I'm playing around with kotlin `contracts` and hav...

# announcements

[bohsen]

I'm playing around with kotlin contracts and have run into a problem I need some expert help with.

I'm trying to validate user input from a method that takes a couple of nullable parameters and would like to use contracts and the below checkParamIsNotNullOrBlank-function to avoid null checks at the end of my expression. Here's a simplified example of what I'm trying to achieve:

@ExperimentalContracts
    override fun validateInput(
        firstParam: String?,
        secondParam: String?,
        thirdParam: String?,
        listener: ValidationListener?) {
        var error: Boolean
        error = isParamNullOrBlank(firstParam) { view?.firstParamEmptyError() }
        if (secondParam.isNullOrBlank()) {
            view?.secondParamEmptyError()
            error = true
        }
        if (thirdParam.isNullOrBlank()) {
            view?.thirdParamEmptyError()
            error = true
        }
        if (error == false) {
            val validObject = Object.Builder()
                .withFirstParam(firstParam)
                .withSecondAndThirdParam("$secondParam^$thirdParam")
                .create

            listener?.onValidated(validObject)
        }
    }

Here's my "contracts"-method:

@ExperimentalContracts
fun isParamNullOrBlank(param: String?, onInvalid: (() -> Unit)? = null) : Boolean {
    contract {
        returns(false) implies (param != null)
    }
    val result = param.isNullOrBlank()
    onError?.invoke()
    return result
}

This doesn't compile because the compiler can't infer that firstParam is not null. Also every if expression should be evaluated as the view can display multiple errors.

[kralli]

Well, the compiler simply cannot infer that firstParam is not null. It could do it if you would use the result of checkParamIsNotNullOrBlank in an if expression directly, but it cannot do it here because error being mutable and modified breaks the contract. In fact creating a variable out of the result of contract methods breaks it already.

[bohsen]

@kralli Ahh of course. Thanks.