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An optional series of elements, separated from each other using delimiters
occurs frequently in programming languages. For example, bf(C++) functions
have parameter lists which may or may not require arguments. Since a parameter
list may be defined empty, an em(empty) alternative is required. However, a
simple generalization of the optional series construction (section
ref(OPTSERIES)) won't work, since that would imply that the em(first) argument
is preceded by a separator, which is clearly not the intention. So, the
following construction is em(wrong):
verb(
opt_parlist:
// empty
|
opt_parlist ',' parameter
)
To define an optional series of delimited elements em(two) rules are
required: one rule handling the optional part, the other the delimited series
of elements. So, the correct definition is as follows:
verb(
opt_parlist:
// empty
|
parlist
;
parlist:
parameter
|
parlist ',' parameter
;
)
Again, the above rule pair can be used as a prototype for recognizing an
optional series of delimited elements. The generic form of these rules could
be formulated as follows:
verb(
opt_series:
// empty
|
series
;
series:
element
|
series delimiter element
)
Note that the tt(opt_series) rules neatly distinguishes the no-element
case from the case were elements are present. Usually these two cases need to
be handled quite differently, and the tt(opt_series) rules empty alternative
easily allows us to recognize the no-elements case.
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