alt Hacker NewsThe origin of all sum types is in "Definition of new data types in ALGOL x", published by John McCarthy in October 1964, who introduced the keyword UNION for such types (he proposed "union" for sum types, "cartesian" for product types, and also operator overloading for custom types).
John McCarthy, the creator of LISP, had also many major contributions to ALGOL 60 and to its successors (e.g. he introduced recursive functions in ALGOL 60, which was a major difference between ALGOL 60 and most existing languages at that time, requiring the use of a stack for the local variables, while most previous languages used only statically-allocated variables).
The "union" of McCarthy and of the languages derived from his proposal is not the "union" of the C language, which has used the McCarthy keyword, but with the behavior of FORTRAN "EQUIVALENCE".
The concept of "union" as proposed by McCarthy was first implemented in the language ALGOL 68, then, as you mention, some functional languages, like Hope and Miranda, have used it extensively, with different syntactic variations.
Definitely if you don't have the C "union" user defined type you should use this keyword for your sum types. Many languages don't have this feature - which is an extremely sharp blade intended only for experts - and that's fine. You don't need an Abrams tank to take the kids to school, beginners should not learn to fly in the F-35A and the language for writing your CRUD app does not need C-style unions.
If Rust didn't have (C-style) unions then its enum should be named union instead. But it does, so they needed a different name. As we work our way through the rough edges of Rust maybe this will stick up more and annoy me, but given Rust 1.95 just finally stabilized core::range::RangeInclusive, the fix for the wonky wheel that is core::ops::RangeInclusive we're not going to get there any time soon.