kotlin-contracts-returns-case.kt

/**
 * @author Adib Faramarzi (adibfara@gmail.com)
 */
fun Any?.isAValidName():Boolean{
    return this!=null && this is String && this.length > 3
}

fun getName():String? { ... }

fun testUserName(){
    val name = getName()

    if(name.isAValidName()){
        // compile error: Type mismatch: String is expected but String? was found.
        val fullName:String = name
    }
}

kotlin-contracts-returns-fixed.kt

/**
 * @author Adib Faramarzi (adibfara@gmail.com)
 */
@ExperimentalContracts
fun Any?.isAValidName():Boolean{
    contract {
       returns(true) implies (this@isAValidName is String)
    }
    return this!=null && this is String && this.length > 3
}

fun getName():String? { ... }

@ExperimentalContracts
fun testUserName(){
    val name = getName()
    
    if(name.isAValidName()){
        // No compile error! The compiler knows that because we told it with `implies`.
        val fullName:String = name
    }
}