Do you mean like this?

(m/rewrite {:key "val1"
            :key2 {:key3 ["it1" "it2" "it3"]}}
  {:key ?a
   :key2 {:key3 [!b ...]}}
  {:key ?a
   :key2 {:key3 [(m/app string/reverse !b) ...]}})
;; {:key "val1", :key2 {:key3 ["1ti" "2ti" "3ti"]}}

Maybe you mean like this:

(ns scratch
  (:require [meander.epsilon :as m]
            [clojure.string :as string]))

(defn my-fun
  [a b]
  {:a a
   :b b})

(m/rewrite {:key "val1"
            :key2 {:key3 ["it1" "it2" "it3"]}}
  {:key ?a
   :key2 {:key3 [!b ...]}}
  {:key ?a
   :key2 {:key3 [(m/app #(my-fun ?a %) !b) ...]}})
;; {:key "val1", :key2 {:key3 [{:a "val1", :b "it1"} {:a "val1", :b "it2"} {:a "val1", :b "it3"}]}}e

hmm, maybe using examples from specter would be easier:

(specter/transform [:sites s/MAP-VALS :services s/ALL] #(meander/match ~~fairly simple map trasformation here, just structure, not using values~~))

Can you give an example input and output?

sites:
  site1:
    services:
      mainService:
        name: "name1"
      sidecar1:
        name: "name2"
      sidecar2:
        name: "name3"
  site2:
    services:
      mainService:
        name: "name4"
      sidecar1:
        name: "name5"
      sidecar2:
        name: "name6"

sites:
  site1:
     httpService:
       name: "name1"
     sidecars:
        - name: "name2"
        - name: "name3"
  site2: 
     httpService:
       name: "name4"
     sidecars:
        - name: "name5"
        - name: "name6"    

the number/name of sites can be variable, we can't depend of them, they need to be maintained

same for number of sidecars

out of services one has special status and is treated as main service, the rest are sidecars

This is still a simplified example. Right now I'm tempted to have few transformations using combination of specter and meander, ie using transform to select appropriate places in the tree to update, use meanders match to do the transformation

(def ex1 {:sites {:site-one {:services {:main-service {:name "s1-main"}
                                        :side-car-one {:name "s1-one"}
                                        :side-car-two {:name "s1-two"}}}
                  :site-two {:services {:main-service {:name "main"}
                                        :side-car-one {:name "s2-one"}
                                        :side-car-two {:name "s2-two"}}}}})

(m/rewrite ex1
  {:sites (m/map-of !k {:services {:main-service !main-service
                                   & (m/app seq ([_ !sidecar] ..!n))}})}
  {:sites (m/map-of !k {:http-service !main-service
                        :sidecars [!sidecar ..!n]})})

;; {:sites {:site-one {:http-service {:name "s1-main"}
;;                     :sidecars [{:name "s1-one"}
;;                                {:name "s1-two"}]}
;;          :site-two {:http-service {:name "main"}
;;                     :sidecars [{:name "s2-one"}
;;                                {:name "s2-two"}]}}}

Thanks a lot! That looks near 🙂

What if I wanted to extend the example with main-service having a key, eg environment that is supposed to go up one level. From being subkey of :main-service to being a sibling of :http-service

(def ex1 {:sites {:site-one {:services {:main-service {:name "s1-main" :environment {:env-var1 "env-var1"}}
                                        :side-car-one {:name "s1-one"}
                                        :side-car-two {:name "s1-two"}}}
                  :site-two {:services {:main-service {:name "main" :environment {:env-var2 "env-var2"}}
                                        :side-car-one {:name "s2-one"}
                                        :side-car-two {:name "s2-two"}}}}})

(m/rewrite ex1
  {:sites (m/map-of !k {:services {:main-service !main-service
                                   & (m/app seq ([_ !sidecar] ..!n))}})}
  {:sites (m/map-of !k {:http-service !main-service
                        :sidecars [!sidecar ..!n]})})

;; {:sites {:site-one {:http-service {:name "s1-main"}
;;                     :environment {:env-var1 "env-var1"}                  
;;                     :sidecars [{:name "s1-one"}
;;                                {:name "s1-two"}]}
;;          :site-two {:http-service {:name "main"}
;;                     :environment {:env-var2 "env-var2"}
;;                     :sidecars [{:name "s2-one"}
;;                                {:name "s2-two"}]}}}