A Kotlin extension function to Retrofit Call class

Call.kt

inline fun <T> Call<T>.enqueue(crossinline onResult: Result<T>.() -> Unit) {
    enqueue(object : Callback<T> {
        override fun onFailure(call: Call<T>?, t: Throwable?) {
            onResult(Result.Error(ApiError("Network error")))
        }

        override fun onResponse(call: Call<T>?, response: Response<T>) {
            onResult(response.result())
        }
    })
}

fun <T> Response<T>.result(): Result<T> {
    if (isSuccessful) {
        val body = body()
        return if (body != null) {
            Result.Success(body)
        } else {
            Result.Error(ApiError("Empty body"))
        }
    }

    return Result.Error(ApiError.from(errorBody()))
}

Result.kt

//this is inspired(!) from https://github.com/nickbutcher/plaid
sealed class Result<out T> {

    data class Success<out T>(val data: T) : Result<T>()
    data class Error(val error: ApiError) : Result<Nothing>()

    override fun toString(): String {
        return when (this) {
            is Success<*> -> "Success[data=$data]"
            is Error -> "Error[error=$error]"
        }
    }
}

Usage.kt

api.getFoo()
    .enqueue {
        when(this) {
            is Result.Success -> {
              //use data
            }
            is Result.Error -> {
              //use error
            }
        }
    }