> In Haskell land, where the type system is considered sound you have `head` functions of type `List a -> a` that are unsound too, because the list might be empty.
Haskell's `head` not is not an example of the type system being unsound (I stress this point because we've been talking about type system soundness, not something-else-soundness).
From the view of the type system, `head` is perfectly sound: if the list is empty, the resulting value is ⊥ ("bottom"). And ⊥ is an inhabitant of every type. Therefore, `head` returning ⊥ when given an empty list is perfectly fine. When you force ⊥ (i.e. use it any way whatsoever), an exception is thrown. See https://wiki.haskell.org/Bottom
This is very much not the same thing (or remotely analogous) to what we have in my TypeScript example. There, the code fails at runtime when I attempt to call `toLowerCase`, yes; what's worse is the slightly different scenario where we succeed in calling something we shouldn't:
class Person {
name: string;
constructor(name: string) {
this.name = name;
}
kill() {
console.log("Killing: " + this.name);
}
}
class Murderer extends Person { }
class Innocent extends Person { }
function populatePeopleFromDatabase(people: Array): void {
// imagine this came from a real SQL query
people.push(new Innocent("Bob"));
}
function populateMurderersFromDatabase(people: Array): void {
// TODO(Aleck): come back and replace this with a query that only selects murderers.
// i wanted to get the rest of the code in place, and this type checks,
// so I'll punt on this for now and come back later when I wrap my head
// around the proper SQL.
// we're not actually using this anywhere just yet, so no biggie ¯\_(ツ)_/¯
populatePeopleFromDatabase(people);
}
// ... some time later, Bob comes along and implements the murderer execution logic:
const murderers: Array = [];
populateMurderersFromDatabase(murderers);
// Bob is about to have a really shitty day:
murderer.forEach((murderer) => murderer.kill());
It is not possible to write an analogous example in Haskell using `head`.