[kennywinker]

I’m not super familiar with kotlin, but swift does the same thing you’re describing so i get where it’s useful. My beef is with including metadata about the executable block (parameter names) inside the block. So per your example:

    f(5) { square(it) }

My preference would be to tag the fn signature on the outside of the braces, something like:

     f(5) [(x)->int]{ square(x) }

I’d also accept ||, (), or nothing as the delimiters around the fn signature. Key point is that it’s OUTSIDE the executable block.

I’m not opposed to using some smarts to infer/simplify the expression when possible. I.e. if it’s a closure with inferable parameter and return types the “it” construct could be used (in swift they use $0, $1, $2 etc for unnamed parameters). Just the only thing inside an executable block {} should be code that gets executed - not type information about that block