Arrow functions cannot be used as user-defined type guards

[pimterry]

User defined type guards work perfectly well for classic function statements, as in the release notes:

interface Animal {name: string; }
interface Cat extends Animal { meow(); }

function isCat(a: Animal): a is Cat {
 return a.name === 'kitty';
}

However, it appears that the arrow function equivalent isn't acceptable:

// Doesn't compile
var isReallyCat: (a: Animal) => a is Cat = (cat) => cat.name === 'kitty';

This gives "Type '(cat: Animal) => boolean' is not assignable to type '(a: Animal) => a is Cat'. Signature '(cat: Animal): boolean' must have a type predicate". I'm not clear how I would give that arrow function a 'type predicate', beyond what's already included here.

Is there a good reason for this? I can't seem to find it documented anywhere.

[Arnavion]

Using a type assertion instead of type annotation does work

var isReallyCat = <(a: Animal) => a is Cat>((cat) => cat.name === 'kitty');

so I imagine it's just an oversight rather than deliberate.

[DanielRosenwasser]

The issue is that you don't want a boolean-returning function to be assignable to a type-guard.

function isHippopotamusFood(a: Animal): boolean {
 // ...
}

let isReallyCat: (a: Animal) => a is Cat = isHippopotamusFood;

For that reason, type predicates need to be placed on the return type of the function proper. The declaration itself needs to opt in.

The following also works:

```ts
var isReallyCat: (a: Animal) => a is Cat = (cat): cat is Cat => cat.name === 'kitty';

or just

var isReallyCat = (cat): cat is Cat => cat.name === 'kitty';

---

We could contextually type the return type to be a type predicate when appropriate, but I don't know if it'd really be worth it, and we usually never touch return types when performing contextual typing.

[Arnavion]

The issue is that you don't want a boolean-returning function to be assignable to a type-guard.

Why not? Nothing prevents me from implementing a x is Foo function with Math.random() either.

[DanielRosenwasser]

Sure, but you have to intentionally opt in to that, and if you're totally sure you want to sabotage your users' experiences, more power to you. We're trying to avoid people from accidentally assigning an arbitrary function to a type predicate.