Java Reference
In-Depth Information
E
Prefix
(
E
)
v
Tail
f
+
E
v
Tail
Figure 4.2: The parse tree for f ( v + v) .
and its parse tree, a
phrase
of the sentential form is a sequence of symbols
descended from a single nonterminal in the parse tree. A
simple
or
prime
phrase
is a phrase that contains no smaller phrase. That is, it is a sequence of
symbols directly derived from a nonterminal. The
handle
of a sentential form
is the leftmost simple phrase. (Simple phrases cannot overlap, so “leftmost”
is unambiguous.) Given the parse tree of Figure 4.2 and the sentential form
f(vTail), f and vTailare simple phrases and f is the handle. Handles are
important because they represent individual derivation steps, which can be
recognized by various parsing techniques.
Although CFGs serve well to characterize syntax, most programming lan-
guages contain rules that are not expressible using CFGs. For example, the
rule that variables must be declared before they are used cannot be expressed
because a CFG provides no mechanism for transmitting to the body of a pro-
gram the exact set of variables that has been declared. In practice, syntactic
details that cannot be represented in a CFG are considered part of the static
semantics and are checked by semantic routines (along with scope and type
rules).
