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