Is there a way to delegate to a map and initialise it at the

Question

Is there a way to delegate to a map and initialise it at the same time Preferably immutably I m looking at <https kotlinlang org docs delegated properties html storing properties in a map> but I want to initialise inline This pseudo code does not compile is what I want to do ```class User map MutableMap lt String Any gt = mutableMapOf Map lt String Any gt by map val name String = Joe by map val age Int = 3 by map val user = User user toMap == mapOf name to Joe age to 3 user name == Joe user age == 3```

Rob Elliot · Answer

Ideally I don t want to repeat the `name` or `age` key property names

Sam · Answer

Closest I can think of off the top of my head is this ```class User map MutableMap lt String String gt Map lt String Any gt by map var name String by map private set init name = Joe ```

Rob Elliot · Answer

Thanks yes that was where I was heading slightly frustratingly noisy

Sam · Answer

wait a half remembered thought is coming back to me

Sam · Answer

Okay so uh ```class User map MutableMap lt String Any gt Map lt String Any gt by map val name String by map withInitialValue Joe infix fun lt V gt MutableMap lt String V gt withInitialValue initialValue V = PropertyDelegateProvider lt Any Map lt String V gt gt property gt apply put property name initialValue ``` edit omg it can be an infix function too

Sam · Answer

Ever feel like you ve gone too deep and need to take a break from Kotlin joy

Rob Elliot · Answer

Awesome thanks

Rob Elliot · Answer

You can even do this ```class User map MutableMap lt String Any gt Map lt String Any gt by map val name String by map Joe operator fun lt V gt MutableMap lt String V gt invoke initialValue V = PropertyDelegateProvider lt Any Map lt String V gt gt property gt apply put property name initialValue ```

Ivan Pavlov · Answer

If I m not mistaken this <https kotlinlang slack com archives C0B8MA7FA p1675174286818479 thread ts=1675173615 409839 amp cid=C0B8MA7FA> solution initializes values in map when instance is created and other options put value only on the first access to property so they are different

Sam · Answer

I believe that the `PropertyDelegateProvider` is created when the property is declared So that solution would put the value in the map on instance creation too I checked by adding some `println` calls and it does seem to work that way

Sam · Answer

It s different from a normal property delegate which indeed would only be invoked on property access

Ivan Pavlov · Answer

Thanks now I see

Rob Elliot · Answer

< Sam> mind if I write this up in a blog post

Sam · Answer

Go for it Especially if you feel like giving me a shout out wink I m just getting started with my own blog <https medium com sam cooper>

Sam · Answer

Drop a link here if you do I would love to read it

Rob Elliot · Answer

Thanks Here it is <https blog lidalia org uk 2023 02 maps with typed keys in kotlin html>

Sam · Answer

Nice It s really cool to see the use case for it I like how you packaged it up into an abstract class too

Sam · Answer

actually that gives me another idea smile ```abstract class AbstractPropertyMap lt V gt private val properties MutableMap lt String V gt = mutableMapOf Map lt String V gt by properties protected fun mapProperty initialValue V = PropertyDelegateProvider lt Any Map lt String V gt gt property gt properties apply put property name initialValue class Links id String otherId String AbstractPropertyMap lt URI gt val self by mapProperty URI create v1 $id val other by mapProperty URI create v1 other $otherId ```

Sam · Answer

Also can you make it link to my <https medium com sam cooper|medium page> instead of this Slack pray I know shameless self promotion smile

Sam · Answer

I ll link back if I do a post about property delegates the way you ve used it is really cool

Rob Elliot · Answer

Yup that s nicer Done