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Do Scala's match types implement full dependent types?
Do Scala's match types support full spectrum dependent types, and if not, what capabilities do they lack?
1 answer
Dependent types are types that depend on values. Scala's match types enable expressing types that depend on types. Scala's other type system features include path-dependent types, which allow types to depend on paths that represent values, but outside of literals and singletons, there is no way to express values themselves in the type system. So no, these are not full dependent types.
As an example of what isn't possible:
type MyTuple[n, A] = n match
case 0 => Unit
case 1 => A
case 2 => (A, A)
def myfn(n: Int): MyTuple[n, String] = n match
case 0 => ()
case 1 => ""
case 2 => ("", "")
Changing MyTuple[n, String] to MyTuple[n.type, String] doesn't help. n.type is a path-dependent type, so the type system knows that all occurrences of n.type are the same, but it doesn't know the run-time value of n and so can't reduce MyTuple enough to check the three cases.
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