name: clojure-malli description: | Data validation with Malli schemas in Clojure. Use when working with: (0) malli, (1) data validation or coercion, (2) defining schemas for maps, collections, or domain models, (3) API request/response validation, (4) form validation, (5) runtime type checking, or when the user mentions malli, schemas, validation, data contracts, or data integrity.

Malli Data Validation

Malli validates data against schemas. Schemas are Clojure data structures.

Quick Validation

(require '[malli.core :as m])
(require '[malli.error :as me])

;; Validate
(m/validate [:map [:name :string] [:age :int]]
            {:name "Alice" :age 30})
;; => true

;; Get errors
(-> [:map [:name :string] [:age :int]]
    (m/explain {:name "Alice" :age "thirty"})
    (me/humanize))
;; => {:age ["should be an integer"]}

Quick Coercion

(require '[malli.transform :as mt])

;; Decode string input to proper types
(m/decode [:map [:port :int] [:active :boolean]]
          {:port "8080" :active "true"}
          (mt/string-transformer))
;; => {:port 8080, :active true}

;; Coerce = decode + validate (throws on error)
(m/coerce [:map [:id :int]] {:id "42"} (mt/string-transformer))
;; => {:id 42}

Common Schema Patterns

Maps with Required/Optional Keys

[:map
 [:id :uuid]                              ;; required
 [:name :string]                          ;; required
 [:email {:optional true} :string]        ;; optional
 [:role {:default "user"} :string]]       ;; optional with default

Constrained Values

[:string {:min 1 :max 100}]    ;; string length 1-100
[:int {:min 0 :max 150}]       ;; integer range
[:enum "draft" "published"]    ;; one of these values
[:re #".+@.+\..+"]             ;; regex match

Collections

[:vector :int]                 ;; vector of ints
[:set :keyword]                ;; set of keywords
[:map-of :keyword :string]     ;; map with keyword keys, string values
[:tuple :double :double]       ;; fixed [x, y] pair

Unions and Conditionals

;; Simple union
[:or :string :int]

;; Nilable
[:maybe :string]               ;; string or nil

;; Tagged union with dispatch
[:multi {:dispatch :type}
 [:user [:map [:type [:= :user]] [:name :string]]]
 [:admin [:map [:type [:= :admin]] [:role :string]]]]

Nested Structures

[:map
 [:user [:map
         [:name :string]
         [:address [:map
                    [:city :string]
                    [:zip :string]]]]]]

Performance: Cache Validators

;; BAD - creates validator every call
(defn process [data]
  (when (m/validate schema data) ...))

;; GOOD - cached validator
(def valid? (m/validator schema))
(defn process [data]
  (when (valid? data) ...))

;; Same for decoders
(def decode-request (m/decoder schema (mt/string-transformer)))
(def coerce-request (m/coercer schema (mt/string-transformer)))

Key Gotchas

1. decode doesn't validate - returns invalid data as-is. Use coerce for safety.
2. Maps are open by default - extra keys allowed. Use {:closed true} to reject them.
3. Keys are required by default - use {:optional true} for optional keys.

Detailed References

• Schema Syntax - All builtin types, properties, operators
• API Reference - Core functions with signatures
• Registries - Reusable schema patterns