While experimenting with context parameter and bac...

# functional

[jean]

While experimenting with context parameter and backing fields, I noticed that this works

context(some: String)
val test: () -> Int
    get() = { some.toIntOrNull() ?: 0 }

And this seems to be ok for the compiler, it doesn’t complain

val test: context(String) () -> Int = { /*some.toIntOrNull() ?:*/ 0 }

But the problem is that I can’t name the context parameter, I get this error “Named context parameters in function types are unsupported. Use syntax ‘context(Type)’ instead.” Is there a way to access the string from the lambda without naming it?

[jean]

I know at that point I could just use partial application like

fun test(some: String): () -> Int = { some.toIntOrNull() ?: 0 }

But I was not expecting the compiler to be ok with a “non-usable” context parameter

[Youssef Shoaib [MOD]]

This is a general "problem" with any contextual lambda.
That's why the stdlib also includes the contextOf function.
In your example, it can be used like:

val test: context(String) () -> Int = { contextOf<String>().toIntOrNull() ?: 0 }

[jean]

aha I see, I did not know about contextOf<String>(), thanks!

[jean]

Do you think the explicit backing field feature from 2.3 could help?

context(some: String)
val test: () -> Int = { some.toIntOrNull() ?: 0 }

This gives “Property with context parameters cannot be initialized because it has no backing field”.

[Youssef Shoaib [MOD]]

I don't see how that's meant to help.
Do you want this?

context(some: String)
val test: () -> Int get() = { some.toIntOrNull() ?: 0 }

This means that some is eagerly bound when you call test, but the returned value is still a lambda (a no-arg one)

[jean]

Yes, I wondered if there would be a way to be able to declare it like so