# Function Schemas

* [Functions](#functions)
* [Predicate Schemas](#predicate-schemas)
* [Function Schemas](#function-schemas)
  * [Generative Testing](#generative-testing)
  * [Function Guards](#function-guards)
  * [Generating Functions](#generating-functions)
  * [Multi-arity Functions](#multi-arity-functions)
  * [Instrumentation](#instrumentation)
  * [Flat Arrow Function Schemas](#flat-arrow-function-schemas)
* [Defn Schemas](#defn-schemas)
  * [Defining Function Schemas](#defining-function-schemas)
    * [Function Schema Annotations](#function-schema-annotations)
    * [Function Schema Metadata](#function-schema-metadata)
    * [Function Inline Schemas](#function-inline-schemas)
  * [Defn Instrumentation](#defn-instrumentation)
  * [Defn Checking](#defn-checking)
* [Development Instrumentation](#development-instrumentation)
  * [ClojureScript Support](#clojurescript-support)
  * [Static Type Checking](#static-type-checking)
  * [Pretty Errors](#pretty-errors)
* [Defn Schemas via metadata](#defn-schemas-via-metadata)
  * [TL;DR](#tldr)

## Functions

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

```clojure
(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?`:

```clojure
(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:

```clojure
(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](https://stackoverflow.com/questions/1696693/clojure-how-to-find-out-the-arity-of-function-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](https://github.com/metosin/malli#sequence-schemas)) and function return values.

Examples of function definitions:

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

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

;; x:int, xs:int* -> int
[:=> [:catn
      [:x :int]
      [:xs [:+ :int]]] :int]

;; arg:int -> ret:int, arg > ret
(defn guard [[arg] ret]
  (> arg ret))

[:=> [:cat :int] :int [:fn guard]]

;; multi-arity function
[:function
 [:=> [:cat :int] :int]
 [:=> [:cat :int :int [:* :int]] :int]]
```

What is that `:cat` all about in the input schemas? Wouldn't it be simpler without it? Sure, check out [Flat Arrow Function Schema](#flat-arrow-function-schemas).

Function definition for the `plus` looks like this:

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

Let's try:

```clojure
(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:

```clojure
(m/validate =>plus str)
;; => true
```
Bummer.

Enter, generative testing.

### Generative Testing

Like [clojure.spec](https://clojure.org/about/spec) demonstrated, we can use [test.check](https://github.com/clojure/test.check) to check the functions at runtime. For this, there is `:malli.core/function-checker` option.

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

(def =>plus
  (m/schema
    [:=> [: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:

```clojure
(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 generative 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 especially bad for CLJS bundle size.

### Function Guards

`:=>` accepts optional third child, a guard schema that is used to validate a vector of function arguments and return value.

```clojure
;; function schema of arg:int -> ret:int, where arg < ret
;; with generative function checking always enabled
(def arg<ret
  (m/schema
   [:=>
    [:cat :int]
    :int
    [:fn {:error/message "argument should be less than return"}
     (fn [[[arg] ret]] (< arg ret))]]
   {::m/function-checker mg/function-checker}))

(m/explain arg<ret (fn [x] (inc x)))
;; => nil

(m/explain arg<ret (fn [x] x))
;{:schema ...
; :value #object[user$eval19073$fn__19074],
; :errors ({:path [],
;           :in [],
;           :schema ...,
;           :value #object[user$eval19073$fn__19074],
;           :check {:total-nodes-visited 1,
;                   :result false,
;                   :result-data nil,
;                   :smallest [(0)],
;                   :time-shrinking-ms 0,
;                   :pass? false,
;                   :depth 0,
;                   :malli.core/result 0}},
;          {:path [2],
;           :in [],
;           :schema [:fn
;                    #:error{:message "argument should be less than return"}
;                    (fn [[[arg] ret]] (< arg ret))],
;           :value [(0) 0]})}

(require '[malli.error :as me])
(me/humanize *1)
; ["invalid function" "argument should be less than return"]
```

