Validations for maps in clojure
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Mississippi Logo

Mississippi provides validation for maps.


A simple example:

user> (use 'mississippi.core)
user> (def subject {:a nil :b 1})
user> (def validations {:a [(comp not nil?) :msg "required"]
                        :b [number? :msg "not numeric"]})
user> (validate subject validations)
{:a nil, :b 1, :errors {:a ("required")}}

Validations are defined as a map matching the structure of the map or subject to be validated. Each key in the validation map has one or more validations defined.

Defining Validations

A validation is a vector containing:

  • a predicate function determining if the attribute is valid or not (required)
  • a key value pair of :msg => function / string
  • a key value pair of :when => function

For example:

{:foo [numeric? :msg "non-numeric!"]}

Will call the built-in numeric? function with the value of :foo in the subject being validated. If this returns false, then the :msg will be assoc'd into the subject inside an :errors map:

{:foo "not number" :errors {:foo ("non-numeric!")}}

The error value is a list, because there can be multiple validations.

Multiple validations

Provide a vector of validation vectors to define multiple validations for a given attribute:

{:foo [[(comp not nil?) :msg "required"]
       [numeric?        :msg "non-numeric!"]]}

Would produce:

{:foo nil :errors {:foo ("required" "non-numeric!")}}

when applied to:

{:foo nil}

Conditional Validations

It is sometimes useful to be able to validate a given attribute based on another inter-related attribute. This is achieved through the :when option, for example:

user> (def subject {:a 501 :b :low})
user> (def validations {:a [#(< % 500) :msg "too high!" :when #(= :low (:b %))]})
user> (validate subject validations)
{:a 501, :b :low, :errors {:a ("too high!")}}

The :when function takes a single argument: the subject under validation.

Built-in Validators

Several common-case validators are built-in for your convenience! All are functions which return a validation vector and support the :when and :msg options, they do however provide sensible default messages. Validation functions that take arguments are shown with an example.

member-of     ;; (member-of #{:a :b :c})
in-range      ;; (in-range 1 10)
subset-of     ;; (subset-of #{:a :b :c})
matches       ;; (matches #"foo")

An example usage:

user> (def validations {:a [(required)
                            (in-range 1 10)]})
user> (validate {:a nil} validations)
{:errors {:a ("required" "not a number" "does not fall between 1 and 9")}}

The matches validator takes an extra optional argument of :match-fn. This defaults to re-find, but you can override to use others such as re-seq or re-matches if desired.


Mississippi is hosted on Clojars.


Add the following to :dependencies in your project.clj

[mississippi "1.0.1"]


Copyright (C) 2010 Michael Jones, Gareth Jones

Distributed under the Eclipse Public License, the same as Clojure.