Comment by jaen

String literals are structural types which are way more expressive than regular (Haskell) ADTs, which are nominal types.

In TS in particular, in combination with other features (mapped types), they are equivalent to row polymorphism + whatever Haskell/GHC features enable type families to specialize on constant literal arguments (or you can use atomic types, but that's not structural / open-world)... so pretty advanced.

This is valid TS/Python:

    type ABC = "A" |"B" | "C"
    type AB = "A" | "B"
    const x: AB = "A";
    const y: ABC = x;

The equivalent Haskell requires using several extensions.