Syntax for one-line "nonymous" functions in "declaration style"

[Bev in TX]

> On Mar 27, 2019, at 10:41 AM, Antoon Pardon <antoon.pardon at vub.be> wrote:
> 
> I don't know. Something like the following is already legal:
> 
> f(x)[n] = x * n
> 
> And it does something completly different.

Where would I find information on what this does in the documentation?
Bev in TX