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Function Schemas


In Clojure, functions are first-class. Here's a simple function:

(defn plus [x y]
  (+ x y))
(plus 1 2)
; => 3

Predicate Schemas

Simplest way to describe function values with malli is to use predefined predicate schemas fn? and ifn?:

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

(m/validate fn? plus)
; => true

(m/validate ifn? plus)
; => true

Note that ifn? also accepts many data-structures that can be used as functions:

(m/validate ifn? :kikka)
; => true

(m/validate ifn? {})
; => true

But, neither of the predefined function predicate schemas can validate function arity, function arguments or return values. As it stands, there is no robust way to programmatically check function arity at runtime.

Enter, function schemas.

Function Schemas

Function values can be described with :=> and :function schemas. They allows description of both function arguments (as sequence schemas) and function return values.

Examples of function definitions:

;; no args, no return
[:=> :cat :nil]

;; int -> int
[:=> [:cat :int] :int]

;; x:int, xs:int* -> int
[:=> [:catn 
      [:x :int] 
      [:xs [:+ :int]]] :int]
;; multi-arity function
 [:=> [:cat :int] :int]
 [:=> [:cat :int :int [:* :int]] :int]]      

Function definition for the plus looks like this:

(def =>plus [:=> [:cat :int :int] :int])

Let's try:

(m/validate =>plus plus)
; => true

But, wait, as there was no way to know the function arity & other information at runtime, so how did the validation work? Actually, it didn't. By default. :=> validation just checks that it's a fn?, so this holds too:

(m/validate =>plus str)
; => true


Enter, generative testing.

Generative Testing

Like clojure.spec demonstrated, we can use test.check to check the functions at runtime. For this, there is :malli.core/function-checker option.

(require '[malli.generator :as mg])

(def =>plus 
    [:=> [:cat :int :int] :int] 
    {::m/function-checker mg/function-checker}))

(m/validate =>plus plus)
; => true

(m/validate =>plus str)
; => false

Explanation why it is not valid:

(m/explain =>plus str)
;{:schema [:=> [:cat :int :int] :int],
; :value #object[clojure.core$str],
; :errors ({:path [],
;           :in [],
;           :schema [:=> [:cat :int :int] :int],
;           :value #object[clojure.core$str],
;           :check {:total-nodes-visited 0,
;                   :depth 0,
;                   :pass? false,
;                   :result false,
;                   :result-data nil,
;                   :time-shrinking-ms 1,
;                   :smallest [(0 0)],
;                   :malli.generator/explain-output {:schema :int,
;                                                    :value "00",
;                                                    :errors ({:path []
;                                                              :in []
;                                                              :schema :int
;                                                              :value "00"})}}})}

Smallest failing invocation is (str 0 0), which returns "00", which is not an :int. Looks good.

But, why mg/function-checker is not enabled by default? The reason is that it uses generartive testing, which is orders of magnitude slower than normal validation and requires an extra dependency to test.check, which would make malli.core much heavier. This would be expecially bad for CLJS bundle size.

Generating Functions

We can also generate function implementations based on the function schemas. The generated functions check the function arity and arguments at runtime and return generated values.

(def plus-gen (mg/generate =>plus))

(plus-gen 1)
; =throws=> :malli.core/invalid-arity {:arity 1, :arities #{{:min 2, :max 2}}, :args [1], :input [:cat :int :int], :schema [:=> [:cat :int :int] :int]}

(plus-gen 1 "2")
; =throws=> :malli.core/invalid-input {:input [:cat :int :int], :args [1 "2"], :schema [:=> [:cat :int :int] :int]}

(plus-gen 1 2)
; => -1

Multi-arity Functions

Multi-arity functions can be composed with :function:

;; multi-arity fn with function checking always on
(def =>my-fn
    [:function {:registry {::small-int [:int {:min -100, :max 100}]}}
     [:=> [:cat ::small-int] :int]
     [:=> [:cat ::small-int ::small-int [:* ::small-int]] :int]]
    {::m/function-checker mg/function-checker}))

