What is the expected way to check for a function? Something like [:map [:handler fn?]] It looks like fn? is not one of the symbols exported in https://github.com/borkdude/sci/blob/cb1dc3139a53c680b2bdc1038d2c6e26b975ee8e/src/sci/impl/namespaces.cljc#L96 I could do something like [:map [:handler [:fn fn?]]], but I'm wondering if this is something that will be supported for serialization.

I think @borkdude knows if sci could support fn?

sure. PR welcome, it should be added about here: https://github.com/borkdude/sci/blob/ab5ba82fa61ab47b7ace8b3e58bfa633ae8e1f72/src/sci/impl/namespaces.cljc#L201

slipped through, don't know why

Awesome, [:map [:handler [:fn 'fn?]]] would work after that & survive the serialization