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Spec Coercion

Like Plumatic Schema, Spec-tools differentiates specs (what) and transformers (how). This enables spec values to be transformed between different formats like JSON and EDN. Transformers are implemented using the spec-tools.core/Transformer protocol.

(defprotocol Transformer
  (-name [this])
  (-options [this])
  (-encoder [this spec value])
  (-decoder [this spec value]))

Spec-tools ships with following transformers:

Name Description
string-transformer String-formats like properties files, query- & path-parameters.
json-transformer JSON format, like string, but numbers and booleans are supported
strip-extra-keys-transformer Decoding strips out extra keys of s/keys specs.
strip-extra-values-transformer Decoding strips out extra values of s/tuple specs.
fail-on-extra-keys-transformer Decoding fails if s/keys specs have extra keys.
nil No transformations, e.g. EDN & Transit.

Coercion

(require '[spec-tools.core :as st])

For simple transformations, there is st/coerce. It takes a spec, a value and a transformer and uses Spec Walker to walk over the specs and transform the value using the transformer. It transforms all values it can, leaving non-coercable parts untouched. Behind the scenes, specs are walked using their s/form & spec-tools form parser. Coercion is inspired by spec-coerce.

NOTE: with the current spec1 design, form-based coercion is just best effort. There is CLJ-2251 which would solve this.

Simple example

(st/coerce int? "1" st/string-transformer) 
; 1

(st/coerce int? "1" st/json-transformer) 
; "1"

Real life example

We have the following specs defined:

(require '[clojure.spec.alpha :as s])

(s/def ::id int?)
(s/def ::description string?)
(s/def ::amount pos-int?)
(s/def ::delivery inst?)
(s/def ::tags (s/coll-of keyword? :into #{}))
(s/def ::item (s/keys :req-un [::description ::tags ::amount]))
(s/def ::items (s/map-of ::id ::item))
(s/def ::location (s/tuple double? double?))
(s/def ::order (s/keys :req-un [::id ::items ::delivery ::location]))

We have a JSON Web Service that reads and writes orders. A valid ::order value:

(def order
  {:id 123,
   :items {1 {:description "vadelmalimsa"
              :tags #{:good :red}
              :amount 10},
           2 {:description "korvapuusti"
              :tags #{:raisin :sugar}
              :amount 20}},
   :delivery #inst"2007-11-20T20:19:17.000-00:00",
   :location [61.499374 23.7408149]})

(s/valid? ::order order)
; true

If we can use EDN or Transit, we can encode and decode the values automatically. Formats like JSON can't represent all required data types. We can simulate a JSON round-tripping using the Muuntaja library:

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

(def json-order
  (->> order
       (m/encode "application/json")
       (m/decode "application/json")))

json-order
;{:id 123,
; :delivery "2007-11-20T20:19:17Z",
; :items {:1 {:description "vadelmalimsa"
;             :tags ["good" "red"]
;             :amount 10},
;         :2 {:description "korvapuusti"
;             :tags ["raisin" "sugar"]
;             :amount 20}}
; :location [61.499374 23.7408149]}

(s/valid? ::order json-order)
; false

The roundtripped data is not correct anymore: the ::delivery is now just a ISO 8601 Date string, ::tags are reduced to strings and the item ::ids are keywords. Bummer.

We could transform all the invalid values manually, but it would add a lot of boilerplate and would be fragile as the transformations need to be in sync with the potentially evolving data models.

As the specs already contain all the required type information, we can derive the transformations for free. Coercion using the st/json-transformer:

(st/coerce ::order json-order st/json-transformer)
;{:id 123,
; :items {1 {:description "vadelmalimsa"
;            :tags #{:good :red}
;            :amount 10},
;         2 {:description "korvapuusti"
;            :tags #{:raisin :sugar}
;            :amount 20}},
; :delivery #inst"2007-11-20T20:19:17.000-00:00",
; :location [61.499374 23.7408149]}

Data is now valid:

(s/valid? ::order (st/coerce ::order json-order st/json-transformer))
; true

Strict coercion

At system boundaries, like in web apis with external clients and in front of data stores, it is important to check data no extra data is carried in. In clojure.spec, maps are open by design. We could add extra constrains to s/keys with s/and not to contain extra keys, but there are no utilities for it in Spec1 and it would be a static constraint on spec, effecting all call sites. Spec should be open within the boundaries.

We can use coercion for this.

