Map Lambda with Side-Effects

["Nordlöw" via Digitalmars-d-learn]

On Wednesday, 14 January 2015 at 01:15:57 UTC, Tobias Pankrath 
wrote:
>
>> Is it possible to
>> - detect that a lambda is has-side-effects and that
>> - the map hasn't been used?
>
> Thing is: I'm regularly doing that on purpose.

Could you show me a code example? I'm curious.

> Actually, isn't your closure even weakly pure in your example, 
> because arr is part of the closure and thus an argument to it.

I'm fully understand the meaning of weakly and strongly pure 
functions in D. However I'm not enough familiar with closure 
purity semantics in D. Could you elaborate a bit, please?