What do I use if I just want to ensure that a fn returns a string or something?

(s/fdef ::string-fn
        :args (s/cat)
        :ret string?)
(s/explain ::string-fn #(identity "string"))

It wants me to add test.check as a dependency

I think you’re mixing syntax here and that’s leading you down a path trying to gen anonymous functions with check

(s/def ::string-fn (s/fspec :args (s/cat) ret string?)) would be better for this

When function specs are validated they use the generator for the anonymous function to invoke the function being validated and verify the outputs. Using that generator requires test.check

Ah hah. That makes sense. Thank you, Alex.