Define "checked" in Terminology Section

[EvanBurchard]

Example usage:

No parts of f's return value should be checked.

[Avaq]

Any kind of inspection of the value, for example value != null, value instanceof Type, typeof value, value.hasOwnProperty(...) etc.

[SimonRichardson]

We should re-iterate what that actually means in the spec. A lot of libraries don't adhere to that part of the spec, so some clarity might be very useful here.

[EvanBurchard]

@SimonRichardson can you give an example from a library that doesn't conform?

I'm struggling to understand how the prescriptive "Don't use the kinds of things @Avaq pointed out" is more clear than just the signature itself.

map :: Functor f => f a ~> (a -> b) -> f b

I was confused because yeah, it needs a f b, but nothing in the type signature indicates anything in particular about the type of b. Isn't that enough info?

I'm probably wrong on this, but I feel like:

1. It doesn't add more info than the type signature
2. It's oddly framed to be prescriptive, rather than describing the already descriptive type signature.
3. Its prescription is vague (@Avaq's explanation in definition might be aid in clarity if being prescriptive is desirable)
4. There's no clear contrasting scenarios/prescriptions for when something "should be checked," so the terminology seems lopsided and out of place.

I don't know. I just found it confusing. Maybe it's just me.

[rjmk]

@EvanBurchard I think it's to avoid implementations like

MyFunctor.prototype.map = f =>
  MyFunctor.of(f(typeof this.value === 'number' ? this.value + 1 : this.value))

(Though in reality things like null checking are more common)

[SimonRichardson]

Exactly what @rjmk shown, I've seen this in the wild a couple of times (I need to keep a list of this stuff tbh!).