ITCS 6010
GSL Grammars
What is a Grammar?

Specifies what can be said—all the possible
sentences and phrases that can be recognized
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Developer’s goal to:
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Predict set of phrases
Encode phrase set
Non-procedural
Written in Grammar Specification Language
(GSL)
Writing a Good Grammar

Broad coverage

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People express themselves in a variety of
ways
Recognizer cannot recognize anything not in
the grammar
But not too broad

Recognition accuracy can be adversely
affected
Writing Grammars
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Grammar writing is an iterative process
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Make best guess
Collect data
Update grammar
Out-of-grammar (OOG)

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When user’s phrase cannot be parsed
5 – 10 % OOG rate acceptable
Writing Grammars (cont’d)

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Focus on most common ways users will
respond to question/prompt
DON’T attempt to figure out ALL possible
responses – wasted effort
Two most common response types:

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Information item
Literal response to question wording
Writing Grammars (cont’d)
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Examples:
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Question: What is the departure city?
Common responses:
 Charlotte
 My departure city is Charlotte
 Departing from Charlotte
Question: What city would you like?
Common responses:
 Charlotte
 I’d like Charlotte
 Departing from Charlotte
Writing Grammars (cont’d)
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Important:
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Word prompts carefully
Coordinate grammars and prompts
If prompt changes, change grammar to
suit
Writing Grammars (cont’d)

Process for developing grammar:
1.
2.
3.
4.
5.
6.
Define dialog
Identify information items and define slots
Design prompts
Anticipate caller responses
Identify “core” and “filler” portions
Write GSL code
Defining Dialog


Good understanding of dialog required
before grammar written
Answer following questions:
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What pieces of information required to
complete task?
What order will information be requested?
Will one piece of information be requested at
a time (directed dialog), or several pieces
(mixed initiative)?
Information and Slot Identification


Allocate one slot for each piece of
required information
Slot has:
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Name
Value format
Value type
Slot Example

Air travel application
Item
Slot Name
Value Format
Value Type
city #1
origin
3-letter code
string
city #2
destination
3-letter code
string
date
date
[<month> <day>]
NL structure
time
time
0 – 2359
integer
yes/no
confirm
“yes’ or “no”
string
Prompt Design



Design prompt before writing grammar
Prompt wording greatly affects user
response
Match prompt to slot
Prompt Design Example
Directed dialog
Prompt
Slot
What city would you like to leave from?
origin
What city would you like to fly to?
destination
What date would you like to leave?
date
What time would you like to depart?
time
You’re going from <origin> to <dest> on <date> at
<time>. Is this correct?
confirm
Prompt Design Example
Mixed initiative
Prompt
Slot
Where would you like to travel?
origin, dest, date, time
What city would you like to leave from?
origin
What city would you like to fly to ?
destination
What date would you like to leave?
date
What time would you like to depart?
time
You’re going from <origin> to <dest> on <date> at
<time>. Is this correct?
Confirm
Anticipate Caller Response
----------------------------------------------------------------------------What city would you like to leave from?
Charlotte
[city name alone]
I’d like to leave from Charlotte
[literal response]
Uh, Charlotte
[initial hesitation]
Charlotte, please
[final “please”]
(I’m) leaving from Charlotte
(I’m) departing from Charlotte
[additional possibilities]
-----------------------------------------------------------------------------------------------------What city would you like to fly to?
New York
[city name alone]
I’m flying to New York
[literal response]
Um, New York
[initial hesitation]
New York, please
[final “please”]
(I’d) like to fly to New York
(I’m) going to New York
[additional possibilities]
Identify Grammar Core and Filler

Core

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Portion with most important meaning-bearing
words
Highly reusable => defined as subgrammar
Filler

Depends largely on prompt wording
Core and Filler Example

Core
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CITY – Charlotte, Raleigh, New York, Miami, San Diego
DATE
TIME
Filler
What city would you like to leave from?
CITY
I’d like to leave from CITY
Uh, CITY
CITY, please
(I’m) leaving from CITY
(I’m) departing from CITY
Grammar Specification Language

Create grammar file
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Text file
File extension .grammar
Can contain more than one grammar
definition
Grammar definition contains the grammar
name and description
GSL (cont’d)
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Grammar definition format:
GrammarName