Client sends ::order with some extra data:

(def evil-json-order
  (->> (-> order
           (assoc :owner "ikitommi")
           (assoc :LONGSTRING ".................................")
           (assoc-in [:items 1 :discount] 80)
           (assoc-in [:items 2 ::discount] 80))
       (m/encode "application/json")
       (m/decode "application/json")))

evil-json-order
;{:id 123
; :owner "ikitommi"
; :LONGSTRING "................................."
; :items {:1 {:description "vadelmalimsa"
;             :tags ["good" "red"]
;             :discount 80
;             :amount 10},
;         :2 {:description "korvapuusti"
;             :tags ["raisin" "sugar"]
;             ::discount 80
;             :amount 20}}
; :delivery "2007-11-20T22:19:17+02:00"
; :location [61.499374 23.7408149]}

Let's coerce the data in:

(st/coerce
  ::order
  evil-json-order
  st/json-transformer)
;{:id 123,
; :owner "ikitommi"
; :LONGSTRING ".................................",
; :delivery #inst"2007-11-20T20:19:17.000-00:00",
; :items {1 {:description "vadelmalimsa"
;            :tags #{:red :good}
;            :discount 80
;            :amount 10}
;         2 {:description "korvapuusti"
;            :tags #{:raisin :sugar}
;            ::discount 80
;            :amount 20}},
; :location [61.499374 23.7408149]}

Also, the invalid data is still there. And according to clojure.spec, it's still valid:

(s/valid?
  ::order
  (st/coerce
    ::order
    evil-json-order
    st/json-transformer))
; true

To strip out values not defined in the specs, we can compose a custom transformer that does both JSON->EDN and strips out all extra keys from s/keys and s/tuple values:

(def strict-json-transformer
  (st/type-transformer
    st/json-transformer
    st/strip-extra-keys-transformer
    st/strip-extra-values-transformer))

Let's coerce again:

(st/coerce
  ::order
  evil-json-order
  strict-json-transformer)
;{:id 123,
; :items {1 {:description "vadelmalimsa"
;            :tags #{:good :red}
;            :amount 10},
;         2 {:description "korvapuusti"
;            :tags #{:raisin :sugar}
;            :amount 20}},
; :delivery #inst"2007-11-20T20:19:17.000-00:00",
; :location [61.499374 23.7408149]}

Extra data is gone. And the value is still valid:

(s/valid?
  ::order
  (st/coerce
    ::order
    evil-json-order
    strict-json-transformer))
; true

Custom coercion

By default, coercion uses the spec parser :type information to apply the coercion. Type-information is exctracted automatically from all/most clojure.core predicates. If you want to support custom predicates, there are multiple options

Composite specs

Simplest way is to create composite spec with s/and. Coercion reads the types from left to right and applies all found coercions.

(def adult? (s/and int? #(>= % 18)))

(st/coerce adult? "20" st/string-transformer)
; 20

Manually typed

We can use st/spec to annotate specs and add a :type hint manually:

(def adult? 
  (st/spec 
    {:spec #(>= % 18)
     :type :long}))

(st/coerce adult? "20" st/string-transformer)
; 20

Spec-based transformations

Transformations can be included in the spec annotations. Below is a example of an simple keyword, with custom encode and decode transformer attached, together with other documentation:

(require '[clojure.string :as str])

(s/def ::my-keyword
  (st/spec
    {:spec #(and (simple-keyword? %) (-> % name str/lower-case keyword (= %)))
     :description "a lowercase keyword, uppercase in string-mode"
     :json-schema/type {:type "string", :format "keyword"}
     :json-schema/example "kikka"
     :decode/string #(-> %2 name str/lower-case keyword)
     :encode/string #(-> %2 name str/upper-case)}))

(st/coerce
  ::my-keyword
  "OLIPA.KERRAN/AVARUUS"
  st/string-transformer)
; :olipa.kerran/avaruus

Roadmap

Web-libs using spec-tools

You can plug-in spec-based coercion easily into any Clojure web app. To get more batteries, you can pick any of the pre-integrated solution from the list below. They provide tools for transparent parameter & response validation based on content-negotiation, extraxt JSON Schema / Swagger documentation out of specs and more:

  • Reitit, fast data driven routing for Clojure/Script. Supports single-page-apps, Ring and Pedestal/Interceptors. The reference implementation for spec-tools based coercion.
  • compojure-api sweet routing macros for the JVM.
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