Meaningful error message for Clojure.Spec validation in :pre

Question

I used the last days to dig deeper into clojure.spec in Clojure and ClojureScript.

Until now I find it most useful, to use specs as guards in :pre

Answer 1

Without taking into account if you should use pre and post conditions to validate function arguments, there is a way to print somewhat clearer messages from pre and post conditions by wrapping your predicate with clojure.test/is, as suggested in the answer below:

How can I get Clojure :pre & :post to report their failing value?

So then your code could look like this:

(ns pre-post-messages.core
  (:require [clojure.spec :as s]
            [clojure.test :as t]))

(defn person-name [person]
  {:pre [(t/is (s/valid? ::person person))]
   :post [(t/is (s/valid? string? %))]}
  (str (::first-name person) " " (::last-name person)))

(def try-1
  {:first-name "Anna Vissi"})

(def try-2
  {::first-name "Anna"
   ::last-name "Vissi"
   ::email "Anna@Vissi.com"})

(s/def ::person (s/keys :req [::first-name ::last-name ::email]))

Evaluating

pre-post-messages.core> (person-name  try-2)

would produce

"Anna Vissi"

and evaluating

pre-post-messages.core> (person-name  try-1)

would produce

FAIL in () (core.clj:6)

expected: (s/valid? :pre-post-messages.core/person person)

  actual: (not (s/valid? :pre-post-messages.core/person {:first-name "Anna Vissi"}))

AssertionError Assert failed: (t/is (s/valid? :pre-post-messages.core/person person))  pre-post-messages.core/person-name (core.clj:5)