1\DOC delta
2
3\TYPE {type 'a delta}
4
5\SYNOPSIS
6A type used for telling when a function has changed its argument.
7
8\DESCRIBE
9The {delta} type is declared as follows:
10{
11   datatype 'a delta = SAME | DIFF of 'a
12}
13The {delta} type may be used in applications where it is
14important to tell if a function has changed its argument or not. As an
15example of this, consider mapping a function over a large collection of
16elements. If only a few elements are changed, it makes sense to re-use
17all those that were not changed. This can of course be handled on an
18ad hoc basis; the {delta} type provides a mechanism for doing this
19systematically.
20
21\COMMENTS
22The {delta} type is an example of polytypism.
23
24\SEEALSO
25Lib.delta_apply, Lib.delta_map, Lib.delta_pair.
26\ENDDOC
27