Kotlinで作れなかったST

Kotlinで作るST.md

Kotlinで作るST

//TODO 解説

ST.kt

@DslMarker
annotation class STDsl

fun main() {
    val ref = runST {
        object : ST<STRef<*, String>> {
            override fun <S> STScope<S>.invoke() = run {
                val ref = newSTRef("hello world")
                val result = ref.readSTRef()
                println(result)
                ref
            }
        }
    }
    // ref.readSTRef() CE!!!!!!
}

sealed interface STRef<S, A>

private class STRefImpl<S, A>(var a: A) : STRef<S, A>

@STDsl
interface ST<A> {
    @STDsl
    operator fun <S> STScope<S>.invoke(): A
}

@STDsl
data class STScope<S>(val s: S) {
    @STDsl
    fun <A> newSTRef(a: A): STRef<S, A> = STRefImpl(a)

    @STDsl
    fun <A> STRef<S, A>.modifySTRef(f: (A) -> A) {
        when (this) {
            is STRefImpl -> a = f(a)
        }
    }

    @STDsl
    fun <A> STRef<S, A>.readSTRef(): A = when (this) {
        is STRefImpl -> a
    }

    @STDsl
    fun <A> ST<A>.bind(): A = run {
        STScope(Unit)()
    }
}

@STDsl
fun <A> runST(block: () -> ST<A>): A = block().run {
    STScope(Unit)()
}


@STDsl
fun <A, B> ST<A>.map(f: (A) -> B): ST<B> = object : ST<B> {
    override fun <S> STScope<S>.invoke(): B =
        runST { this@map }.let(f)
}

@STDsl
fun <A, B> ST<A>.flatMap(f: (A) -> ST<B>): ST<B> = object : ST<B> {
    override fun <S> STScope<S>.invoke(): B =
        runST { this@flatMap }.let(f).bind()
}