There is really no difference between doing this and returning a `Maybe`, which is the standard Haskell pattern, except that the `Maybe` result also allows the result to be structurally different rather than simply a refinement of the input type. In a sense, the TypeScript approach is a convenience feature that allows you to write a validation function that returns `Bool`, which normally erases the gained information, yet still preserve the information in the type system.
This is quite nice in situations where the type system already supports the refinement in question (which is true for this NonEmpty example), but it stops working as soon as you need to do something more complicated. I think sometimes programmers using languages where the TS-style approach is idiomatic can get a little hung up on that, since in those cases, they are more likely to blame the type system for being “insufficiently powerful” when in fact it’s just that the convenience feature isn’t sufficient in that particular case. I presented an example of one such situation in this followup blog post: https://lexi-lambda.github.io/blog/2020/08/13/types-as-axiom...
This is quite nice in situations where the type system already supports the refinement in question (which is true for this NonEmpty example), but it stops working as soon as you need to do something more complicated. I think sometimes programmers using languages where the TS-style approach is idiomatic can get a little hung up on that, since in those cases, they are more likely to blame the type system for being “insufficiently powerful” when in fact it’s just that the convenience feature isn’t sufficient in that particular case. I presented an example of one such situation in this followup blog post: https://lexi-lambda.github.io/blog/2020/08/13/types-as-axiom...