my assumption was that the lambda in getOrElse sho...

# codereview

[amit]

my assumption was that the lambda in getOrElse should need to return Heros object as default value

[Shawn]

unsure if you’ve realized yet, but this is why age1 is typed as Any instead of Hero - you’re supplying an Int as a default for a get that returns Hero?

[Shawn]

you’re conflating steps - getting an object out of the map and accessing a property in that property

[Shawn]

you’re only doing the former

[Shawn]

this is still being statically checked, by the way, but the type system inference sorta relies on you knowing what you’re doing

[amit]

Thanks @Shawn for reply....but getOrElse accepts lambda of type () -> V where V should ideally be match with value type present in Map<K, V>.....so effectively getOrElse should accepts () -> Heros but it is also accepting () -> Int

[amit]

I am a beginner so I am little bit confused here....but can be trivial question

[amit]

Can you please direct me to any resource which talks more about this?

[Shawn]

Basically the short answer is that this is happening due to variance, specifically because the signature of the map interface isn’t Map<K, V>, it’s Map<K, out V>. When calling .getOrElse() with a lambda that returns a type that isn’t V or a subtype, the type system infers that you intend to operate on a map where K is the same, but V is now a supertype of the original map’s V, since it’s a covariant type parameter

[Shawn]

in this case V is resolved to be the only thing it can be, which is Any

[Shawn]

if you try calling getOrElse with explicit type parameters like this

mapByName.getOrElse<String, Any>("unknown") { 0 }

IntelliJ will gray out the parameters and indicate that they are redundant since this is what the type system has already resolved

[Shawn]

you can also see this in action by just assigning the map to a valid supertype

val superMap: Map<String, Any> = mapByName

[Shawn]

if you explicitly annotate age1 as Hero, or change the invocation to .getOrDefault<String, Hero>() then the type checker will raise an error

[amit]

yes that I tried...thanks for the information 🙂

[Shawn]

https://kotlinlang.org/docs/reference/generics.html

[amit]

thanks...Need to go through it....much to digest