There's lots of factors: * GC: it probably had a very simple algorithm, rather than a fancy generational GC * Data structures: it wouldn't support modern func

There's lots of factors: * GC: it probably had a very simple algorithm, rather than a fancy generational GC * Data structures: it wouldn't support modern functional structures like HAMT * Compilation: you probably used an interpreter, rather than compiling the code to bytecode or native code ...and of course, the algorithm itself. It may not have been designed for functional data structures, or you may not even have implemented it in a functional style in the first place (Scheme supports mutation).