Support for Nullable Tables

[JonathanxD]

Given the following code:

class TomlReadTest : FunSpec({
    test("toml read nullable") {
        @Serializable
        data class Key(val value: Long)
        @Serializable
        data class Config(val key: Key?)

        val mapper = Toml(
            config = KtomlConf(
                ignoreUnknownNames = true,
                emptyValuesAllowed = true
            )
        )
        val toml = mapper.decodeFromString<Config>("""            
            [key]
            value = 1            
        """.trimIndent())

        assertNotNull(toml)
        assertEquals(1L, toml.key?.value)
    }
})

It is expected to deserialize Config with key = Key(1L), however ktoml fails with:

This kind of node should not be processed in TomlDecoder.decodeValue(): com.akuleshov7.ktoml.parsers.node.TomlTable@6d9c9c74
com.akuleshov7.ktoml.exceptions.InternalDecodingException: This kind of node should not be processed in TomlDecoder.decodeValue(): com.akuleshov7.ktoml.parsers.node.TomlTable@6d9c9c74
	at com.akuleshov7.ktoml.decoders.TomlDecoder.decodeKeyValue(TomlDecoder.kt:96)
	at com.akuleshov7.ktoml.decoders.TomlDecoder.decodeValue(TomlDecoder.kt:25)
	at com.akuleshov7.ktoml.decoders.TomlDecoder.decodeNotNullMark(TomlDecoder.kt:36)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeNullableSerializableElement(AbstractDecoder.kt:79)

Changing to a property with default value does work:

class TomlReadTest : FunSpec({
    test("toml read nullable") {
        @Serializable
        data class Key(val value: Long)
        @Serializable
        data class Config(val key: Key = Key(0L))

        val mapper = Toml(
            config = KtomlConf(
                ignoreUnknownNames = true,
                emptyValuesAllowed = true
            )
        )
        val toml = mapper.decodeFromString<Config>("""            
            [key]
            value = 1            
        """.trimIndent())

        assertNotNull(toml)
        assertEquals(1L, toml.key.value)
    }
})

All my code base uses Kotlinx Serialization, only the configuration parsing is using Jackson and I would like to remove this.

[orchestr7]

Hello @JonathanxD! Thank you for this issue! I even did not think that someone will use a nullable table… I will try to support it in the nearest release.

But it looks that I will need to change the logic significantly.

PS I am actually confused that the default value for tables worked. I have supported it by chance🤦‍♂️

UPD: fixed in #69