is there a function for `?:` similar to how there's `get()` for `[]` ? if not, why not ?

# announcements

[LastExceed]

is there a function for ?: similar to how there's get() for [] ? if not, why not ?

[Big Chungus]

No. [] Is syntactic sugar for operator function linked to a specific type, wheras ?: Is language level sugar for if(x ==null) (note how there's no function here)

[Cyril Find]

you can probably use getOrDefault

[LastExceed]

thats for optionals, elvis is for nullables

[LastExceed]

i figured i can just implement it myself

Untitled.kt

[Big Chungus]

getOrDefault("key", "default value") is the same as get("key") ?: "defaultValue" given Map<String, String>

[LastExceed]

oh, so collections have it too. TIL. still can't use it on a nullable variable though

[ephemient]

there's no function for && || either. those and ?: do not evaluate the right hand side unless needed; that is fundamentally different than a function, where all arguments are evaluated before the call.

[LastExceed]

good point, is there a way i can adjust my implementation for that ?

[LastExceed]

nvm figured it out:

Untitled.kt

[Big Chungus]

You can always write an inline function for custom syntactic sugar like that. Then instead f you having to be verbose about it in all usages, you only need to wrappit once and let compiler unwrap it in all call sites

[LastExceed]

i dont understand, can u make an example ?

[Big Chungus]

inline fun <T> T.elvis(fallback: () -> T) = this ?: fallback()

val x = null
val y = x ?: 42
val z = x.elvis { 42 }

[Big Chungus]

It's just a matter of inline keywoard. Read on inline functions in kotlin docs 😉 Your case is one of the main use-cases for inline functions

[LastExceed]

i'm familiar with inline, i just dont see the point of using it here, nor how it makes the result any less verbose. technically you even made it more verbose by adding 1 more word

[LastExceed]

maybe you meant to add inline to my first implementation rather than my second, but i just tried that and it still evaluates the parameter in advance

[Big Chungus]

The point is not that. It's a matter of optimisation. Without inline, you'll get a lambda function allocated for each call. With inline it just unwraps into call sites and the function itself disappears

[Big Chungus]

So you get nicer api without any tradeoffs

[LastExceed]

ah i see, didn't know inline also unwraps the lambda parameter. but how does this make api nicer ? isnt this just a performance optimiuation ?

[Big Chungus]

nicer as compared to not having this function at all and using ?:

[LastExceed]

now im confused

[LastExceed]

oh wait, i think i get it

[Big Chungus]

I'm not comparing inline vs regular function

[Big Chungus]

I'm talking about wrapper function vs just cofing in the check in all call sites

[Big Chungus]

normally you have slight performance tradeoff between the two. With inline you don't

[LastExceed]

you meant that getText().elvis{"no text"}.toUpper() is nicer than (getText() ?: "no text").toUpper() right ?

[Big Chungus]

From your perspective, yes. I personally prefer just using ?:

[Big Chungus]

Basically if you MUST have .elvis() function, at least make it inline

[LastExceed]

alright got you. i agree that in this example ?: is clear enough, but i think it becomes a bit messy in nested scenarios (which to be fair is quite the edge case)

1 more question: why doesnt inlining the first implementation do the trick? inlining basically means pasting the content of the function at the call site which would leave us with just ?: which shouldn't evaluate the fallback unless needed, no ?

[Big Chungus]

correct

[Big Chungus]

it should work fine on first version of .elvis() too

[LastExceed]

well it doesnt

[Big Chungus]

with inline?

[LastExceed]

yes

[Big Chungus]

How do you decide it's not working?

[LastExceed]

Untitled.cpp

Untitled.cpp

[Big Chungus]

Works fine on playground: https://pl.kotl.in/qOlkxbQKw
As you can see run{} is not getting evaluated

[LastExceed]

that doesnt mean the parameter passed to the function isnt evaluated

[LastExceed]

i edited my example to be more complete and readable, try that

[Big Chungus]

Ah, right. No all the parameters passed in the function are resolved beforehand. Basically your execution flow is as follows:
1. Find that fn elvis need an argument of type Int
2. Evaluate fm f to get Int result (which is now compatible with inline
3. Unwrap inline function with that result repeated in all references within function body

[Big Chungus]

Inline only unwraps its body, not the call site stuff like argument resolution

[LastExceed]

welp, guess i'll stick with the other solution then

[Big Chungus]

You still gain benefits of inlining that other version of elvis

[LastExceed]

yeah i see that now

[LastExceed]

for potential future readers: final version is this.
T : Any is needed to guarantee that the lambda doesn't return null
inline makes sure we don't loose any performance as it unwraps the lambda at compile time

Untitled.cpp

[Big Chungus]

you might even experiment with crossinline to unwrap argument lambda too

[LastExceed]

oh, i've never heard of that keyword. gotta look this up

[LastExceed]

ok i looked it up. ?: allows using return as fallback, so elvis should too. therefore no crossinline needed

[Big Chungus]

That wasn't the idea. I was thinking of getting rid of allocating mem for that fallback lambda and instead unwrapping it after ?: via crossinline

[LastExceed]

doesnt inline already do that ?

[LastExceed]

i thought that was the whole point of using inline in the first place

[Big Chungus]

Honestly, I'm not sure anymore 😄 I thought inline just unwraps the function body, but not the arguments

[LastExceed]

it also unwraps the lambda

[LastExceed]

crossinline just prevents non-local returns

[Big Chungus]

Hey, I've learned something new!

[LastExceed]

me too!