[roenxi]

Spec will give you stronger feedback than a docstring or function signature. It can tell you (in code terms, with a testable predicate) if a call to an interface wouldn't make sense.

Eg, spec can warn you when an argument doesn't make sense relative to the value of a second argument. Eg, with something like (modify-inventory {:shoes 2} :shoes -3) spec could pick up that you are about to subtract 3 from 2 and have negative shoes (impossible!) well before the function is called - so you can test elsewhere in the code using spec without having to call modify-inventory or implement specialist checking methods. And a library author can pass that information up the chain without clear English documentation and using only standard parts of the language.

You can't do that with defrecord, but it is effectively a form of documentation about how the arguments interact.

[valenterry]

> Eg, spec can warn you when an argument doesn't make sense relative to the value of a second argument.

That's something that dependently-typed typesystems easily do as well if not better because inside of the function or data definition, the information is still available and used for code-completion, other compiletime checks etc.

[modernerd]

Does the spec logic typically live inside the modify-inventory function, or elsewhere? If elsewhere, what triggers it before the function is called?

[teataster]

There is very little spec logic. It looks a lot like type declarations in typed languages.

It's usually outside the scope of functions, since you are likely going to want to reuse those declarations. For example, you can use spec to generate test cases for something like quick-check.

You can add pre and post conditions to clojure function's metadata that test wether the spec complies with the function's input/output.