I was writing an expression which had to end up wi...

# getting-started

[Rob Elliot]

I was writing an expression which had to end up with a cast, then a method call and wrapping the whole thing in brackets was looking really ugly by the time ktlint had finished with it:

private val jwtVerifier = (
  JWT.require(Algorithm.RSA256(jwkRSAProvider))
    .acceptLeeway(3) as JWTVerifier.BaseVerification
)
  .build(clock)

So I pulled out an extension function as so:

private val jwtVerifier = JWT
  .require(Algorithm.RSA256(jwkRSAProvider))
  .acceptLeeway(3)
  .castTo<JWTVerifier.BaseVerification>()
  .build(clock)

@Suppress("UNCHECKED_CAST")
private fun <B> Any.castTo(): B = this as B

Is there an existing equivalent of castTo I'm missing?

Is there any way to ensure that the receiver is a supertype of the target type, without having to pass both as type arguments at the call site?

[Sam]

I think your best option would just be to use let.

.acceptLeeway(3)
.let { it as JWTVerifier.BaseVerification }
.build(clock)

That way you get the formatting you want, but still get the IDE warnings and other inspections that come with using as.

[Rob Elliot]

That's nice, thanks

[Sam]

I think something like what you're asking for is possible with some annotation hackery:

@Suppress("INVISIBLE_REFERENCE", "INVISIBLE_MEMBER")
fun <A, B : A> @kotlin.internal.Exact A.castTo() = this as B

Then using _ at the call site:

.acceptLeeway(3)
.castTo<_, JWTVerifier.BaseVerification>()
.build(clock)

But the resulting behaviour is still less intelligent than the built-in IDE inspections 😞

Any().castTo<_, String>() // ✅
"Hello".castTo<_, Int>() // ❌
("Hello" as Any).castTo<_, Int>() // ✅

At the end of the day, you'll always be able to come up with some example of a downcast that looks fine but fails at runtime

[Rob Elliot]

Though now it occurs to me that perhaps this is nicer still:

private val jwtVerifier = JWT
  .require(Algorithm.RSA256(jwkRSAProvider))
  .acceptLeeway(3)
  .withAudience(audience)
  .build(clock)

fun Verification.build(clock: Clock): JWTVerifier = (this as JWTVerifier.BaseVerification).build(clock)

[Sam]

😄 good point

[Sam]

Easy to get carried away with cool Kotlin features and forget about the good old fashioned solutions

[Sam]

I like it 👍