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Christian Jaeger
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FP::Abstract::Pure - functional purity protocol


 use FP::List;
 my $v= list(1); # or any other data structure that implements
                 # FP::Abstract::Pure
 is UNIVERSAL::isa($v, "FP::Abstract::Pure"), 1;
 # Or use Safe::Isa instead of UNIVERSAL::isa, but I don't understand
 # why overriding isa is useful (why fake inherit as opposed to real
 # inheriting but then shadowing what needs to be shadowed? NEXT method
 # and that needs to be supported in mock classes? TODO figure out.)

 # but usually:
 use FP::Predicates;
 is_pure ($v) # true if $v is (officially) immutable, detectable from
              # inheritance of FP::Abstract::Pure or other means


Base class for all data structures that don't allow mutation (by ordinary programs), i.e. are persistent (1).

(1) as in https://en.wikipedia.org/wiki/Persistent_data_structure

More precisely, those objects that don't have methods that when called make other methods non-functions.