SYNTAX ANO SEMANTICS

THE METALANGUAGE: LANGUAGE PROCESSING

5.1 SYNTAX ANO SEMANTICS

Perhaps the best description begins with an example •

. In BIBLIO,

we would I ike to ask the question

Generalization of grammar?

to find the list of subjects which have been stated to immediately include the subject grammar in their extent. {By the examples of Chapter

III,

these would be syntax directed interpretation and compi

I

ing.) Clearly, we would find a language implementation based strictly on simple patterns rather strained, so the rules of grammar wi 11 be sufficiently general to accept not onJy relatively simple queries

I

ike the above, but much more complex, composed queries. Below

is

a

portion of the syntax of BIBLIO.

The distinguished symbol of BIBLIO, as of every language defined using LWL, is <sentence>. Thus, one rule wi 11 be

<sentence>::= <query>?

which sp

e

cifies that one valid kind of sentence of BIBLIO is a <query>

fol lowed by a question mark. Since the answers to queries wi

I I

be

I

ists of authors,

I

ists of publications or

I

ists of subjects, we need corresponding syntactic rules •

.

For the above query, we require only

<query>::= <q_subject>

where <q_subject> is

a

list of subjects. Then,

·

to aliow

a

simple query Ii ke*

*Recal I from Chapter III that a query merely naming a subject

asks for a

Ii st of al I subjects covered by it.

Grammar?

we wi I I have a rule

<q_subject> ::= <subject>

Finally, to define the "general ization

¹¹

query, we need

<q_subject> ::=generalization of <q_subject>

With the above fragmentary grammar, and assuming a lexical rule corresponding to

<subject>::= grammar

our example sentence is analyzed in the fol lowing form:

<sentence>

<query>

<q_subject>

<q_sub j ec t>

<subject>

·

General izati~n of grammar ?

Notice that the generality we have introduced creates som~ extra levels of ana I ys is in the grammar; however, it immediate I

y

a I I ows more comp I

ex

queries like

Generalization of generalization of grammar?

Generalization of generalization of generalization of grammar?

Al though the grammar above determines the structure of our

sentence, it says nothing about how its meaning (in this case, a reply

to the user) is to

.

be computed. We rectify this

by

associating 1-1ith

each rule the name of a function which wil I carry out the computation imp I ied by the syntactic transformation.

fragment, as actually written in

LWL,

is below.

The resulting grammar

query_forming rule <sentence>::= <query>'?' (print}

subject_query rule <query>::= <q_subject>: (format_subjects)

generalization ru~e <q_subject> ::= 'generalization o f ' <q_subject>

: (genera Ii ze) ·

primitive_subject rule <q_subject> ::=<subject>: {single_subject) Each rule is named, so that it may later be referenced for modification or debugging. Text that is literally mentioned in the left or right hand side of a rule {i.e., terminal symbols) is _expressed by quoted strings, since spaces in the object language are significant but spaces in

LWL

are not. The i n for ma t i on fo I I ow i ng the " :I' i s the · s em ant i c specification; in this case, the names of the appropriate functions.

These ru I es have the side effect of specifying the resu It types and constituent types of the functions they mention. In genera I rewr i te rules, one semantic specification appears for each non-terminal phrase in the left hand side. An omitted semantic specification implies the identity semantic function.

is

With the above grammar, the meaning of the question Generalization of grammar?

print( produce_subjects( generalize(. single_subject{grammar) ) ) );

The phrase marker which represents this computation is produced

by

the I anguage processor as a resu It of the syntactic ana I ys is be fore any

actual evaluatio~ is attempted; thus, syntactic and semantic processing do not ord i nar i I y proceed in para I I e I • This may save cons i derab I e semantic computation if spurious part i a I parses can be rejected for purely syntactic reasons before any semantic computation takes place.

A I so, this convention minimizes the prob I em of "undoing" which can haunt many syntax directed compilers; having to undo falsely hypothesized act i ans based on spurious parses is usua I I y av~ i ded by adopting very

. simple bounded context grammars. As we shal I see below, the processing

of semantics synchronously with syntax is possible, though not mandatory.

Let us return again to Chomsky's I ist, and take up the discussion more specifically. His first question, in our context, asks,

"What are the strings of symbols to be considered?" The terminal symbols of every object language are the printable characters of the host computer (the EBCDIC printable characters), augmented by the symbols <string_begin>, <string_end>, <input_terminator> and

<carriage_return>. <s tr i ng_beg in> and <s tr i ng_end> are automa ti ca I I y appended around the input string, for the convenience of syntactic ana I ys is. < i nput_term i nator> is a REL-recognized symbo I by which the user indicates the end of his input sentence, and <carr i age_return> ·is the end of I ine.* The complete vocabulary of every language is encoded

*Carri age return is ord i nar i I y a nor·ma I input character, and a spec i a I

<input_terminator> is required to initiate processing of an input sentence. In TSO, this is (control)-S. Note that most EBCDIC devices do not have lower case characters or some of the special symbols used in the description of LWL. For

LWL,

a transl iteration similar to that

as

.

objects of the LWL type char; thus, the non-terminal vocabulary is made to correspond to unprintable characters of EBCDIC. These do

not

over

I

ap the codes for the four characters defined above or the char constants described in the section on List Processing in Chapter IV.

Turning to Chomsky's second question, we understand "structural description" by the description introducing functions in the previous chapter. The structural description is the phrase marker, representing both the sentence's syntactic analysis

_

and the composition of functions required to compute its meaning.

Corresponding to Chomsky's third question, about "generative

grammar", we need to define our complete notion of "analytical grammar

^11•

The fundamentals of the rule statement appear above; in the succeeding

sections, we elaborate the definition of rules by introducing useful

extensions.