Beautiful Validation... Why have I never thought of this before?!

readme.md

At a certain point you just want pattern matching.

example-user.clj

(defn validates-credentials [username password]
  (let [uc (count username)
        pc (count password)]
    (match [username uc password pc]
       [(:or nil "") _ _ _]         {:error "No username given" :field "name"}
       [_ _ (:or nil "") _]         {:error "No password given" :field "pass"}
       [_ (_ :guard #(< % 3)) _ _]  {:error "Username less than 3 characters" :field "pass"}
       [_ _ _ (_ :guard #(< % 4))]  {:error "Password less than 4 characters" :field "pass"}
       [#"^([a-z0-9-_]+)$" _ _ _]   {:error "Username contains invalid characters" :field "name"}
       :else true)))

example-user.js

  var user = {
    validateCredentials: function (username, password) {
      return (
          (!(username += '') || username === '') ? { error: "No Username Given.", field: 'name' }
        : (!(username += '') || password === '') ? { error: "No Password Given.", field: 'pass' }
        : (username.length < 3)                  ? { error: "Username is less than 3 Characters.", field: 'name' }
        : (password.length < 4)                  ? { error: "Password is less than 4 Characters.", field: 'pass' }
        : (!/^([a-z0-9-_]+)$/i.test(username))   ? { error: "Username contains invalid characters.", field: 'name' }
        : false
      );
    }
  };

  var results = user.validateCredentials('Nijikokun','somepassword');
  console.log(results);

I'm not sure I like pattern matching for this use case. Suppose you add another parameter, say, because the requirements change such that you need to verify the password. You can create a guard to ensure that both passwords match, but now I have to maintain all the other patterns by adding _ and keeping track of those. At some point, you just want composable functions.

I don't see how verifying a password requires another another pattern. You test the password, and it's either equal to the thing you want to verify or not.

At some point you just want composable rules - core.logic.

Sure, but now you have error handling outside of the match statement, and you've added an if statement, etc. Validation is tricky. Pattern matching doesn't make it any less so.

Don't disagree. But for simple cases like this pattern matching is fine. For something more sophisticated ... yes, you need something more sophisticated :)

this is pretty gouda :3