Like interpreter and compiler architectures, type systems kinds and their meaning, ways of specifying the semantics formally, and also the structure of novel kinds of abstractions proposed by researchers in some experimental languages.
If you learn all of that today, the next time you'd need to learn anything new about a mainstream PL would be in 10 years, if not twice that.
Example: many modern languages only recently started supporting some of the Algol-68 features. That's 50 years to get a lambda into the language...
Like interpreter and compiler architectures, type systems kinds and their meaning, ways of specifying the semantics formally, and also the structure of novel kinds of abstractions proposed by researchers in some experimental languages.
If you learn all of that today, the next time you'd need to learn anything new about a mainstream PL would be in 10 years, if not twice that.
Example: many modern languages only recently started supporting some of the Algol-68 features. That's 50 years to get a lambda into the language...