Record Constructors

Some more Clojure Do’s and Don’ts for you. This week: record constructors.

Don’t use interop syntax to construct records

defrecord and deftype compile into Java classes, so it is possible to construct them using Java interop syntax like this:

(defrecord Foo [a b])

(Foo. 1 2)
;;=> #user.Foo{:a 1, :b 2}

But don’t do that. Interop syntax is for interop with Java libraries.

Since Clojure version 1.3, defrecord and deftype automatically create constructor functions. Use those instead of interop syntax.

For records, you get two constructor functions: one taking the values of fields in the same order they appear in the defrecord:

(defrecord Foo [a b])

(->Foo 1 2)
;;=> #user.Foo{:a 1 :b 2}

And another taking a map whose keys are keywords with the same names as the fields:

(map->Foo {:b 4 :a 3})
;;=> #user.Foo{:a 3, :b 4}

deftype only creates the first kind of constructor, taking the field values in order.

(deftype Bar [c d])

(->Bar 5 6)
;;=> #<Bar user.Bar@2168aeae>

Constructor functions are ordinary Clojure Vars. You can pass them to higher-order functions and :require :as or :refer them into other namespaces just like any other function.

Do add your own constructor functions

You cannot modify or customize the constructor functions that defrecord and deftype create.

It’s common to want additional functionality around constructing an object, such as validation and default values. To get this, just define your own constructor function that wraps the default constructor.

(defrecord Customer [id name phone email])

(defn customer
  "Creates a new customer record."
  [{:keys [name phone email]}]
  {:pre [(string? name)
         (valid-phone? phone)
         (valid-email? email)]}
  (->Customer (next-id) name phone email))

You don’t necessarily have to use :pre conditions for validation; that’s just how I wrote this example.

It’s up to you to maintain a convention to always use your custom constructor function instead of the automatically-generated one.1

I frequently define a custom constructor function for every new record type, even if I don’t need it right away. That gives me a place to add validation later, without searching for and replacing every instance of the default constructor.

Even custom constructor functions should follow the rules for safe constructors. In general, that means no side effects and no “publishing” the object to another place before the constructor is finished. Keep the “creation” of an object (the constructor) separate from “starting” or “using” it, whatever that means for your code.

Footnotes:

1

Theoretically you could make the default constructors private with alter-meta!, but I’ve never found it necessary.

2 thoughts on “Record Constructors”

  1. Can you clarify what you mean by “You cannot modify or customize the constructor functions that defrecord and deftype create.” ?

    The following works in Clojure 1.6.0:


    (defrecord Foo [num])

    (defn ->Foo
    [num]
    {:pre [(pos? num)]}
    (Foo. num))

    (defn -main
    [& args]
    (->Foo 1) ;; works
    (->Foo -100)) ;; assertion error

  2. That is an implementation detail which I would not recommend relying upon.

    Besides, it would be really confusing to someone who expects the default constructors to have the default behavior.

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