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Purely functional is a term in computing used to describe algorithms or data structures that exclude destructive modifications (updates). As a consequence of this restriction, variables refer to immutable, persistent values. This means that old values available prior to a change continue to be accessible and may be updated following a modification.Such persistence can be advantageous in the development of many applications.
Moreover, the inherent referential transparency tends to make purely functional computation more amenable to analysis, both formal and informal.
Since every value in a purely functional computation is built up out of existing values, it is impossible to create a cycle of references. The result is that the reference graph, where there are edges from objects to the objects they reference, is a directed acyclic graph.
Functional programming
