If I am mistaken about how these terms are used, then the first code comment needs to be changed. Either way, the current code comments are inconsistent and confusing. The way they currently read sounds like it is applying covariant logic to both explanations, even though the parameter/consumer type is and should be contravariant and the return/producer type is and should be covariant.

expected here refers to the definition off:

valf:Function[Buyable,Buyable]= b=> b

So it both expects and returns aBuyable.
Does that make more sense now?

You can try to extend the sentence with the verb used:

// OK to return a Buyable where an Item is expected **to be returned**// OK to provide a Book where a Buyable is expected **to be provided**

It seems to me that "expected" should consistently refer either a type inf or a type in the line of code following the comment (a type ing orh). I don't think it is good for the meaning of the term "expected" to change in these two comments.

Here is the complete example with both comments:

val f: Function[Buyable, Buyable] = b => b// OK to return a Buyable where a Item is expectedval g: Function[Buyable, Item] = f// OK to provide a Book where a Buyable is expectedval h: Function[Book, Buyable] = f

The first comment is comparing the return type ofg to the return type off. If "expected" refers to the type ing it is correct, but if it refers to the type inf it is wrong.

The second comment is comparing the parameter type ofh to the parameter type off. If "expected" refers to the type inh, it is wrong, but if it refers to the type inf it is correct.

I don't think swapping the meaning of "expected" between these two comments is helpful because it makes it sound like a line of reasoning rather than a statement of what is or how the language works. Then it can become confusing because it sounds like it is using covariant reasoning to defend contravariant types by always putting the more specific type first. This could imply that the reason it makes sense is because a "Buyable" is an "Item" (which intuitively makes sense), and that "Book" is a "Buyable" (which also intuitively makes sense, and thus is misleading).

Perhaps the types should be flipped in the first comment instead of the second. Also, changing the word "OK" to "Allowed" would go even further in the direction of making these sound like statements of what is / how it works, rather than a line of reasoning.

@joroKr21 care to respond further?

Well I think it's precisely the variance that determines in which direction to apply "expected". Let's say we expect a typeF[A] and provide a typeF[B]

• ifF[+_] is covariant, we expect thatA <:< B
• ifF[-_] is contravariant we expect thatB <:< A
• ifF[_] is invariant we expect thatA =:= B