Partially also said in the issue, I believe a generic solution that will work in every context is not possible , so better not to pretend it is. It would be nice to have the :invoke-callback hook and some control over lazy sequences (via :realize-max). This control over lazy sequences could however also be implemented via the bindings so this doesn’t have to ship with Sci directly actually. Especially since range is not the only case where something is realized this might reflect the actual situation better.