Identical schema using the Schema AST syntax:

```clojure
(m/from-ast
 {:type :=>
  :input {:type :cat
          :children [{:type :int}]}
  :output {:type :int}
  :guard {:type :fn
          :value (fn [[[arg] ret]] (< arg ret))
          :properties {:error/message "argument should be less than return"}}}
 {::m/function-checker mg/function-checker})
```

### 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.

<!-- :test-doc-blocks/skip -->
```clojure
(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`:

<!-- :test-doc-blocks/skip -->
```clojure
;; multi-arity fn with function checking always on
(def =>my-fn
  (m/schema
    [: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}))

(m/validate
  =>my-fn
  (fn
    ([x] x)
    ([x y & z] (apply - (- x y) z))))
; => true

(m/validate
  =>my-fn
  (fn
    ([x] x)
    ([x y & z] (str x y z))))
; => false

(m/explain
  =>my-fn
  (fn
    ([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:

<!-- :test-doc-blocks/skip -->
```clojure
(def my-fn-gen (mg/generate =>my-fn))

(my-fn-gen)
; =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
```

### Instrumentation

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

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

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

Instrumenting a function with input & return constraints:

<!-- :test-doc-blocks/skip -->
```clojure
(def pow
  (m/-instrument
    {: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:

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

(multi-arity-pow 4)
;; =stdout=> :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)
;; =stdout=> :malli.core/invalid-input {:input [:cat :int :int], :args [5 0.1], :schema [:=> [:cat :int :int] [:int {:max 6}]]}
;; :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:

<!-- :test-doc-blocks/skip -->
```clojure
(def pow-gen
  (m/-instrument
    {: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}]]]}
```

### Flat Arrow Function Schemas

Function schema `:=>` requires input arguments to be wrapped in `:cat` or `:catn`. Since `0.16.2` there is also flat arrow schema: `:->` that allows input schema to be defined as flat sequence:

```clojure
;; no args, no return
[:-> :nil]

;; int -> int
[:-> :int :int]

;; arg:int -> ret:int, arg > ret
(defn guard [[arg] ret]
  (> arg ret))

[:-> {:guard guard} :int :int]

;; multi-arity function
[:function
 [:-> :int :int]
 [:-> :int :int [:* :int] :int]]
```

Technically `:->` is implemented as a proxy to `:=>`. To get the actual schema:

```clojure
(m/deref [:-> :int :int])
; [:=> [:cat :int] :int]
```

This can be seen also in explain results:

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

## Defn Schemas

### Defining Function Schemas

There are three ways to add function schemas to function Vars (e.g. `defn`s):

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.

```clojure
(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:

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

Listing the current accumulation of function (Var) schemas:

```clojure
(m/function-schemas)
;{user {plus1 {:schema [:=> [:cat :int] [:int {:max 6}]]
;              :ns user
;              :name plus1}}}
```

Without instrumentation turned on, there is no schema enforcement:

```clojure
(plus1 10)
;; => 11
```

Turning instrumentation on:

<!-- :test-doc-blocks/skip -->
```clojure
(require '[malli.instrument :as mi])

(mi/instrument!)
; =stdout=> ..instrumented #'user/plus1

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

Note that vars already containing a primitive JVM function will not be instrumented.

#### 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.

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

```clojure
(mi/collect!)
; => #{#'user/minus}

(m/function-schemas)
;{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:

<!-- :test-doc-blocks/skip -->
```clojure
(mi/instrument!)
; =stdout=> ..instrumented #'user/plus1
; =stdout=> ..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:

| key             | description |
| ----------------|-------------|
| `:malli/schema` | function schema
| `:malli/scope`  | optional set of scope definitions, defaults to `#{:input :output}`
| `:malli/report` | optional side-effecting function of `key data -> any` to report problems, defaults to `m/-fail!`
| `:malli/gen`    | optional 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](https://github.com/plumatic/schema#beyond-type-hints) schema hints via `malli.experimental` ns:

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

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

Function schema is registered automatically:

