only certain predicates are "built in" - i.e. have an implicit mapping to something. map? gets swapped with [:map], effectively

if you want to use a custom predicate you can either use [:fn object?] - which won't have generation semantics if you use that for your project

there is also a way to make a custom schema key/generator/etc. I just don't know it offhand

https://github.com/metosin/malli#custom-schema-types

it is documented here though

Cheers @U3JH98J4R, this solved it perfectly. I didn't know that it swapped the predicates, this makes sense now!