GrammarDescription
GrammarName
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Character string used to reference grammar
Contains at least one uppercase letter (usually the first letter)
No more 200 characters in length
Can contain:
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Upper and lower-case letters
Digits
- dash
_ underscore
‘ single quote
@ at sign
. period
GSL (cont’d)
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GrammarDescription
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Recognizable word sequence or phrase
Consists of :

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Word tokens
Grammar names
Operators
Word tokens and grammar names separated by white
space character (space, tab, newline)
Word tokens are terminal symbols

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Represent actual word for recognition e.g. dog
Must be lowercase
GSL Code
; This is a simple grammar
Sentence ( good morning )

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A semicolon (;) indicates a comment
Sentence is the name of the grammar
OR Construction
[A B C … D]
A or B or C or … or D
Sentence( good [ morning afternoon evening ] )
Operators

(A B C) A followed by B then C
(good morning)
good morning

[ A B C] A or B or C
(good [morning afternoon evening night])
good morning, good afternoon, good evening, good night
Operators

?A
A is optional
Command ( tell me my balance in checking ?please)
tell me my balance in checking, tell me my balance in checking please
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+A
One or more repetitions of A
Sentence( thanks +very much )
thanks very much, thanks very very much, thanks very ….much

*A
Zero or more repetitions of A
Sentence( thanks *very much )
thanks much, thanks very much, thanks very very much
Natural Language Interpretation
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NL interpretation assigns meaning to word
strings
Many utterances. . .
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“withdraw fifteen hundred bucks from savings”
“take fifteen hundred out of savings”
“give me one thousand five hundred dollars from my
savings account”
. . .may express the same meaning:
<action "withdrawal">
<source_account "savings">
<amount 1500>
Interpretation
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Slots are ...
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Defined for the domain
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command
amount
source
Associated with word strings in the grammar
Filled with values when the associated word
string is recognized by NL Interpretation
Interpretation

Define the relevant “slots” for the domain
Slot
Value
command
source-account
destination-account
amount
"transfer"
"savings"
"checking"
125.10
“Transfer one twenty five ten from savings to checking”
“I want to transfer to checking from savings one
hundred twenty five dollars and ten cents”
“Please put a hundred twenty five dollars ten cents in
checking from my savings account”
Slot-Filling Commands
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NL commands go between curly braces:
{ … }

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Commands “attach” to the preceding item —
either a word or a grammar construction
NL commands are part of the grammar file:
Command ( withdraw from
[ checking {<source_account "checking">}
savings {<source_account "savings">}
]
) {<action "withdrawal">}
More About Grammars
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Subgrammars
Return Commands
NL Functions
Subgrammars

Subgrammars match a “part” of an utterance
Account
( [ savings
checking
(money market)
]
?account
)

Subgrammars reduce redundancy
Command
[ ( tell me the balance in Account )
( transfer from Account to Account )
( withdraw from Account )
]
Return Commands and Variables

To associate a return value with a grammar:
{return("checking")}
“return” is like other {} commands except no
slot is filled; only the value is defined
Assignment: A higher-level grammar can store
the returned value in a variable:
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<Sub-grammar>:<variable_name>
Example: Account:acct results in the variable acct being
set to the value returned by the grammar Account
Dereferencing: To access a variable’s value,
preface the variable name with ‘$’.
Return Commands and Variables
Command [ ( tell me the balance in Account:acct )
{<account $acct>}
( transfer from Account:src to Account:dest )
{<source-account $src>
<dest-account $dest>}
( withdraw from Account:src )
{<source-account $src>}
]
Account ( [
checking
{return("checking")}
savings
{return("savings")}
( money market ) {return("money_market")}
]
?account
)
NL Functions

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Slot values and return values can be function calls
Available functions:
add
sub
mul
div
neg
strcat

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returns the sum of two integers
returns the result of subtracting the second
integer from the first
returns the product of two integers
returns the truncated integer result of dividing
the first integer by the second (e.g., div(9 5)
returns 1)
returns the negation of an integer
returns the concatenation of two strings
Arguments separated by whitespace, not commas
No space between function name and parenthesis
NL Functions
 Example:
Digit
[
one
two
three
...
{return(1)}
{return(2)}
{return(3)}
twenty
thirty
forty
...
{return(20)}
{return(30)}
{return(40)}
]
Decade [
]
Number ( Decade:d1 Digit:d2 ) {<number add($d1 $d2)>}

Matching the top-level grammar Number fills the slot
number with the sum of NL variables d1 and d2