[lisper]

Because you haven't finished the whole thing but you want to test part of it before you continue.

[iaabtpbtpnn]

There's always an escape hatch for this. Even in Java (and Haskell), you can just throw an unchecked exception from your unimplemented function.

[josephg]

Likewise in Rust you can todo!() in incomplete functions. But it’s still much harder to run incomplete code than it is in javascript, because you still need to satisfy the type checker and borrow checker even for incomplete programs.