    ([x] x)
    ([x y & z] (apply - (- x y) z))))
; => true

    ([x] x)
    ([x y & z] (str x y z))))
; => false

    ([x] x)
    ([x y & z] (str x y z))))
;{:schema [:function
;          {:registry {::small-int [:int {:min -100, :max 100}]}}
;          [:=> [:cat ::small-int] :int]
;          [:=> [:cat ::small-int ::small-int [:* ::small-int]] :int]],
; :value #object[malli.core_test$eval27255$fn__27256],
; :errors ({:path [],
;           :in [],
;           :schema [:function
;                    {:registry {::small-int [:int {:min -100, :max 100}]}}
;                    [:=> [:cat ::small-int] :int]
;                    [:=> [:cat ::small-int ::small-int [:* ::small-int]] :int]],
;           :value #object[malli.core_test$eval27255$fn__27256],
;           :check ({:total-nodes-visited 2,
;                    :depth 1,
;                    :pass? false,
;                    :result false,
;                    :result-data nil,
;                    :time-shrinking-ms 0,
;                    :smallest [(0 0)],
;                    :malli.generator/explain-output {:schema :int,
;                                                     :value "00",
;                                                     :errors ({:path []
;                                                               :in []
;                                                               :schema :int
;                                                               :value "00"})}})})}

Generating multi-arity functions:

(def my-fn-gen (mg/generate =>my-fn))

; =throws=> :malli.core/invalid-arity {:arity 0, :arities #{1 :varargs}, :args nil, :input nil, :schema [:function {:registry {::small-int [:int {:min -100, :max 100}]}} [:=> [:cat ::small-int] :int] [:=> [:cat ::small-int ::small-int [:* ::small-int]] :int]]}

(my-fn-gen 1)
; => -3237

(my-fn-gen 1 2)
; => --543

(my-fn-gen 1 2 3 4)
; => -2326


Besides testing function schemas as values, we can also intrument functions to enable runtime validation of arguments and return values.

Simplest way to do this is to use m/-instrument which takes options map and a function and returns a instrumented function. Valid options include:

:schemafunction schema
:scopeoptional set of scope definitions, defaults to #{:input :output}
:reportoptional side-effecting function of key data -> any to report problems, defaults to m/-fail!
:genoptional function of schema -> schema -> value to be invoked on the args to get the return value

Instrumenting a function with input & return constraints:

(def pow
    {:schema [:=> [:cat :int] [:int {:max 6}]]}
    (fn [x] (* x x))))

(pow 2)
; => 4

(pow "2")
; =throws=> :malli.core/invalid-input {:input [:cat :int], :args ["2"], :schema [:=> [:cat :int] [:int {:max 6}]]}

(pow 4)
; =throws=> :malli.core/invalid-output {:output [:int {:max 6}], :value 16, :args [4], :schema [:=> [:cat :int] [:int {:max 6}]]}

(pow 4 2)
; =throws=> :malli.core/invalid-arity {:arity 2, :arities #{{:min 1, :max 1}}, :args [4 2], :input [:cat :int], :schema [:=> [:cat :int] [:int {:max 6}]]}

Example of a multi-arity function with instrumentation scopes and custom reporting function:

(def multi-arity-pow
    {:schema [:function
              [:=> [:cat :int] [:int {:max 6}]]
              [:=> [:cat :int :int] [:int {:max 6}]]]
     :scope #{:input :output}
     :report println}
      ([x] (* x x))
      ([x y] (* x y)))))

(multi-arity-pow 4)
; =prints=> :malli.core/invalid-output {:output [:int {:max 6}], :value 16, :args [4], :schema [:=> [:cat :int] [:int {:max 6}]]}
; => 16

(multi-arity-pow 5 0.1)
; =prints=> :malli.core/invalid-input {:input [:cat :int :int], :args [5 0.1], :schema [:=> [:cat :int :int] [:int {:max 6}]]}
; =prints=> :malli.core/invalid-output {:output [:int {:max 6}], :value 0.5, :args [5 0.1], :schema [:=> [:cat :int :int] [:int {:max 6}]]}
; => 0.5

