Why when extending a type would I lose properties ...

# getting-started

[Mike R]

Why when extending a type would I lose properties on it? This is my code:

fun Response<*>.createError(): Error {
    val errorBody = this.errorBody() ?: run {
        logger.error { "createError failed: null errorBody found" }
        throw ClassCastException("Null errorBody found on response")
    }

    val error = Gson().fromJson(errorBody.string(), models.Error::class.java)
    return error
}

What is happening is I am losing the contents of errorBody when I try to use the receiver (Response)... I've read over the docs on class extension and I don't see anything about this?

[Mark Murphy]

what do you mean by "losing the contents"?

[Mike R]

Well, what happens is that before I call the extension function I can access the contents of errorBody just fine. Once I am in the scope of the extension function, errorBody no longer has the contents (a string)

[Mark Murphy]

so, you're getting the manually-thrown ClassCastException when you call createError()?

[Mike R]

Nope. I enter the scope of createError() just fine, but once inside when I call errorBody() it is just an empty string

[Mike R]

If it helps, Response is a type class belonging to the retrofit library

[Mark Murphy]

have you put a breakpoint in createError() and stepped into errorBody() to see what's happening?

[Mike R]

I have not, I've just been using the debugger and looking at what it is showing me. I haven't actually stepped in.

[Mike R]

Let me give that a shot

[Mark Murphy]

AFAIK your createError() function should be able to call errorBody() just fine -- if you are trying to call errorBody() multiple times, though, I don't know if there are any Retrofit-specific limitations that you're running into

[Mike R]

That's the thing - I can call errorBody() fine, it's just that it loses any context that it had before it enters the scope of createError()

[Mike R]

Let me take a screenshot, that might help explain

[Mark Murphy]

my point is that your outside-of-scope errorBody() call might be consuming the content, such that by the time your second, inside-of-scope errorBody() call is made, the content is gone -- streaming APIs sometimes have destructive read behaviors like that

[Mike R]

Before entering createError()

[Mike R]

After entering createError()

[Mike R]

Ohhhhh - okay, I see what you mean. That is interesting - I hadn't thought about the fact that it may consume it and then no longer be available!

[Mark Murphy]

I don't think I've ever tried calling errorBody() twice on the same Response, so I'm not sure what the behavior is supposed to be

[Mark Murphy]

but your screenshots feel like "content was consumed" more than "Kotlin extension functions are screwy" 😄

[Mike R]

YUP!

[Mike R]

You were spot on!

[Mike R]

As soon as I removed my original logging function that DID call errorBody() it started working!

[Mike R]

Wow, I've never encountered something like that before. I appreciate the help Mark!

[Mark Murphy]

you're welcome!

[iammini5]

i suppose it is same as - https://youtrack.jetbrains.net/issue/KT-19452