defn-spec

A Clojure(Script) wrapper around defn that optionally adds checking to a function's args and/or return value via assertions.

NOTE: This is an experimental idea. Feedback (good idea, bad idea, this/that doesn't work, etc.) is appreciated.

Installation

[defn-spec "0.1.2"]

Usage

You use defn-spec in the same way you would use defn.

(require '[clojure.spec.alpha :as s])
(require '[defn-spec.core :as ds])

(ds/defn-spec my-inc
  [x]
  (inc x))

Now let's add Spec assertions to the above function definition by adding an attribute map.

(ds/defn-spec my-inc
  {::s/args (s/cat :x int?)
   ::s/ret  nat-int?}
  [x]
  (inc x))

Open a REPL and try calling your new function...

(my-inc 1)
=> 2

(my-inc "a")
ExceptionInfo Call to user/my-inc did not conform to spec:
In: [0] val: "a" fails at: [:x] predicate: int?
  
(my-inc -2)
ExceptionInfo Return value from user/my-inc did not conform to spec:
val: -1 fails predicate: nat-int?

If defn-spec is not working, ensure s/*compile-asserts* is true.

I suggest checking out expound to greatly improve the above error messages.

fdef automatic instrumentation

If you prefer to keep your function's spec separate from its definition and still want instrument to be enabled automatically, take a gander at defn-spec.core/fdef.

(defn instrumented-fn
  [x]
  (inc x))

(ds/fdef instrumented-fn
         :args (s/cat :x number?)
         :ret number?)

ds/fdef works exactly the same as clojure.spec.alpha/fdef, differencing itself by automatically enabling instrumentation for the passed in symbol. Here is a cleaned up version of what the macro expands to.

(let
  [r (s/fdef instrumented-fn
             :args (s/cat :x number?)
             :ret number?)]
  (st/instrument `instrumented-fn)
  r)

If you set s/*compile-assers* to false, fdef will elide the call to instrument.

Keeping your fdef's in a separate namespace from where your function is defined results in you falling susceptible to the initial/global call issue. You Have Been Warned.

In Production

Set s/*compile-asserts* to false for all defn-spec expansions to compile to a regular defn form.

(binding [s/*compile-asserts* false]
  (macroexpand-1 '(ds/defn-spec my-inc
                    {::s/args (s/cat :x int?)
                     ::s/ret  nat-int?}
                    [x]
                    (inc x))))
=> (clojure.core/defn my-inc 
     #:clojure.spec.alpha{:args (s/cat :x int?), :ret nat-int?} 
     [x] 
     (inc x))

FAQs

Why not fdef?

fdef requires that you keep the Specs for your function's args and return value in a separate location. This approach lets you easily collocate the Specs for your function and its definition.

It is not easy to instrument initial/global function calls. This is best explained by example. Imagine you have an API that has a reg-event function. This function takes a keyword ID and a handler function and does some side effecting things (i.e. registers it in a global registrar). Here is our API namespace:

(ns my-project.api
  (:require
    [clojure.spec.alpha :as s]))

(defn reg-event
  [id handler]
  ;; do side effecting stuff
  nil)

(s/fdef reg-event
        :args (s/cat :id keyword? :handler fn?)
        :ret nil?)

(reg-event "my-event" (fn []))

You'll notice we call the reg-event function within the api namespace, but our call to reg-event was incorrect. We do not get a Spec instrumentation error, however. This is confusing because you know that you enabled instrumentation in your dev.user namespace.

(ns dev.user
  (:require
    [clojure.spec.test.alpha :as st]
    [my-project.api]))

(st/instrument)

Thinking carefully about the order things are executed, we discover the problem.

1. Load dev.user in the REPL...
2. Require clojure.spec.test.alpha.
3. Require my-project.api.
4. Run (st/instrument)
5. ... dev.user finished loading.

The call to instrument happens after we require'ed my-project.api. The incorrect call to reg-event in the my-project.api namespace was not instrumented when it was executed so we do not get an instrumentation error.

You may think that you can simply include the instrument call after every fdef but that isn't a great idea for a couple reasons. First, it does not scale well. You'll need to remember to include the call directly after every fdef for every one of your globally called functions. Let's hope you don't forget. Second, you need a way to ensure the instrument call does not occur in production. Maybe you wrap instrument in a macro to elide its call in production.

That seems like an awful lot of work to get instrumentation to behave correctly for initial/global function calls.

How is this different from Orchestra?

Orchestra provides a macro called defn-spec. That macro (and Orchestra itself) aims to extend Spec's instrumentation functionality, not take the assertion approach as this library does. Further, Orchestra's defn-spec macro specifies a new DSL for specifying a function's :args and :ret specs, which does not follow Clojure's defn format. IDEs (like Cursive) do not have support for this new DSL.

Tests

• Clojure: lein test
• Clojurescript: lein doo <js-env> (ex: lein doo phantom)

License

Copyright © 2018 Provisdom

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.