package com.example.aiapp

/**
 * What a screen knows about something it had to fetch: still finding out, got it, or couldn't.
 *
 * Three states rather than a value alongside a nullable error, because "we couldn't find out" must
 * not share a representation with "there is nothing" -- a failed fetch would otherwise render as an
 * empty list, which is the one wrong answer that looks like a right one.
 *
 * [Loading] and [Error] carry no payload, so they are `LoadState<Nothing>` and this is covariant in
 * [T]: one `LoadState.Loading` serves every screen.
 */
sealed class LoadState<out T> {
    data object Loading : LoadState<Nothing>()

    data class Loaded<out T>(val value: T) : LoadState<T>()

    data class Error(val message: String) : LoadState<Nothing>()

    companion object {
        /**
         * The failure a fetch produces. Api.kt writes its messages to be read on this screen, so
         * this passes one through rather than replacing it; the fallback covers only a throwable
         * with no message at all, which [ApiException] never is.
         */
        fun failed(e: ApiException): Error = Error(e.message ?: "Unknown error")
    }
}