This document discusses using Clojure spec to validate form input and generate human-readable error messages. It describes using a multi-method error formatter to match validation predicates and generate customized error messages. Default error formatters are provided that match common predicates like blank values and string lengths. The spec and error formatters can then be used with ClojureScript UI components like text fields to validate user input and display errors. Values can also be normalized during validation using spec conformers.

1. Form Validation with
Clojure Spec
Alexander Kiel
Clojure Meetup Leipzig 2017

2. General Idea
• We like to use the same specs for validating user
input and for server-side validation
• We use an error-formatter to generate human-
readable error messages from explain-data

3. Error Formatter
• multi-method with hierarchic dispatch on
[normalized-pred id via]
[normalized-pred id]
[normalized-pred]
• Predicates are normalised by removing all numbers
(<= (count %) 10) ==> (<= (count %))
• default error formatters are available
• no need to write an error formatter for each input

4. Default Error Formatters
;; Matches on (not (blank? %)) predicate
(defmethod error-formatter '[(not (blank? %))]
[_ _]
"Wert erforderlich”)
;; Matches on normalised (<= (count %)) predicate and
;; uses full predicate and val from problem for error msg
(defmethod error-formatter '[(<= (count %))]
[_ {:keys [pred val]}]
(let [max-length (last pred)]
(gstr/format "Maximale Länge von %s Zeichen um %s
Zeichen überschritten"
max-length (- (count val) max-length))))

5. Match Error Formatters by ID
(defmethod error-formatter '[(re-matches %)]
[_ _]
"Ungültiger Barcode.")
(defmethod error-formatter '[(re-matches %) "in-insert-pos"]
[_ _]
"Ungültige Position.")
(defmethod error-formatter '[(re-matches %) "out-group-id"]
[_ _]
"Ungültige Gruppe.")

6. Usage in TextField
• TextField uses default error formatters which match
the used predicate.
(s/def ::name #(not (blank? %)))
(form/validating-text-field
{:id "sso-name"
:auto-focus true
:floating-label-text "Name"
:spec ::name
:on-change #(update-order! ::name %)})

7. Conforming Values
• one can normalise input values through s/conform
• in this example, users don’t have to input positions uppercase
(def upper-case-conformer
(s/conformer
(fn [x]
(if (string? x)
(str/upper-case x)
::s/invalid))))
;; A 2D position from A1 to D7
(s/def ::pos
(s/and upper-case-conformer
#(re-matches #"[A-D][1-7]" %)))

8. GitHub Project
https://github.com/alexanderkiel/material-comp