With :gen we can omit the function body. Here's an example to generate random values based on the return schema:

(def pow-gen
    {:schema [:function
              [:=> [:cat :int] [:int {:max 6}]]
              [:=> [:cat :int :int] [:int {:max 6}]]]
     :gen mg/generate}))

(pow-gen 10)
; => -253

(pow-gen 10 20)
; => -159

(pow-gen 10 20 30)
; =throws=> :malli.core/invalid-arity {:arity 3, :arities #{1 2}, :args (10 20 30), :input nil, :schema [:function [:=> [:cat :int] [:int {:max 6}]] [:=> [:cat :int :int] [:int {:max 6}]]]}

Defn Schemas

Defining Function Schemas

There are three ways to add function schemas to function Vars (e.g. defns):

  1. Function Schema Annotation with m/=>
  2. Function Schema Metadata via :malli/schema
  3. Function Inline Schemas with mx/defn

Function Schema Annotations

m/=> macro takes the Var name and the function schema and stores the var -> schema mappings in a global registry.

(def small-int [:int {:max 6}])

(defn plus1 [x] (inc x))
(m/=> plus1 [:=> [:cat :int] small-int])

The order doesn't matter, so this also works:

(m/=> plus1 [:=> [:cat :int] small-int])
(defn plus1 [x] (inc x))

Listing the current accumulation of function (Var) schemas:

;{user {plus1 {:schema [:=> [:cat :int] [:int {:max 6}]]
;              :ns user
;              :name plus1}}}

Without instrumentation turned on, there is no schema enforcement:

(plus1 10)
; => 11

Turning instrumentation on:

(require '[malli.instrument :as mi])

; =prints=> ..instrumented #'user/plus1

(plus1 10)
; =throws=> :malli.core/invalid-output {:output [:int {:max 6}], :value 11, :args [10], :schema [:=> [:cat :int] [:int {:max 6}]]}

Function Schema Metadata

defn schemas can be defined with standard Var metadata. It allows defn schema documentation and instrumentation without dependencies to malli itself from the functions. It's just data.

(defn minus
  "a normal clojure function, no dependencies to malli"
  {:malli/schema [:=> [:cat :int] small-int]}
  (dec x))

To collect instrumentation for the defn, we need to call mi/collect!. It reads all public vars from a given namespace and registers function schemas from :malli/schema metadata.

; => #{#'user/minus}

;{user {plus1 {:schema [:=> [:cat :int] [:int {:max 6}]]
;              :ns user
;              :name plus1},
;       minus {:schema [:=> [:cat :int] [:int {:min 6}]]
;              :ns user
;              :name minus}}}

We'll also have to reinstrument the new var:

; =prints=> ..instrumented #'user/plus1
; =prints=> ..instrumented #'user/minus

(minus 6)
; =throws=> :malli.core/invalid-output {:output [:int {:min 6}], :value 5, :args [6], :schema [:=> [:cat :int] [:int {:min 6}]]}

All Var metadata keys with malli namespace are used. The list of relevant keys:

:malli/schemafunction schema
:malli/scopeoptional set of scope definitions, defaults to #{:input :output}
:malli/reportoptional side-effecting function of key data -> any to report problems, defaults to m/-fail!
:malli/genoptional value true or function of schema -> schema -> value to be invoked on the args to get the return value

Setting :malli/gen to true while function body generation is enabled with mi/instrument! allows body to be generated, to return valid generated data.

