Question

Q1)   Convert the following set of BNF rules to a single EBNF rule.

<E> --> <E> + <T>
     |  <E> - <T>
     |  <T>

Q2)   Briefly explain how the expected type and actual type of <expr>

            in the following two BNF rules are determined:

<assign> --> <var> = <expr>  (Rule 1)
<expr>   --> <var> + <var>   (Rule 2)

Answer 1

Answer 1.

BNF:

<E> --> <E> + <T>

     | <E> - <T>

     | <T>

Conversion to EBNF:

<E> --> <E> <O> <T> +

<O> --> ‘+’ | ‘-‘

E equals to ‘E’ ‘O’ ‘T’

Where

E is expression

O is operator which can be ‘+’ or ‘-‘

T is term which is one or more.

Answer 2.

actual-type: a synthesized attribute which stores the actual type of a <VAR> or an <EXPRESSION>. In the case of an <EXPRESSION> , the type is computed given the types of the component <VAR>s.

expected-type: an inherited attribute which stores the expected type of <EXPRESSION> . It is determined by the type of the <VAR> on the left-hand-side of the expression statement.

Answer 1.

BNF:

<E> --> <E> + <T>

     | <E> - <T>

     | <T>

Conversion to EBNF:

<E> --> <E> <O> <T> +

<O> --> ‘+’ | ‘-‘

E equals to ‘E’ ‘O’ ‘T’

Where

E is expression

O is operator which can be ‘+’ or ‘-‘

T is term which is one or more.

Answer 2.

actual-type: a synthesized attribute which stores the actual type of a <VAR> or an <EXPRESSION>. In the case of an <EXPRESSION> , the type is computed given the types of the component <VAR>s.

expected-type: an inherited attribute which stores the expected type of <EXPRESSION> . It is determined by the type of the <VAR> on the left-hand-side of the expression statement.