Java Reference
In-Depth Information
1 Start
→
Digs
ans
$
call
print
(
ans
.
val
)
2 Digs
up
→
Digs
below
d
next
up
.
val
←
below
.
val
×
below
.
base
+
next
up
.
base
←
below
.
base
3
|
SetBase
basespec
up
.
base
←
basespec
up
.
val
←
0
4 Setbase
n
→ λ
n
←
10
5
|
xd
num
n
←
num
x
d
d
d
d
$
(a)
(b)
Figure 7.8: (a) Grammar that avoids global variables; (b) Parse tree
reorganized to facilitate bottom-up attribute propagation.
Global variables are easily introduced into semantic actions, but there are good
reasons to avoid using them:
•
A grammar's rules are often invoked recursively during parsing, and the
use of global variables can introduce unwanted interactions as semantic
actions are applied (see Exercise 12).
•
Global variables can make semantic actions di
cult to write and main-
tain, as any semantic action could read or write a global variable. More-
over, proper initialization and reinitialization of global variables can be
problematic.
•
Global variables may require setting or resetting
A more robust solution attempts to restructure the parse tree so that infor-
mation flows where it is needed for semantic actions. The following steps
are suggested as a mechanism for obtaining a grammar more appropriate for
bottom-up, syntax-directed translation:
1. Sketch the parse tree that would allow bottom-up synthesis and transla-
tion, without use of global variables.
