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This is just a difference between named and anonymous types, really. This is very similar to named functions vs lambda functions, both provide the same exact fu

This is just a difference between named and anonymous types, really. This is very similar to named functions vs lambda functions, both provide the same exact functionality but avoiding superfluous identifiers makes programming more ergonomic. More specifically sum types in Rust must also be tagged, meaning you need to explicitly construct and deconstruct them. This aspect is along the type-alias vs newtype axis which gets into structural vs nominative typing and the trade-offs therein. So yes, Rust enums and Typescript union types are both exactly sum-types and the differences are due to the surrounding decisions made about the languages. Rust's sum types are named and tagged, Typescript's are anonymous and untagged, but they're both sum types.

nybble41 6y ago
> So yes, Rust enums and Typescript union types are both exactly sum-types… As others have pointed out, untagged unions are not sum types because the union of a type with itself has no effect, whereas adding a type to itself yields twice as many possible values. Untagged unions can function as sum types when there is no overlap between the members, but not in the general case. To illustrate the difference: You can construct every possible algebraic type as some combination of void (no values), unit (one value), sum (|A + B| = |A| + |B|), and product (|A * B| = |A| * |B|). This does not work if the sum type is replaced with an untagged union. You can't even get as far as constructing the equivalent of the boolean type; while |Unit + Unit| has two distinct values, |Unit ⋃ Unit| only has one value.