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delta                                                            (Lib)
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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.

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