Ok, thanks!

In the Java world, the assertions is also something that need to be turn on 
In that sence, they are kind of not mandatory to be executed (or at least 
signals that to the reader of the code).

I would be happier if you guys could add another method, that I can use in 
my :pre conditions, that leverage
the same amount of details in the error messages, but that is always 
"turned on".

In the meanwhile, I will use s/assert ;-)

Joakim Tengstrand

On Wednesday, 14 September 2016 15:59:09 UTC+2, Alex Miller wrote:
> Another option that has been added since the guide was written is s/assert 
> which seems closer to what you're suggesting.
> (defn name [user]
>   {:pre [(s/assert :common/user user)]}
>   (-> user :user/name))
> ;; need to enable assertion checking - this can also be enabled globally 
> with system property clojure.spec.check-asserts
> (s/check-asserts true)
> (name {:user/name "Elon"})
> "Elon"
> (name {:x "Elon"})
> ExceptionInfo Spec assertion failed
> val: {:x "Elon"} fails predicate: (contains? % :user/name)
> :clojure.spec/failure  :assertion-failed
>   clojure.core/ex-info (core.clj:4725)
> Rather than use it in a precondition, you can also use s/assert directly 
> in the code.
> On Wednesday, September 14, 2016 at 7:37:24 AM UTC-5, joakim.t...@nova.com 
> <javascript:> wrote:
>> (ns spec-test.core
>>   (:require [clojure.spec :as s]))
>> (s/def :user/name string?)
>> (s/def :common/user (s/keys :req [:user/name]))
>> ; first version of name (using :pre)
>> (defn name [user]
>>   {:pre [(s/valid? :common/user user)]}
>>   (-> user :user/name))
>> ; This statement works ok and returns "Elon":
>> (name {:user/name "Elon"})
>> ; but this statement...
>> (name {:x "Elon"})
>> ;...will throw:
>> CompilerException java.lang.AssertionError:
>> Assert failed: (s/valid? :common/user user)
>> ; ...but then I don't get as much information
>> ; about the error as if I would have called:
>> (s/explain :common/user {:x "Elon"})
>> ;...which also contains the predicate:
>> val: {:x "Elon"} fails spec: :common/user
>> predicate: (contains? % :user/name)
>> ; (second version of name - more verbose)
>> ; or do I need to wite it like this:
>> (defn name [user]
>>   (let [parsed (s/conform :common/user user)]
>>     (if (= parsed ::s/invalid)
>>       (throw (ex-info "Invalid input" (s/explain-data :common/user user)))
>>       (-> user :user/name))))
>> ; so that:
>> (name {:x "Elon"})
>> ; ...will return:
>> CompilerException clojure.lang.ExceptionInfo:
>>   Invalid input #:clojure.spec{:problems}
>>     ({:path [], :pred (contains? % :user/name),
>>       :val {:x "Elon"}, :via [:common/user], :in []})
>> ; It should be nice if I could be able to write it like this
>> ; (or similar, to get a better error message):
>> (defn name [user]
>>   {:pre [(s/explain :common/user user)]}
>>   (-> user :user/name))

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