Nice! I was actually talking to David a while back about whether this would be possible - like implementing an Interface in code, but as a sort of cluster on the GH canvas. Great to see you’ve made it work.
I think the general principle allowing to define functions without code could also be applied to several other types of functions beyond Fields. For example, defining custom Goals for Kangaroo, the mathematical Functions in GH2, root finding…
I guess performance will always be an issue to some degree, but the advantages for accessibility and quick prototyping could outweigh that.