```clojure
(m/function-schemas)
;{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:

```clojure
(times 10 10)
;; => 100
```

You can enable instrumentation with `mi/instrument!`:

<!-- :test-doc-blocks/skip -->
```clojure
(mi/instrument!)
; =stdout=> ..instrumented #'user/plus1
; =stdout=> ..instrumented #'user/minus
; =stdout=> ..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`:

```clojure
(mx/defn ^:malli/always times :- :int
  "x times y"
  [x :- :int, y :- small-int]
  (* x y))
```
<!-- :test-doc-blocks/skip -->
```clojure
user=> (times 10 5)
50
user=> (times 10 10)
Execution error (ExceptionInfo) at malli.core/-exception (core.cljc:138).
:malli.core/invalid-input
```

### 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 instrumentation  is intended for development time, but can also be used for production builds.

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

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

<!-- :test-doc-blocks/skip -->
```clojure
(m/=> power [:=> [:cat :int] [:int {:max 6}]])
(defn power [x] (* x x))

(power 6)
; => 36

;; instrument all registered vars
(mi/instrument!)

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

(mi/unstrument!)

(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.
<!-- :test-doc-blocks/skip -->
```clojure
(mi/instrument!
 {: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)
; =stdout=> :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:

```clojure
(mi/check)
;{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.
<!-- :test-doc-blocks/skip -->
```clojure
(m/=> plus1 [:=> [:cat [:int {:max 5}]] [:int {:max 6}]])

(mg/check)
; => 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.

<!-- :test-doc-blocks/skip -->
```clojure
;; 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}]]}

(mi/instrument!)

;; 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 `malli.dev` namespace.

```clojure
(require '[malli.dev :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.

<!-- :test-doc-blocks/skip -->
```clojure
(defn plus1 [x] (inc x))
(m/=> plus1 [:=> [:cat :int] [:int {:max 6}]])

(dev/start!)
; malli: instrumented 1 function var
; malli: dev-mode started

(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])
; =stdout=> ..instrumented #'user/plus1

(plus 6)
; => 7

(dev/stop!)
; malli: unstrumented 1 function vars
; malli: dev-mode stopped
```

## ClojureScript support

See the document: [docs/clojurescript-function-instrumentation.md](clojurescript-function-instrumentation.md)

### Static Type Checking

Running `malli.dev` instrumentation also emits [clj-kondo](https://github.com/metosin/malli#clj-kondo) type configs for all `defn`s, enabling basic static type checking/linting for the instrumented functions.

Here's the above code in [Cursive IDE](https://cursive-ide.com/) with [clj-kondo](https://github.com/clj-kondo/clj-kondo) enabled:

<img src="img/clj-kondo-instrumentation.png">

### Pretty Errors

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

```clojure
(require '[malli.dev.pretty :as pretty])
```

<!-- :test-doc-blocks/skip -->
```clojure
(defn plus1 [x] (inc x))
(m/=> plus1 [:=> [:cat :int] [:int {:max 6}]])

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

(plus1 "2")
; =stdout=>
; -- 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:
;
;   https://cljdoc.org/d/metosin/malli/LATEST/doc/function-schemas
;
; --------------------------------------------------------------------
; =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:

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

Pretty printer uses [fipp](https://github.com/brandonbloom/fipp) under the hood and has lot of configuration options:

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

### TL;DR

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

<!-- :test-doc-blocks/skip -->
```clojure
(ns malli.demo)

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

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

(plus1 "123")

(comment
  (dev/stop!))
```

Here's the same code in [Cursive IDE](https://cursive-ide.com/) with [clj-kondo](https://github.com/clj-kondo/clj-kondo) enabled:

<img src="img/defn-schema.png"/>

## Future work

* [support Schema defn syntax](https://github.com/metosin/malli/issues/125)
* better integration with [clj-kondo](https://github.com/clj-kondo/clj-kondo) and [clojure-lsp](https://github.com/clojure-lsp/clojure-lsp) for enhanced DX.