In the Fexl project I have a couple of tests b15 and index_C which render output with nested indentation. I was using a "pure functional" style, but not anymore. I just published this commit of the Fexl project which eliminates that complexity, simplifying the code by using stateful output routines.
That eliminates the "pure functional" output style, which took great pains to build a nested list structure that was only sent to the output device at the very end. This required passing around a hidden "tab" parameter in a monadic fashion, the use of a "next" continuation parameter, and semicolons to ensure that the tail of a structure was threaded in properly.
Now when you call routines like say, nl, put, and indent, the effects of those calls are going directly into a file handle or other target as they run. You are no longer building up a lazy nested piece of data which is later sent to an output target. All the complexity of the "pure" method vanishes like a bad dream.
This drastically simplifies the code supporting the b15 and index_C tests. I also use the technique to great advantage in the demo.ray project.
This illustrates a principle which I title:
That's clearly a riff on Dijkstra's famous Go To Statement Considered Harmful. I consider the use of pure functions as sometimes harmful.
The pure functional style still has its place in certain aspects of a program, but for input and output it's easier just to cut to the chase and deal with the machine on its own terms. In those cases, "var" is your friend — and it can even create more opportunities for implementing routines directly in C, as I did with the parsing primitives in stream.c in the Fexl project.
I am also finding the var technique useful for "throughput". In one major project I was passing a single immutable state parameter around everywhere, and I greatly simplified the code by keeping that state in a handful of vars.
The good thing about "var" is that it is controlled mutability, not the haphazard and dangerous kind where you modify nested data structures in place. In Fexl the data structures are still immutable, but you use a var to point to a new version when you make a state change. I think this strikes the right balance between pure functions and routines with side effects.