How to properly filter an Enumerator? (onchain filter on stackitem Enumerator)

[igormcoelho]

This appeared here: #877 and on NFT discussions, and many places else...

Suppose I have an Enumerator stack item:

Enumerator e = GetEnumerator();

How can I filter it? My first thought went back to OPEVAL discussions, but perhaps we need something less dangerous and already existing on neovm... delegates?

Enumerator e2 = e.Filter(x => hasProperty());

Could we pass this property to bool function as a delegate on C#? @lightszero @erikzhang @lock9 @shargon

Perhaps this could generate something like:
System.Enumerator.Filter [parameter method position in code]

example (draft):

delegate filterfunc(x) : int -> bool = x > 5; // don't know how to write it...

Enumerator e;
e.Filter(filterfunc);
// filterfunc is compiled to position 0001

System.Enumerator.Filter 0001

[igormcoelho]

Agree with @shargon, json is very useful in.this context.

[lock9]

Is this a duplicate of #880 ? Do you want to close this and continue only in #880 ?

[igormcoelho]

@erikzhang I'll reopen this.

Now we can finally do it, using neo-project/neo-vm#190

[erikzhang]

No, you can't use it for data filtering. Because it cannot be passed to other contracts. So we need #284. That's why I close this.