Function Inline Schemas

Malli also supports Plumatic Schema -style schema hints via malli.experimental ns:

(require '[malli.experimental :as mx])

(mx/defn times :- :int
  "x times y"
  [x :- :int, y :- small-int]
  (* x y))

Function schema is registered automatically:

;{user {plus1 {:schema [:=> [:cat :int] [:int {:max 6}]]
;              :ns user
;              :name plus1},
;       minus {:schema [:=> [:cat :int] [:int {:max 6}]]
;              :ns user
;              :name minus},
;       times {:schema [:=> [:cat :int [:int {:max 6}]] :int]
;              :ns user
;              :name times}}}

... but not instrumented:

(times 10 10)
; => 100

You can enable instrumentation with mi/instrument!:

; =prints=> ..instrumented #'user/plus1
; =prints=> ..instrumented #'user/minus
; =prints=> ..instrumented #'user/times

(times 10 10)
; =throws=> :malli.core/invalid-input {:input [:cat :int [:int {:max 6}]], :args [10 10], :schema [:=> [:cat :int [:int {:max 6}]] :int]}

... or by using metadata ^:malli/always:

(mx/defn ^:malli/always times :- :int
  "x times y"
  [x :- :int, y :- small-int]
  (* x y))
user=> (times 10 5)
user=> (times 10 10)
Execution error (ExceptionInfo) at malli.core/-exception (core.cljc:138).

Defn Instrumentation

The function (Var) registry is passive and doesn't do anything by itself. To instrument the Vars based on the registry, there is the malli.instrument namespace. Var instrumentations focus is for development time, but can also be used for production builds.

(require '[malli.instrument :as mi])

Vars can be instrumented with mi/instrument! and the instrumentation can be removed with mi/unstrument!.

(m/=> power [:=> [:cat :int] [:int {:max 6}]])
(defn power [x] (* x x))

(power 6)
; => 36

;; instrument all registered vars

(power 6)
; =throws=> :malli.core/invalid-output {:output [:int {:max 6}], :value 36, :args [6], :schema [:=> [:cat :int] [:int {:max 6}]]}


(power 6)
; => 36

Instrumentation can be configured with the same options as m/-instrument and with a set of :filters to select which Vars should be instrumented.

 {:filters [;; everything from user ns
            (mi/-filter-ns 'user)
            ;; ... and some vars
            (mi/-filter-var #{#'power})
            ;; all other vars with :always-validate meta
            (mi/-filter-var #(-> % meta :always-validate))]
  ;; scope
  :scope #{:input :output}
  ;; just print
  :report println})

(power 6)
; =prints=> :malli.core/invalid-output {:output [:int {:max 6}], :value 36, :args [6], :schema [:=> [:cat :int] [:int {:max 6}]]}
; => 36

Defn Checking

We can also check the defn schemas against their function implementations using mi/check. It takes same options as mi/instrument!.

Checking all registered schemas:

;{user/plus1 {:schema [:=> [:cat :int] [:int {:max 6}]],
;             :value #object[user$plus1],
;             :errors ({:path [],
;                       :in [],
;                       :schema [:=> [:cat :int] [:int {:max 6}]],
;                       :value #object[user$plus1],
;                       :check {:total-nodes-visited 12,
;                               :depth 4,
;                               :pass? false,
;                               :result false,
;                               :result-data nil,
;                               :time-shrinking-ms 0,
;                               :smallest [(6)],
;                               :malli.generator/explain-output {:schema [:int {:max 6}],
;                                                                :value 7,
;                                                                :errors ({:path [],
;                                                                          :in [],
;                                                                          :schema [:int {:max 6}],
;                                                                          :value 7})}}})}}

It reports that the plus1 is not correct. It accepts :int but promises to return [:int {:max 6}]. Let's fix the contract by constraining the input values.

(m/=> plus1 [:=> [:cat [:int {:max 5}]] [:int {:max 6}]])

; => nil

All good! But, it's still wrong as the actual implementation allows invalid inputs resulting in invalid outputs (e.g. 6 -> 7). We could enable instrumentation for the function to fail on invalid inputs at runtime - or write similar range checks ourselves into the function body.

A pragmatically correct schema for plus1 would be [:=> [:cat :int] [:int]]. It also checks, but would fail on Long/MAX_VALUE as input. Fully correct schema would be [:=> [:cat [:int {:max (dec Long/MAX_VALUE)}] [:int]]]. Generative testing is best effort, not a silver bullet.

