Much more is written about how easy or hard some technology is to learn than about how hard it is to relearn. Maybe this is because people are more eager to write about something while the excitement or frustration of their first encounter is fresh.
The ease of relearning a technology is under-rated. As you’re learning a programming language, for example, it may be difficult to imagine forgetting it and needing to relearn it down the road. But you might ask yourself
If I put this down for a couple years and then have to come back to it, what language would I wish I’d written it in?
A while back I debated relearning Perl for the kind of text munging projects that Perl was designed for. But not only would I have to relearn Perl once, I’d have to relearn it every time I revisit the code. Perl does not stick in my head without constant use. Awk, on the other hand, is small and simple, and has a lot of the benefits of Perl. You can learn the basics of Awk in a day, and so if you have to, you can relearn it in a day.
Something easy to learn is also easy to relearn.
However, the converse isn’t necessarily true. Some things may be hard to learn but easy to pick back up. For example, I found LaTeX hard to learn but easy to relearn after not using it for several years. A lot of other tools seem almost as hard to relearn every time I pick them up. I think part of what made LaTeX easy to pick back up was its internal consistency. It’s a little quirky, but it has conceptual integrity.
I’ve used Mathematica off and on ever since it came out three decades ago. Sometimes I’d go for years without using it, but it has always been easy to pick back up. Mathematica is easy to return to because its syntax is consistent and predictable. Mathematica has conceptual integrity. I find R much harder to use because the inconsistent syntax fades from my memory between uses.
Conceptual integrity comes from strong leadership, even a “benevolent dictator.” Donald Knuth shaped TeX and Stephen Wolfram shaped Mathematica. R has been more of an egalitarian effort, and it shows.
The “Tidyverse” of libraries on top of R is more consistent than the base language, due to the Hadley Wickham doing so much of the work himself. In fact, the Tidyverse was initially called the “Hadleyverse,” though Hadley didn’t like that name.