Type Refinements with Type Proofs in Kotlin

TwitterHandle.kt

/* Coming up ~ April 2020 */

package test
import arrow.*

inline class TwitterHandle
@Suppress("NON_PUBLIC_PRIMARY_CONSTRUCTOR_OF_INLINE_CLASS")
private constructor(val handle: String) {
  companion object : Refined<String> {
    override val validate: String.() -> Map<String, Boolean> = {
      mapOf(
        "Should start with '@'" to startsWith("@"),
        "Should have length <= 16" to (length <= 16),
        "Should not contain the word 'twitter'" to !contains("twitter"),
        "Should not contain the word 'admin'" to !contains("admin")
      )
    }
  }
}

/**
 * Similar extensions can target Validation and Either not just nullable types
 * Proofs `String` can be coerced safely into a TwitterHandle
 */
@Proof(TypeProof.Extension, coerce = true)
fun String.twitterHandle(): TwitterHandle? =
  if (TwitterHandle.isValid(this)) TwitterHandle(this)
  else null

val admin = "@admin"
val whatever = "@whatever"

/* Fails to Compile:
 * "@adminxxxxxxxxxxxxxxxxxxxxxxxx" is not a valid TwitterHandle because:
 * e: Should not contain the word 'admin'
 * e: Should have length <= 16 
 */
TwitterHandle("@adminxxxxxxxxxxxxxxxxxxxxxxxx") 

TwitterHandle("@whatever") 
//TwitterHandle("@whatever") 

val ok: TwitterHandle = "@whatever" 
//TwitterHandle("@whatever") //ok

val nullableOk: TwitterHandle? = "@whatever" 
//TwitterHandle("@whatever") 

val runtimeAdminWillBeNull: TwitterHandle? = admin 
//null

val runtimeWhateverIsValidated: TwitterHandle? = whatever 
//TwitterHandle("@whatever")