Concepts of Programming Languages

138 Chapter 3 Describing Syntax and Semantics

expected_type

<assign>

<var>[3]

B

<var>[2]

=+A

<var>

A

<expr>

actual_type

actual_type actual_type actual_type

<assign>

<var>[3]

B

actual_type =

int_type

actual_type =

real_type

<var>[2]

=+A

<var>

A

actual_type =

real_type

<expr> expected_type = real_type

actual_type = real_type

Figure 3.7

The flow of attributes
in the tree

Figure 3.8

A fully attributed
parse tree

3.4.7 Evaluation

Checking the static semantic rules of a language is an essential part of all com-

pilers. Even if a compiler writer has never heard of an attribute grammar, he

or she would need to use their fundamental ideas to design the checks of static

semantics rules for his or her compiler.

One of the main difficulties in using an attribute grammar to describe all of

the syntax and static semantics of a real contemporary programming language

is the size and complexity of the attribute grammar. The large number of attri-

butes and semantic rules required for a complete programming language make

such grammars difficult to write and read. Furthermore, the attribute values on

a large parse tree are costly to evaluate. On the other hand, less formal attribute