.
Recording sentence Structure while parsing in ATN.
• Collect the structure of legal sentence in order to further
analyze them.
• For instance we can identify one particular noun phase as
the syntactic subject (SUBJ) of a sentence and another as the
syntactic object of the verb (OBJ)
• Within noun phrase we might identify the det structure,
adjective, the head noun and so on.
• Thus the sentence “Jack found a bag” might be represented
by the structure
(S (
SUBJ (NP NAME jack)
MAIN-V
found
TENSE
PAST
OBJ (NP DET a
HEAD bag
)
)
• Such a structure is created using RTN parser by allowing
each network to have a set of registers.
• Registers are local to each network. Thus each time a new
network is pushed and new set of empty registers are created.
• When the network is popped, the registers disappear.
• Registers can be set to the values and these can be retrieved
from registers.
• NP network has registers names such as, DET, ADJS, HEAD
and NUM.
• Registers are set by actions that can be specified on the arc.
• When an arc is followed, the actions associated with it are
executed.
• The most common act involves setting the register to a
certain value.
• When a pop arc is followed, all the registers set in the current
network are automatically collected to form a structure
consisting of the network followed by a list of the registers with
their values.
• When a category arc, such as name or verb etc. is
followed, the word in the input is put into a special variable
named as *.
name
S1
S2
• Thus the plausible action on the arc from S1 to S2 would
be to set NAME register to the current word.
• It is written as
NAME  *
• The push arcs such as NP must be treated differently.
• Typically many words will be used by the network called using
push arc.
• The network used in push would have set of registers that
capture the structure of the constituent that was parsed.
• The structure built by the pushed network is returned in the
value *.
NP
S
S1
• Thus the action on the arc from S to S1 might be SUBJ  *
• Therefore, a RTN with registers and tests with actions on
those registers, is an Augmented Transition network (ATN).
NP
S:
S
verb
S1
NP
S2
pop
S3
jump
det
NP:
noun
NP
NP1
adj
name
pop
NP2
Arc
NP/1
Test
none
Actions
DET  *
NUM  NUM*
NP/2
none
NAME  *
NUM  NUM*
NP1/1
NUM  NUM* 
{then action is taken
otherwise it fails }
HEAD  *
NUM  NUM  NUM*
ADJS Append (ADJS, *)
NP1/2
none
S/1
none
SUBJ  *
S1/1
NUMSUBJ  NUM* 
{then action is taken
otherwise it fails }
MAIN_V  *
NUM  NUM SUBJ NUM*
S2/1
none
OBJJ  *
Notations:
• NUM*
• NUMSUBJ
- is the NUM register of the structure in *
- is the NUB register of the structure in SUBJ
• The values of the registers are often viewed as sets and the
intersection () and union () of sets are allowed to combine
the values of different registers.
• For the registers that may take a list of values, an append
function is permitted.
• Append (ADJS,*) returns the list that is the list in the
register ADJS with the value of * appended on the end.
• The sentence, with the word positions indicated, is as
follows:
1
The 2 dogs 3 love 4 john 5.
A simple Lexicon:
Word
dogs
dog
the
Love
John
Representation
(NOUN
ROOT dog
NUM {3p} )
(NOUN
ROOT dog
NUM {3s} )
(DET
ROOT the
NUM (3s, 3p} )
(VERB
ROOT love
NUM {3p} )
(NAME
ROOT john )
Trace of S Network
Step
Node
Position
Arc followed
Registers
1
2
S
NP
1
1
S/1
NP / 1
[ DET  the,
NUM  {3s, 3p}]
3
NP1
2
NP / 1
{check {3s, 3p }  {3p}  }
4
NP2
3
NP2 / 1
5
-
3
S / 1 succeeds
S1
3
S1 / 1
{check {3p }  {3p}  }
[HEAD  dogs,
NUM  {3p}]
return structure
{NP {
DET  the
HEAD  dogs
NUM  {3p}
}
}
SUBJ 
{NP {
DET  the
HEAD  dogs
NUM  {3p}
} }
[MAIN_V  love,
NUM  {3p}]
6
7
S2
NP
4
4
8
NP2
5
9
S3
5
OBJ  *
{ NAME  john
NUM  {3p}
}
NP2 / 1
return structure
OBJ 
{NP { NAME  john
NUM  {3p}
}
}
S3 / 1
return
succeeds {S {SUBJ 
{NP { DET  the
HEAD  dogs,
NUM  {3p}
}
MAIN_V  love,
NUM  {3p}]
OBJ 
{NP { NAME  john
NUM  {3p} }
})
S2 / 1
NP / 2
Implementation of ATN in Prolog
• Database clauses will be used to store and read the registers.
run :- set_state (s0) /* initialize state */,
write (“ATN analyses your sent”), nl,
write (“Please type in your sentence”),
readln (Sent), analyse (Sent),
write (“Your sent is syntactically correct), nl ,
clear_dbase.
run :- write(“ your sent is syntactically wrong”),
clear_dbase.
clear_dbase :- retract(_), fail.
clear_dbase.
analyse (S) :- S = “.”, final_state(_).
analyse(S) :- current_state(N_state), !,
transition(N_state, S, S1), !,
analyse(S1).
/* main transistions */
transition(s0, A, B) :- get_token(A, W, B),
word_type(W, verb),
asserta(type_reg (“QUEST”)),
asserta(verb_reg(W)), set_state(s2).
transition(s0, A, B) :- check_np(np, A, B),
asserta(type_reg (“DECL”),
build_phrase(np, STR),
assert(subj_reg (STR)), set_state(s1).
/* NP transition */
check_np(np2, C, B) :- B = C, !.
check_np(np3, C, B) :- B = C, !.
check_np(St, A, B) :- transition(St, A, C),
get_state(N), check_np(N, C, B).
/* Build Phrases */
build_phrase(np, STR) :- det_reg(DET),
adj_reg(ADJ), noun_reg(NOUN),
get_template(np, T),
fill_template(T, AUX, T1),
fill_template(T1, VERB, STR).