We redefined plus1 function schema and the instrumentation is now out of sync. We have to call mi/instrument! to re-instrument it correctly.

;; the old schema & old error
(plus1 6)
; =throws=> :malli.core/invalid-output {:output [:int {:max 6}], :value 9, :args [8], :schema [:=> [:cat :int] [:int {:max 6}]]}


;; the new schema & new error
(plus1 6)
; =throws=> :malli.core/invalid-input {:input [:cat [:int {:max 5}]], :args [6], :schema [:=> [:cat [:int {:max 5}]] [:int {:max 6}]]}

This is not good developer experience.

We can do much better.

Development Instrumentation

For better DX, there is namespace.

(require '[ :as dev])

It's main entry points is dev/start!, taking same options as mi/instrument!. It runs mi/instrument! and mi/collect! (for all loaded namespaces) once and starts watching the function registry for changes. Any change that matches the filters will cause automatic re-instrumentation for the functions. dev/stop! removes all instrumentation and stops watching the registry.

(defn plus1 [x] (inc x))
(m/=> plus1 [:=> [:cat :int] [:int {:max 6}]])

; =prints=> ..instrumented #'user/plus1
; =prints=> started instrumentation

(plus1 "6")
; =throws=> :malli.core/invalid-input {:input [:cat :int], :args ["6"], :schema [:=> [:cat :int] [:int {:max 6}]]}

(plus1 6)
; =throws=> :malli.core/invalid-output {:output [:int {:max 6}], :value 9, :args [8], :schema [:=> [:cat :int] [:int {:max 6}]]}

(m/=> plus1 [:=> [:cat :int] :int])
; =prints=> ..instrumented #'user/plus1

(plus 6)
; => 7

; =prints=> ..unstrumented #'user/plus1
; =prints=> stopped instrumentation

ClojureScript support

See the document: docs/

Static Type Checking

Running instrumentation also emits clj-kondo type configs for all defns, enabling basic static type checking/linting for the instrumented functions.

Here's the above code in Cursive IDE with clj-kondo enabled:

Pretty Errors

For prettier runtime error messages, we can swap the default error printer / thrower.

(require '[ :as pretty])
(defn plus1 [x] (inc x))
(m/=> plus1 [:=> [:cat :int] [:int {:max 6}]])

(dev/start! {:report (pretty/reporter)})

(plus1 "2")
; =prints=>
; -- Schema Error ----------------------------------- malli.demo:13 --
; Invalid function arguments:
;   ["2"]
; Input Schema:
;   [:cat :int]
; Errors:
;   {:in [0],
;    :message "should be an integer",
;    :path [0],
;    :schema :int,
;    :type nil,
;    :value "2"}
; More information:
; --------------------------------------------------------------------
; =throws=> Execution error (ClassCastException) at malli.demo/plus1 (demo.cljc:13).
;           java.lang.String cannot be cast to java.lang.Number

To throw the prettified error instead of just printint it:

(dev/start! {:report (pretty/thrower)})

Pretty printer uses fipp under the hood and has lot of configuration options:

(dev/start! {:report (pretty/reporter (pretty/-printer {:width 80
                                                        :print-length 30
                                                        :print-level 2
                                                        :print-meta true}))})


Example of annotating function with var meta-data and using for dev-time function instrumentation, pretty runtime exceptions and clj-kondo for static checking:

(ns malli.demo)

(defn plus1
  "Adds one to the number"
  {:malli/schema [:=> [:cat :int] :int]}
  [x] (inc x))

;; instrument, clj-kondo + pretty errors
(require '[ :as dev])
(require '[ :as pretty])
(dev/start! {:report (pretty/reporter)})

(plus1 "123")


Here's the same code in Cursive IDE with clj-kondo enabled:

Future work

Can you improve this documentation? These fine people already did:
Tommi Reiman, Joel Kaasinen, Stig Brautaset, Daniel Vingo, hgranthorner & Carsten Behring
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