having trouble converting a small bit of java to kotlin. ```for (Map<String, String> serviceO...

# announcements

[snowe]

having trouble converting a small bit of java to kotlin.

for (Map<String, String> serviceOutputs; (serviceOutputs = resultReader.read(header)) != null; ) {
    try {
        final double elapsed = Double.parseDouble(serviceOutputs.get("__elapsed"));

gets converted to

var serviceOutputs: Map<String, String>
while (resultReader.read(*header).also { serviceOutputs = it } != null) {
  try {
    val elapsed = it["__elapsed"]!!.toDouble()

which doesn’t work because it can be null, and serviceOutputs can’t be null.

I can correct this to:

var serviceOutputs: Map<String, String>?
while (resultReader.read(*header).also { serviceOutputs = it } != null) {
    serviceOutputs?.let {
      try {
        val elapsed = it["__elapsed"]!!.toDouble()

but that is pretty messy and not indicative at all of what is happening. serviceOutputs can never be null in the block, and using a let is messy. Any way to accomplish this a bit cleaner?

[Nir]

I guess you actually need the value of serviceOutputs after the while loop ends?

[Nir]

Because, it does not serve any other purpose that I can see, does it?

[Nir]

Actually, serviceOutputs will always be null when the loop exits. so why have it as a variable at all?

[snowe]

ah yeah that was automatic conversion. it’s such a large method I wasn’t looking through to see what was actually happening. EOD and I wasn’t thinking.

[snowe]

let me refactor that out and just use a loop with it and see what I get.

[Nir]

I guess it's still a bit tricky because you can't declare the variable in a while loop itself

[nanodeath]

while (true) {
	val serviceOutputs = resultReader.read(header) ?: break
	val elapsed = serviceOutputs["__elapsed"]?.toDouble() ?: error("No __elapsed field")

[Nir]

yeah, that's where i was going next

[snowe]

that’s a good solution.

[Nir]

while (true) doesn't usually make me feel great but it's pretty practical 🙂

[nanodeath]

I find myself doing while (true) kinda surprisingly often

[Nir]

nicest is probably honestly to factor out this pattern

[Nir]

it's a very obvious pattern. python actually provides a special construct, that lets you transform something like this from a while loop to a for loop

[Nir]

ohhhhh

[Nir]

I know

[nanodeath]

you could probably make some kind of resultReader.readToSequence(header).forEach {

[snowe]

yeah I was playing with a foreach, wasn’t having any luck.

[Nir]

this already exists i think generateSequence

[Nir]

generateSequence { resultReader.read(*header) }.forEach { ... }

[Nir]

Or even:

for (x in generateSequence { resultReader.read(*header) } ) { ... }

[Nir]

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/generate-sequence.html

[snowe]

both of those seem good. As long as I can avoid a bunch of elvis operators, since I don’t care about the null case anyway I think it’ll be good. Let me play around with both of those.

[Nir]

yeah i mean those two its just th eusual for loop vs for each trade-off, pretty orthogonal to everything else

[Nir]

important thing is to get the sequence that we want

[snowe]

yeah, eventually I’m trying to migrate this to a bunch of coroutines, just trying to make sure I don’t break anything in the interim.

thanks for all the help you two!

[Nir]

well, using a sequence is basically a coroutine, in a sense 🙂 Sure, np!