Add a decoder which splits the string at the separator. Then it's a multi schema for a tuple which dispatches of first

You mean a custom transformer like in https://github.com/metosin/malli/blob/master/docs/tips.md#trimming-strings ?

(require '[malli.transform :as mt])
(require '[malli.core :as m])
(require '[clojure.string :as str])

;; a decoding transformer, only mounting to :string schemas with truthy :string/trim property
(defn string-trimmer []
  (mt/transformer
    {:decoders
     {:string
      {:compile (fn [schema _]
                  (let [{:string/keys [trim]} (m/properties schema)]
                    (when trim #(cond-> % (string? %) str/trim))))}}}))

;; trim me please
(m/decode [:string {:string/trim true, :min 1}] " kikka  " string-trimmer)
; => "kikka"

But then, it's the responsibility of the validate callers to explicitly mention my custom transformer. Is there a way to have the custom transformer somehow embedded in the schema?

(def Composite [:tuple {:decode/string #(str/split % #"/") A B])

You can do some sort of schema constructor, too: (def Composite (-simple-schema (fn [A B] [:tuple {:decode/string #(str/split % #"/") A B])))

Then use it like (Composite [:= domain] Domain)

sort of type-constructors

Thanks @UK0810AQ2 I'll give it a try

The way I found works well for me is starting from an ideal representation then working backwards with transformers to get there