Type checking function body via generators

Type checking function body via generators.md

Problem

Given some function specs we would like to know if there are bugs in a function definition associated with invalid function calls.

Example

(s/fdef foo :args (s/cat :x number? :y number?) :ret string?)
(s/fdef bar :args (s/cat :z coll?) :ret number?)

(defn baz []
  (let [a (foo 1 2)]
        b (bar a)]
    b))

In this specific example, we would like to check that a (the value returned from the call to foo) will always satisfy the :arg spec for bar.

Reframing as generative check

Stated another way: we need to check that the string? generator produces values which satisfies the coll? predicate.

We can do that by generating values and testing them using s/exercise and s/valid?.

Cache facts & examples

We can also cache the output in case we come across this combination again.

Additionally, we can keep examples of values which don't validate to improve error messages with a concrete example.

Unresolved

• What about transitive stuff? Are specs insufficient to catch case where a function's input type relates to it's output type.
• Specs are typically defined/available after function is declared. That means analysis would need to be a separate sweep.