On Wednesday 28 September 2016 15:27, Gregory Ewing wrote: > * No side effects (new variable bindings may be created, but > existing ones cannot be changed; no mutable data structures).
As I understand it, for some advanced functional languages like Haskell, that is only true as far as the interface of the language (the API) is concerned. Implementation-wise, the language may in fact use a mutable data structure, provided it is provable that only one piece of code is accessing it at the relevant times. The analogy with Python is the string concatenation optimization. Officially, writing: a + b + c + d + e in Python has to create and destory the temporary, intermediate strings: (a+b) (a+b+c) (a+b+c+d) before the final concatenation is performed. But, provided there is only one reference to the initial string a, and if certain other conditions related to memory management also hold, then this can be optimized as an in-place concatenation without having to create new strings, even though strings are actually considered immutable and growing them in place is forbidden. My understanding is that smart functional languages like Haskell do that sort of thing a lot, which avoids them being painfully slow. -- Steven git gets easier once you get the basic idea that branches are homeomorphic endofunctors mapping submanifolds of a Hilbert space. -- https://mail.python.org/mailman/listinfo/python-list
