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In html you can only use Strings in your attributes like in the value attribute of input {}. To use other data types in your model you have to specify how to represent a specific value as String (e.g. Number, Currency, Date). When you work with input {} you also need parse the entered text back to your data type. fritz2 provides a special function format for this, which creates a Lens<P, String> for you:

fun <P> format(parse: (String) -> P, format: (P) -> String): Lens<P, String>

The following validation example demonstrates its usage:

import dev.fritz2.lenses.format

object Formats {
    private val dateFormat: DateFormat = DateFormat("yyyy-MM-dd")
    val date: Lens<Date, String> = format(
        parse = { dateFormat.parseDate(it) },
        format = { dateFormat.format(it) }

When you have created a special Lens for your own data type like, you can then use it to create a new SubStore: concatenate your Lenses before using them in the sub() method e.g. sub(L.Person.birthday + Fromat.dateLens) or call the method sub() on the SubStore of your custom type P with your formatting Lens e.g sub(Format.dateLens).

Here is the code from the validation example which uses the special Lens in the Formats object specified above for the com.soywiz.klock.Date type:

import com.soywiz.klock.Date

val personStore = object : RootStore<Person>(Person(createUUID()))
val birthday = personStore.sub(Person.birthday +
// or
val birthday = personStore.sub(Person.birthday).sub(
input("form-control", id = {
    value =
    type = const("date")

    changes.values() handledBy birthday.update

The resulting SubStore is a Store<String>.

You can of course reuse your custom formatting Lens for every SubStore of the same type (in this case com.soywiz.klock.Date).

Distributed by an MIT license.