Hi I just ran into this quirky behavior combining if stateme kotlinlang #getting-started

val nonNullVal = 
if(condition1) { 
    someOperation.get() // null 
} else if (condition2) { 
    someOperationTwo.get() // null 
} else { 
    null 
} ?: defaultVal

When condition 1 is true our

nonNullVal

evaluates to

null

. When condition 1 is false and condition 2 is true our

nonNullVal

evaluates to

defaultVal

What is happening? X_x I would just like to get a grasp of what is going on internally. Any help is appreciated! Thanks!

Jacob

04/27/2022, 4:49 AM

my guess is that everything after the first else is like its wrapped in {}

I can't find a lot of docs about else-if seems to me every else-if is really just

else {if...}

ephemient

04/27/2022, 5:54 AM

https://kotlinlang.org/spec/expressions.html#conditional-expressions yes

so what happens here is that you have

if (condition1) { block } else expression

where

expression

if (condition2) { block } else { block } ?: defaultVal

Goldcrest

04/27/2022, 6:03 AM

I was just typing this but you were faster 😄 : So what I thought was

val nnv = 
( if(a) null 
else if(b) null 
else null )
?: defaultVal

is actually

val nnv = if(a) null else (if(b) null else null ?: defaultVal)

?! That explains it! Thanks you so much! I’m glad I asked 😃

ephemient

04/27/2022, 6:04 AM

try using a single

when

to get your desired behavior,

when {
    condition1 -> ...
    condition2 -> ...
    else -> null
} ?: defaultVal

the precedence in the grammar here does seem confusing, though. it may be worth filing a YouTrack issue to ask if they can modify the grammar to account for if-else chains

I've brought it up in https://kotlinlang.slack.com/archives/C0B9K7EP2/p1651041143051719, and in the meanwhile, you can either use

when

inline fun <R> runIf(cond: Boolean, then: () -> R): R? = if (cond) then() else null

val nonNullVal = runIf(condition1) {
    someOperation.get() // null
} ?: runIf(condition2) {
    someOperationTwo.get() // null
} ?: defaultVal

as borrowed from https://youtrack.jetbrains.com/issue/KT-43712

Goldcrest

04/27/2022, 8:37 AM

Thanks! For now I’ll just call it a TIL and carve it onto my heart to use

when

expression instead of

else if

whenever possible.

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