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\name{Utility classes}
\alias{named.list}
\alias{item.list}
\alias{named.list-class}
\alias{item.list-class}
\alias{initialize,item.list-method}
\alias{initialize,named.list-method}
\alias{coerce,data.set,named.list-method}
\alias{show,named.list-method}
\alias{atomic-class}
\alias{double-class}
\title{Named Lists, Lists of Items, and Atomic Vectors}
\description{
The classes "named.list" and "item.list" are merely some
'helper classes' for the construction of the classes "data.set"
and "importer".
Class "named.list" extends the basic class "list" by an additional
slot "names". Its \code{initialize} method assures that the names
of the list are unique.
Class "item.list" extends the class "named.list", but does not
add any slots. From "named.list" it differs only by the
\code{initialize} method, which calls that for "named.list"
and makes sure that all elements of the list belong to
class "\link{item}".
Classes "atomic" and "double" are merely used for
method selection.
}
\examples{
new("named.list",a=1,b=2)
# This should generate an error, since the names
# are not unique.
try(new("named.list",a=1,a=2))
# Another error, one name is missing.
try(new("named.list",a=1,2))
# Also an error, the resulting list would be unnamed.
try(new("named.list",1,2))
new("item.list",a=1,b=2)
# Also an error: "item.list"s are "named.lists",
# and here the names would be non-unique.
try(new("item.list",a=1,a=2))
}
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