Require generator for :gen/fmap

[frenchy64]

:gen/fmap is fed nil if its schema cannot create a generator. This does not seem like desirable semantics because silent failures could happen if the :gen/fmap function is permissive, like first or a keyword, making the entire generator return nils.

Could we reconsider this? Perhaps :gen/fmap predated :gen/return and :gen/elements, but we can certainly now force the user to be explicit here. Importantly, is this worth the risk breaking things by fixing it?

Also includes a fix for the CLJS version of alphanumeric-char? in the tests.

[ikitommi]

I think this is a good idea.

[ikitommi]

Perhaps :gen/fmap predated :gen/return and :gen/elements

it is, but I as the orginal test showed, I think this was just not thought of. clojure.test.check.generators/fmap always requires a generator, so malli should too.

[ikitommi]

thanks!