Why does Kotlin for Android Developers (the book) need to add extensions parseList again?

Question

I know Anko provides the functions parseSingle, parseOpt and parseList , I don't understand why the code of Android Developers (the book) need to design extensions parseList again.

Could you tell me? Thanks!

https://github.com/antoniolg/Kotlin-for-Android-Developers/blob/master/app/src/main/java/com/antonioleiva/weatherapp/data/db/ForecastDb.kt

override fun requestForecastByZipCode(zipCode: Long, date: Long) = forecastDbHelper.use {

        val dailyRequest = "${DayForecastTable.CITY_ID} = ? AND ${DayForecastTable.DATE} >= ?"
        val dailyForecast = select(DayForecastTable.NAME)
                .whereSimple(dailyRequest, zipCode.toString(), date.toString())
                .parseList { DayForecast(HashMap(it)) }

}

https://github.com/antoniolg/Kotlin-for-Android-Developers/blob/master/app/src/main/java/com/antonioleiva/weatherapp/extensions/DatabaseExtensions.kt

fun <T : Any> SelectQueryBuilder.parseList(parser: (Map<String, Any?>) -> T): List<T> =
        parseList(object : MapRowParser<T> {
            override fun parseRow(columns: Map<String, Any?>): T = parser(columns)
})

Malagasy

Answer 1

Anko's parseList takes a MapRowParser, not a function. This simplifies usage. With Anko version you'd write

.parseList { mapRowParser { DayForecast(HashMap(it)) } }

instead. That's assuming there is a constructor function like mapRowParser which I can't find in their sources; otherwise, you could write it quite trivially.

Or rather, it's already written for you in the example code, just not as a separate function:

fun <T> mapRowParser(parser: (Map<String, Any?>) -> T): MapRowParser<T> = 
    object : MapRowParser<T> {
        override fun parseRow(columns: Map<String, Any?>): T = parser(columns)
    }

I am honestly really surprised if this function doesn't exist already (maybe called something else, but what?). OTOH, if it does exist, Leiva should have used it.

Carbonic

Thanks! What does HashMap(it) mean? – Malagasy

It's how constructors are called in Kotlin. It copies the input map into a new HashMap (I don't know why, instead of just DayForecast(it)). – Carbonic

Thanks! but .parseList { mapRowParser { DayForecast(HashMap(it)) } } don't work, and .parseList { MapRowParser { DayForecast(HashMap(it)) } } don't work too! Could you give me a full code? – Malagasy

Edited to insert the function. – Carbonic

Sorry, but your code don't work yet, I get the error Error:(28, 19) None of the following functions can be called with the arguments supplied: public final inline fun <T : Any> parseList(parser: MapRowParser<???>): List<???> defined in org.jetbrains.anko.db.SelectQueryBuilder public final inline fun <T : Any> parseList(parser: RowParser<???>): List<???> defined in org.jetbrains.anko.db.SelectQueryBuilder – Malagasy

I guest that there are some problems with your codre fun <T> mapRowParser(parser: (Map<String, Any?>) -> T): MapRowParser<T> = ... – Malagasy

Answer 2

If you expected simple single column rows result, you can use ready-made Anko parsers:

• ShortParser
• IntParser
• LongParser
• FloatParser
• DoubleParser
• StringParser
• BlobParser

Your code might be:

val dailyForecast = select(DayForecastTable.NAME)
                .whereSimple(dailyRequest, zipCode.toString(), date.toString())
                .parseList(StringParser)

Or you can implement your own parser. Here below sample for three columns result (Int, String, String):

val userParser = rowParser { id: Int, name: String, email: String ->
    Triple(id, name, email)
}

Emulsify