Immutable state does not necessarily imply 20mb of numbers guaranteed. It depends on the compiler and on the implementation of data structures. Functional data

Immutable state does not necessarily imply 20mb of numbers guaranteed. It depends on the compiler and on the implementation of data structures. Functional data structures can use mutation behind the scenes to produce a facade of immutability. Clojure has an efficient "immutable" array data structure offering constant-time (~log32 n) performance and better space usage. Shallow binding is another general technique that produces space and time performance nearly identical to corresponding mutable data structures by wrapping the mutable data structure with older versions storing a pointer to the incrementally newer version plus the incremental change. Both of these use mutation behind the scenes. It's not necessary to sacrifice performance to program in a functional way.