Skip to content
Define Clojure specs inline with your function definitions
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


CircleCI Clojars Project

defn-spec lets you create Clojure Specs inline with your defn. The syntax (and implementation) has been borrowed from Schema, so if you've used that before, this should be very familiar.

You can define your argument and return specs inline with your defn:

(ns my.ns
  (:require [net.danielcompton.defn-spec-alpha :as ds]
            [clojure.spec.alpha :as s]
            ; ...

;; Predicate definitions elided for brevity
(s/def ::instant instant?)
(s/def ::zone-id zone-id?)
(s/def ::zoned-date-time zoned-date-time?)

(ds/defn to-zoned-dt :- ::zoned-date-time
  [instant :- ::instant
   zone-id :- ::zone-id]
  (ZonedDateTime/ofInstant instant zone-id))

Instead of writing your fdef separately from your defn:

(defn to-zoned-dt
  [instant zone-id]
  (ZonedDateTime/ofInstant instant zone-id))

(s/fdef to-zoned-dt
        :args (s/cat :instant ::instant :zone-id ::zone-id)
        :ret ::zoned-date-time)


Add the defn-spec-alpha dependency to your project's dependencies. Don't add it to your development dependencies unless you are only speccing development time functions.


[net.danielcompton/defn-spec-alpha "0.1.0"]


 {net.danielcompton/defn-spec-alpha {:mvn/version "0.1.0"}}}

Note that clojure.spec is required to use defn-spec. This requires Clojure 1.9.0 or greater.


The basic syntax is the same as clojure.core/defn, but you can optionally add 'spec hints' with :- to any of the arguments and the return value. Any valid spec can be used, e.g. functions, sets, registered spec identifiers.

(ds/defn my-fn-name :- <:ret spec>
  [arg1 :- <spec for single arg>
   arg2 :- <spec for single arg>
   arg3] ;; Not all args need to be specced
   ; ...

The API is very minimal, there is just a single defn macro in the public API.


  • Multiple arity functions are not yet supported. #2
  • & rest, & [a b], and :keys destructuring are not yet supported. #3, #4, #10
  • :fn specs are not supported yet, as I'm not sure where to put the :fn spec yet. #6
  • ClojureScript is not supported yet. #7
  • Using an attr-map after the function definition is not supported. Looking at defn's docstring: (defn name doc-string? attr-map? ([params*] prepost-map? body) + attr-map?) it is the final + attr-map? that isn't supported. I've never seen this used in the wild, and didn't even know this was a thing until investigating the defn macro.


I've been using Clojure spec for a while, but I found that I often resisted writing specs for functions. This was mostly because I didn't want to have to duplicate a bunch of information into the fdef. It's not a huge deal, but in my experience it was enough to deter me from writing specs while I was heavily working on an area of code.

I created defn-spec to increase the locality of the spec definitions, and to reduce the activation energy to start adding specs to your codebase. I wanted to make something that was small enough to drop into any project or library I worked on.

defn-spec doesn't introduce any concepts or features to spec. The only deviation from vanilla spec is that defn-spec will automatically build the s/cat form for you from the arguments that you provide.


  • Orchestra has a defn-spec macro. Cursive doesn't yet have support for this macro though, and it requires you to provide specs for every function argument. Orchestra is great though, and I recommend everyone check it out, especially for instrumenting their :ret and :fn specs.
  • Ghostwheel has a defn macro. It also introduces a new way of describing specs, along with many other useful features.
  • Provisdom/defn-spec is more focused on instrumenting functions. It does allow inline definitions of specs, though the syntax is much the same as in an fdef.

Try them all out though, and see which one works best for you.

Benefits and tradeoffs

Like all things in life, defn-spec has benefits and tradeoffs:


  • Makes it easy to incrementally spec your functions. It lowers the activation energy needed to add a spec, perhaps just for a single arg or return value.
  • Makes it easier to see more code on one screen.
  • Makes it harder for your specs to get out of sync with the function definition, as they are linked together.
  • Avoids repeating all argument names in the s/cat form.


  • For some specs, particularly complex :args specs with many branches, it may be simpler to define the fdef separately. defn-spec is designed for the 80-90% of Clojure functions that have simple argument lists and return types.
  • Making it easier to change specs means that it is easier to accidentally break your callers. If your fdef is defined separately, it forces you to think more about growing the spec.

Implementation notes

  • defn-spec uses your :ret spec as-is, but constructs an :args spec for you based on the function's argument names and the 'spec hints' that you provide.
  • defn-spec should preserve all metadata, docstrings, and type hints in the same way that Clojure's defn does. If you spot anything that isn't being retained, this is a bug, let me know!
  • If you use the ds/defn macro, but don't define any 'spec hints' for the arguments or return value then no fdef spec is defined.
  • When expanding the ds/defn macro, the fdef is defined immediately after the clojure.core/defn. If you declare fdefs after this point, they will overwrite the defn-spec fdef. You cannot merge 'spec hints' defined on a function and other spec definitions in a standalone fdef.
  • Unlike schema, defn-spec doesn't control when/if function specs are checked. I recommend using orchestra in development to instrument the :args, :fn, and :ret specs for your fdef's.


This library is currently in alpha preview and is soliciting feedback on functionality and syntax from interested parties before publishing an official release. In the meantime, you can use a SNAPSHOT build at [TODO].

defn-spec-alpha follows clojure.spec.alpha. When clojure.spec-alpha2 is released, the plan is to publish a new artifact ID and set of defn-spec-alpha2 namespaces, so you can use both versions side-by-side as you migrate to spec-alpha2.

Long-term I would like defn-spec to be so stable that it is safe to include as a library dependency. There is only a single macro in this library to minimise the risk consumers are taking on. While I strongly want to keep source compatibility, I can't guarantee this in the short-term. Until this warning is removed I would recommend only using this in applications or libraries where you control all of the consumers. There have also been rumblings that eventually there may be something similar to this built into Clojure's core defn macro.

Cursive integration

You can tell Cursive to resolve defn-spec's defn macro like the schema defn macro. See the Cursive setup page for full details on how to do this.


Copyright © 2017-2019 Daniel Compton

defn-spec contains extensive copying from plumatic/schema, and couldn't have been completed without their great work.

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

You can’t perform that action at this time.