The dependent type question is, I think, subtly different: "a function whose return type is either a number or a string depending on the boolean argument."
The union type (or the more or less equivalent Either ADT) don't give you type checking because you can take either (sic) branch no matter what your boolean value was. You're free to write a function that takes false and returns an int, and the type checker will accept that function as correct.
No matter how you twist and turn in GHC Haskell or TypeScript, you can't prevent that just by the type signature alone, so you have to "trust me bro" that the code does what the comment says.
For this example that's trivial and laughable, but being able to trust that code does what the compiler-enforced types say and nothing more is perhaps a step towards addressing supply chain attacks.
The union type (or the more or less equivalent Either ADT) don't give you type checking because you can take either (sic) branch no matter what your boolean value was. You're free to write a function that takes false and returns an int, and the type checker will accept that function as correct.
No matter how you twist and turn in GHC Haskell or TypeScript, you can't prevent that just by the type signature alone, so you have to "trust me bro" that the code does what the comment says.
For this example that's trivial and laughable, but being able to trust that code does what the compiler-enforced types say and nothing more is perhaps a step towards addressing supply chain attacks.