am i understanding it right that using `withDefaul...

# stdlib

[bbaldino]

am i understanding it right that using withDefault for a map doesn't actually insert that default into the map when a key which isn't present is found, it just returns an instance of that default?

[nfrankel]

this is the code for the function:

public fun <K, V> Map<K, V>.withDefault(defaultValue: (key: K) -> V): Map<K, V> =
    when (this) {
        is MapWithDefault -> this.map.withDefault(defaultValue)
        else -> MapWithDefaultImpl(this, defaultValue)
    }

so your assumption is correct

[bbaldino]

do you know of an API/idiom to get the other behavior? (insert a initial value the first time a key which isn't present is retrieved)

[karelpeeters]

That's just the factory function, you need to look at the getValue source @nfrankel

[karelpeeters]

This is so easy to test though:

val map = mutableMapOf<String, Int>().withDefault { 5 }
println(map.getValue("test"))
println(map.keys)

[karelpeeters]

No, it doesn't insert it.

[bbaldino]

yes, and i had seen that behavior in code i wrote, i just wanted to double check i wasn't nuts/missing something

[karelpeeters]

I see.

[bbaldino]

in my head 'withDefault' strongly suggested it would be inserted, separating it from getOrDefault

[karelpeeters]

For the other behavior you need getOrPut although you then have to repeat the default value.

[bbaldino]

great, i had missed getOrPut. i think i can live with that, though the other style would've been nice. i run into this somewhat often when having a map of some key to a list

[diesieben07]

A map which inserts when you call get violates the Map interface contract and thus is a bad idea.

[bbaldino]

sure, i'd be fully fine with a separate map type that did this. i'm thinking of DefaultDict in python (that was actually my initial interpretation of what withDefault did: return me a wrapped map type which had that behavior)