The problem here is not the absence of pattern matching, it is the absence of the "tag" part of tagged unions. Union types like the one you presented (union res

The problem here is not the absence of pattern matching, it is the absence of the "tag" part of tagged unions. Union types like the one you presented (union result = user | Errors) are different from sum types, as they don't have a tag. Without the tag you need a way to know the type of the result at runtime. For example in TypeScript, you can use a type assertion and control-flow analysis to emulate sum types and pattern matching: https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#union-types https://www.typescriptlang.org/docs/handbook/2/everyday-type.... While sum types and union types are close, they are not exactly the same. Let's take an example: union bools = bool | bool enum Bools = A(bool) | B(bool) The example is a bit weird but shows an important point: bools has 2 values, true and false. Bools has 4 values, A(true), A(false), B(true), B(false). In bools (the union), true and true is always the same. In Bools (the sum/enum), A(true) is not the same as B(true). I don't know enough about type theory to know if union types have a relationship with sum types. They can be used the same for most cases but they have differences around the edges.