MutationReceiver

internal val customClientKey = ContextPropertyKey<CustomClient>()

val ContextReceiver.customClient: CustomClient?
    get() = get(customClientKey)

var ContextReceiverBuilder.customClient: CustomClient
    get() = error("You cannot retrieve a builder property directly.")
    set(value) = set(customClientKey, value)

inline fun <T, S> buildCustomMutationKey(
    id: MutationId<T, S>,
    crossinline mutate: suspend CustomClient.(variable: S) -> T
): MutationKey<T, S> = buildMutationKey(
    id = id,
    mutate = {
        val client = checkNotNull(customClient) { "customClient isn't available. Did you forget to set it up?" }
        with(client) { mutate(it) }
    }
)

MutationReceiver {
    customClient = newCustomClient()
}

class CreatePostKey : MutationKey<Post, PostForm> by buildCustomMutationKey(
    mutate = { body -> // CustomClient.(PostForm) -> Post
        doSomething(body)
    }
)