Scala Option's collect method doesn't like my PartialFunction

I think I'm missing something:

scala> S开发者_StackOverflow中文版ome(1) collect ({ case n if n > 0 => n + 1; case _ => 0})
res0: Option[Int] = Some(2)

scala> None collect ({ case n if n > 0 => n + 1; case _ => 0})   
<console>:6: error: value > is not a member of Nothing
       None collect ({ case n if n > 0 => n + 1; case _ => 0})
                                 ^
<console>:6: error: value + is not a member of Nothing
       None collect ({ case n if n > 0 => n + 1; case _ => 0})

Why is this error happening? I think I'm misunderstanding how collect works...

---

Unless you specify, the literal None is of type Option[Nothing]. This is necessary, since None has to be a valid member of all types Option[_]. If you instead wrote

(None:Option[Int]) collect ({ case n if n > 0 => n + 1; case _ => 0}) 

or

val x:Option[Int] = None
x collect ({ case n if n > 0 => n + 1; case _ => 0}) 

then the compiler would be able to type check your collect call

---

None collect ({ case n if n > 0 => n + 1; case _ => 0}) 

Why would n have the method >? There's nothing in there to allow the compiler to assume that. So, try changing that to:

None collect ({ case n: Int if n > 0 => n + 1; case _ => 0})

And you'll get the following error message:

<console>:8: error: pattern type is incompatible with expected type;
 found   : Int
 required: Nothing
       None collect ({ case n: Int if n > 0 => n + 1; case _ => 0}) 
                               ^

Meaning, basically, that the compiler knows an Int is impossible here, since you are just passing None. As it happens, None is of type Option[Nothing].