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]
> <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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