Optional element in sequence

Question

I am trying to match the following sequences using Prismatic / Schema:

[{:n "some text"}] ; => valid

and

[{:k "some text"} {:n "some text"}] ; => valid

What I tried:

(s/def Elem3
  {:k s/Str})

(s/def Elem2
  {:n s/Str})

(s/def Elem
  [(s/optional Elem2 "elem2") Elem3])

(s/validate Elem [{:k "huji"}])
;; =>
;; Value does not match schema: [(named {:n missing-required-key, :k
;; disallowed-key} "elem2")]

(s/def Elem
  [(s/maybe Elem2) Elem3])

(s/validate Elem [{:k "huji"}])
;; =>
;; [(maybe {:n Str}) {:k java.lang.String}] is not a valid sequence
;; schema; a valid sequence schema consists of zero or more `one`
;; elements, followed by zero or more `optional` elements, followed by
;; an optional schema that will match the remaining elements.

(s/defrecord ElemOption1
  [elem3 :- Elem3])

(s/defrecord ElemOption2
    [elem2 :- Elem2
     elem3 :- Elem3])

(s/def Elem
  (s/conditional
   #(= 2 (count %)) ElemOption2
   :else ElemOption1))

(s/validate Elem [{:k "huji"}])
;; =>
;; Value does not match schema: (not (instance?
;;  peg_dsl.standard_app.ElemOption1 [{:k "huji"}]))

The main problem is that I do not understand how to write a scheme that allows you to omit the first element of the specified vector. What is the correct way to combine both vectors from above?

Answer 1

The problem with your first attempt is that starting with does not necessarily mean that he expects {: ks / Str} or nothing, and he sees {:n s/Str}, so that is clearly not correct.

. Maybe nil, . . s/one* s/optional*, s/optional s/one.

, , , , .

:

(def ElemKNList [(s/one {:k s/Str} "k") (s/one {:n s/Str} "n")])
(def ElemNList [(s/one {:n s/Str} "n")])

(def Elem (s/conditional #(= 2 (count %)) ElemKNList 
                         :else ElemNList))

(s/validate Elem [{:k "huji"} {:n "huji"}])    
    => [{:k "huji"} {:n "huji"}]
(s/validate Elem [{:n "huji"}])
    => [{:n "huji"}]          
(s/validate Elem [{:k "huji"}])
    => ExceptionInfo Value does not match schema: [(named {:n missing-required-key, :k disallowed-key} "n")]  schema.core/validator/fn--18435 (core.clj:151)