Wouldn't it be easy for Python to implement generating functions so that the iterators they return are equipped with a __reset__() method?
Here is the context of this question. Python documentation defines a "iterator" as an object ITERATOR having methods __next__() and __iter__() such that the call ITERATOR.__iter__() returns the object itself, and once a call ITERATOR. __next__() raises StopIteration every such subsequent call does the same. Python iterators model "generating Turing machines", i.e. deterministic Turing machines which take no input, which have a separation symbol # in the output alphabet; have an one-way-infinite output tape on which the output head prints and moves only to the right; The machine may have a halting state; a word is said to be "generated by M" if it appears on the output tape delimited by separation symbols # (independently of whether M halts or computes infinitely). Generating Turing machines provide a characterization of recursively enumerable languages: a language is generated by a generating Turing machine iff it is accepted by a Turing machine. Turing machines can take as input and run other Turing Machines -- similarly Python functions can take other functions (including iterators) as parameters and call them. HOWEVER, this symmetry breaks down: a Turing machine which takes a generating Turing machine M as input can run M for a number of steps and then RESET M and run it again, while iterators as currently defined in Python do not have a reset method. (I realize that instead of reseting an old iterator, one can make a new iterator but it is not as elegant.) (Contrary to Python's philosophy that there should be one-- and preferably only one --obvious way to do it) there are several ways to define iterators: -- http://mail.python.org/mailman/listinfo/python-list
