Hey, is there a way to generally reference the value inside a value class? I'm trying to use type-sa...

# getting-started

[ursus]

Hey, is there a way to generally reference the value inside a value class?
I'm trying to use type-safe database ids, and therefore I need a column adapter

class BusinessMessageIdColumnAdapter : ColumnAdapter<BusinessMessageId, String> {
    override fun decode(databaseValue: String): BusinessMessageId = BusinessMessageId(databaseValue)
    override fun encode(value: BusinessMessageId): String = value.value
}

for every type of id, which is annoying

I know I could get it via reflection, but .. probably looking for a "hack" like this (the enumValues function)

@Suppress("FunctionName") // Emulating a constructor.
inline fun <reified T : Enum<T>> EnumColumnAdapter(): EnumColumnAdapter<T> {
  return EnumColumnAdapter(enumValues())
}

[ephemient]

on JVM you can do

class EnumColumnAdapter<T : Enum<T>>(private val javaClass: Class<T>) {
    fun decode(databaseValue: String): T = java.lang.Enum.valueOf(javaClass, databaseValue)
    fun encode(value: T): String = value.name
}

EnumColumnAdapter(BusinessMessageId::class.java) // etc.

[ephemient]

on all platforms you can do

class EnumColumnAdapter<T : Enum<T>> @PublishedApi internal constructor(
    private val valuesByName: Map<String, T>
) {
    fun decode(databaseValue: String): T = valuesByName.getValue(databaseValue)
    fun encode(value: T): String = value.name
}

inline fun <reified T : Enum<T>> EnumColumnAdapter(): EnumColumnAdapter<T> =
    EnumColumnAdapter(valuesByName = enumValues<T>().associateBy { it.name })

EnumColumnAdapter<BusinessMessageId>() // etc.

[ephemient]

you can't make a inline reified constructor, but you can make a constructor-like function

[Jonathan Mew]

We made all our typesafe strings extend a single StringMicrotype class, and made a single StringMicrotype ColumnAdapter that'll work for all of them.

It does mean introducing some class hierarchy which might feel a bit artificial, and a separate adapter for each type of data required. Might work for you though!

[ursus]

how do you instantiate it back from string if the adapter only sees the StringMicrotype base class? like @ephemient did?

[ursus]

@ephemient I dont think I follow, the id is not enum, or r u hijacking it somehow?

[Jonathan Mew]

how do you instantiate it back from string

could you give me an example, please?

[ursus]

well how do you instantiate BusinessMessageId from string generically, if presume you BusinessMessageId : StringMicrotype and the adapter has a StringMicrotype parameter

[ephemient]

I assumed BusinessMessageId was an enum

[ephemient]

when used genetically a value class is just a normal class

[ephemient]

so you could build something like

class ColumnAdapter<T>(
    private val encode: (String) -> T,
    private val decode: (T) -> String,
)

ColumnAdapter(
    encode = ::BusinessMessageId,
    decode = BusinessMessageId::value,
)

but you can't get a ::T constructor for a generic type <T>, so that's about as far as you can go

[ursus]

yea I wanted to get typeinference like in the enumcolumnadapter, shame

[Jonathan Mew]

Sorry, I was being daft - our StringMicrotypeColumnType is abstract, we have separate column classes, each like

class EmailAddressColumnType : StringMicrotypeColumnType<EmailAddress>() {
    override fun fromString(s: String) = EmailAddress(s)
}

A decent bit of logic is still shared, but not as neat as you're after

[ephemient]

with some horrible reflection,

@JvmInline
value class Id(val value: String)

val converter = Converter<Id>()
println(converter.encode(Id("Hello, ")) + converter.decode("world!"))
// => Hello, Id(value=world!)

Untitled.cpp

[ephemient]

note that the value-ness of Id doesn't come into play at all: when generic, you're dealing with the class Id , not the underlying String representation, just like how <T : Int> is also always boxed

[ursus]

thank you