Do you find it acceptable that this code passes type-checking?
function endlessLoop(): never {
while (true) {
}
}
let y = endlessLoop();
Why does y exist and fall under the never type category?
Do you find it acceptable that this code passes type-checking?
function endlessLoop(): never {
while (true) {
}
}
let y = endlessLoop();
Why does y exist and fall under the never type category?
never is a specific type that can be assigned to any variable in TypeScript. When you declare let x = infiniteLoop(), TypeScript automatically determines that the variable x has the type never, which is perfectly valid.
However, the usefulness of this lies in the fact that it prevents you from using the variable x later on in your code, thereby avoiding potential semantic errors.
To explore more examples and explanations regarding the never type in TypeScript, check out this resource.
It's important to note that TypeScript cannot statically detect infinite loops due to the undecidability of such problems in type systems.
By explicitly indicating the never type, TypeScript enables developers to highlight and prevent unintended behaviors related to computations that are declared to never end.
In attempting to address my own query, it appears that the concept of never in TypeScript is akin to a "bottom" type. However, "bottom" is considered a sub-type of all other types:
function f() : never
{
return (1 as never); // although this doesn't hold true for "as void".
}
Bottom serves as a member of every type (though with certain restrictions related to unboxed kinds), making it usable wherever a value of that type would be found (source).
Therefore, the function infiniteLoop() has the ability to yield never, which can exist as a subtype within any other type and even be explicitly declared like so:
let x:never;
Due to its nature as a sub-type of all types, variables can be "casted" (using as) to the never. Nonetheless, it is not permissible to assign a super-set to its corresponding sub-set; for instance, assigning the type Triangle to never results in an error, as demonstrated below:
type Shape = Square | Rectangle | Circle | Triangle;
function assertNever(x: never): never {
throw new Error("Unexpected object: " + x);
}
function area(s: Shape) {
switch (s.kind) {
case "square": return s.size * s.size;
case "rectangle": return s.height * s.width;
case "circle": return Math.PI * s.radius ** 2;
default: return assertNever(s); // error occurs if cases are missing
}
}
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