\DOC delta

\TYPE {type 'a delta}

\SYNOPSIS
A type used for telling when a function has changed its argument.

\DESCRIBE
The {delta} type is declared as follows:
{
   datatype 'a delta = SAME | DIFF of 'a
}
The {delta} type may be used in applications where it is
important to tell if a function has changed its argument or not. As an
example of this, consider mapping a function over a large collection of
elements. If only a few elements are changed, it makes sense to re-use
all those that were not changed. This can of course be handled on an
ad hoc basis; the {delta} type provides a mechanism for doing this
systematically.

\COMMENTS
The {delta} type is an example of polytypism.

\SEEALSO
Lib.delta_apply, Lib.delta_map, Lib.delta_pair.
\ENDDOC