Create subschemas of malli-schemas using a spec2-inspired select notation.
It's based on Rich Hickey's ideas from his talk "Maybe Not" about how spec-alpha2 might allow for schema reuse.
Follow along using deps-try:
$ deps-try io.github.eval/malli-select metosin/malli
# no install/docker
$ docker run -it --rm -v deps-try:/home/deps-try-user ghcr.io/eval/deps-try io.github.eval/malli-select metosin/malli
user=> (require '[malli-select.core :as ms]
'[malli.core :as m :refer [form] :rename {form p}])
user=> (def Person
[:map
[:name string?]
[:age pos-int?]
[:addresses [:vector [:map
[:street string?] [:zip string?]]]]])
;; require :name, everything else is optional
user=> (p (ms/select Person [:name]))
[:map
[:name string?]
[:age {:optional true} pos-int?]
[:addresses
{:optional true}
[:vector
[:map
[:street {:optional true} string?]
[:country {:optional true} string?]]]]]
;; *if* any address is provided, it should at least have :street
user=> (p (ms/select Person [{:addresses [:street]}]))
[:map
[:name {:optional true} string?]
[:age {:optional true} pos-int?]
[:addresses
{:optional true}
[:vector
[:map [:street string?] [:country {:optional true} string?]]]]]
;; example valid data:
;; {}, {:addresses []}, {:addresses [{:street "Main"}]}
;;
;; example invalid data:
;; {:addresses nil}, {:addresses [{}]}, {:addresses [{:street "Foo" :country :se}]}
;; any address provided should be a full address
user=> (p (ms/select Person [{:addresses ['*]}]))
;;
;; require all attributes of a person (shallow, i.e. address attributes become optional)
user=> (p (ms/select Person ['*]))
;; example valid data:
;; {:name "Foo" :age 18 :addresses [{}]}
;; remove any optional attribute
user=> (p (ms/select Person [{:addresses ['*]}] {:prune-optionals true}))
;; or shorter:
user=> (p (ms/select Person ^:only [{:addresses ['*]}]))
;; example valid data:
;; {:name :not-a-string}
;;
;; Typically you'd use this to generate only specific data:
user=> (require '[malli.generator :as mg])
user=> (mg/generate (ms/select Person ^:only [:name]))
{:name "sNeLdUI5KtPw"}
;; selecting something not contained in the schema:
user=> (ms/select Person [:a])
Execution error (AssertionError) at dk.thinkcreate.malli-select/select (malli_select.clj:175).
Assert failed: Selection contains unknown paths: ([:a])
Available:
([:addresses] [:age] [:name] [:addresses :street] [:addresses :zip])
(empty? invalid-selection-paths)
;; bypass this check:
user=> (ms/select Person [:a] {:verify-selection false})See the tests for more.
Copyright (c) 2026 Gert Goet, ThinkCreate. Distributed under the MIT license. See LICENSE.