And it is important to consider that once you commit to, say, iterating over an entire file, that the file must be reasonably small anyway to get decent performance. The same issue exists with collections. Who cares if collect: creates an extra collection or if WriteStream wastes space at the end of a long underlying collection; if these concerns are really so important then probably this huge collection should not exist and/or you should not be iterating over the entire thing anyway.

To put it very simply: you just can not expect a program to work on large data structures just because you micro optimized everywhere. If you want to handle large data structures then it takes planning and specialized algorithms and test cases. If you are not going to put in that effort, then don’t sweat the small stuff. It is very liberating to code with an eye towards correctness and towards algorithmic performance, and not to worry about getting down the constant factor. It seems to lead to lower stress, faster code production, and fewer bugs generated.