Preconditions are already assertions, so it makes more sense to use
s/assert in your code body than in a precondition.
On Wednesday, June 7, 2017 at 8:12:22 AM UTC-5, David Goldfarb wrote:
>
> One big downside of using s/assert in a precondition: It does not work
> with (s/nilable ...) specs, since s/assert returns valid values.
>
> I fell into this trap for a moment of head-scratching just now.
>
> On Wednesday, September 14, 2016 at 4:59:09 PM UTC+3, 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,
>> [email protected] 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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