Smarter way to reload Clojure code. Clj-Reload tracks namespace dependencies, unloads namespaces, and then loads them in the correct topological order.

This is only about namespace dependencies within a single project. It has nothing to do with Leiningen, Maven, JAR files, or repositories.

Dependency

io.github.tonsky/clj-reload {:mvn/version "0.4.3"}

The problem

Do you love interactive development? Although Clojure is set up perfectly for that, evaluating buffers one at a time can only get you so far.

Once you start dealing with the state, you get data dependencies, and with them, evaluation order starts to matter, and now you change one line but have to re-eval half of your application to see the change.

But how do you know which half?

The solution

Clj-reload to the rescue!

Clj-reload scans your source dir, figures out the dependencies, tracks file modification times, and when you are finally ready to reload, it carefully unloads and loads back only the namespaces that you touched and the ones that depend on those. In the correct dependency order, too.

Let’s do a simple example.

a.clj:

(ns a
  (:require b))

b.clj:

(ns b
  (:require c))

c.clj:

(ns c)

Imagine you change something in b.clj and want to see these changes in your current REPL. What do you do?

If you call

(clj-reload.core/reload)

it will notice that

• b.clj was changed,
• a.clj depends on b.clj,
• there’s c.clj but it doesn’t depend on a.clj or b.clj and wasn’t changed.

Then the following will happen:

Unloading a
Unloading b
Loading b
Loading a

So:

• c wasn’t touched — no reason to,
• b was reloaded because it was changed,
• a was loaded after the new version of b was in place. Any dependencies a had will now point to the new versions of b.

That’s the core proposition of clj-reload.

Usage

Initialize:

(require '[clj-reload.core :as reload])

(reload/init
  {:dirs ["src" "dev" "test"]})

:dirs are relative to the working directory.

Use:

(reload/reload)
; => {:unloaded [a b c], :loaded [c b a]}

Works best if assigned to a shortcut in your editor.

Usage: recovering from errors

reload can be called multiple times. If reload fails, fix the error and call reload again.

Alternatively, you can call unload which will unload all changed code and will not try loading it back. Fix the code and call reload again to load it back.

Usage: Return value

reload returns a map of namespaces that were reloaded:

{:unloaded [<symbol> ...]
 :loaded   [<symbol> ...]}

By default, reload throws if it can’t load a namespace. You can change it to return exception instead:

(reload/reload {:throw false})

; => {:unloaded  [a b c]
;     :loaded    [c b]
;     :failed    b
;     :exception <Throwable>}

Usage: Choose what to reload

By default, clj-reload will only reload namespaces that were both:

• Already loaded
• Changed on disk

If you pass :only :loaded option to reload, it will reload all currently loaded namespaces, no matter if they were changed or not.

If you pass regexp to :only, clj-reload will search for matching namespaces and load them. This is useful, for example, if you want to find all tests namespaces:

(reload/reload {:only #".*\.-test"})

This will find and load all namespaces, no matter if they were loaded before, that match this regexp.

Finally, if you pass :only :all option to reload, it will reload all namespaces it can find in the specified :dirs, no matter whether loaded or changed.

Usage: Skipping reload

Some namespaces contain state you always want to persist between reloads. E.g. running web-server, UI window, etc. To prevent these namespaces from reloading, add them to :no-reload during init:

(reload/init
  {:dirs ...
   :no-reload '#{user myapp.state ...}})

Alternatively, if you want to never unload some namespace, but still reload it (e.g. it contains important state, but only in defonce-s), use :no-unload:

(reload/init
  {:dirs ...
   :no-unload '#{app.main ...}})

:no-reload implies :no-unload.

Usage: Unload hooks

Sometimes your namespace contains stateful resource that requires proper shutdown before unloading. For example, if you have a running web server defined in a namespace and you unload that namespace, it will just keep running in the background.

To work around that, define an unload hook:

(def my-server
  (server/start app {:port 8080}))

(defn before-ns-unload []
  (server/stop my-server))

before-ns-unload is the default name for the unload hook. If a function with that name exists in a namespace, it will be called before unloading.

You can change the name (or set it to nil) during init:

(reload/init
  {:dirs [...]
   :unload-hook 'my-unload})

For symmetry, there’s also :reload-hook 'after-ns-reload that triggers after reload.

Usage: Keeping vars between reloads

One of the main innovations of clj-reload is that it can keep selected variables between reloads.

To do so, just add ^:clj-reload/keep to the form:

(ns test)

(defonce x
  (rand-int 1000))

^:clj-reload/keep
(def y
  (rand-int 1000))

^:clj-reload/keep
(defrecord Z [])

and then reload:

(let [x test/x
      y test/y
      z (test/->Z)]
  
  (reload/reload)
  
  (let [x' test/x
        y' test/y
        z' (test/->Z)]
    (is (= x x'))
    (is (= y y'))
    (is (identical? (class z) (class z')))))

Here’s how it works:

• defonce works out of the box. No need to do anything.
• def/defn/deftype/defrecord/defprotocol can be annotated with ^:clj-reload/keep and can be persisted too.
• Project-specific forms can be added by extending clj-reload.core/keep-methods multimethod.

Why is this important? With tools.namespace you will structure your code in a way that will work with its reload implementation. For example, you’d probably move persistent state and protocols into separate namespaces, not because logic dictates it, but because reload library will not work otherwise.

clj-reload allows you to structure the code the way business logic dictates it, without the need to adapt to developer workflow.

Simply put: the fact that you use clj-reload during development does not spill into your production code.

Usage: init-less workflow

Sometimes it might be useful to integrate clj-reload into your system-wide profile or into tool like CIDER to be available in all your projects without explicitly adding it as a dependency.

To support that, clj-reload:

• Lets you skip init, in which case it’ll initialize with every directory it can find on classpath,
• Supports :clj-reload/no-reload and :clj-reload/no-unload meta on namespace symbol, like this:

(ns ^:clj-reload/no-reload no-reload
  (:require ...))

In that case, you can just call clj-reload.core/reload and it should work with default settings.

Comparison: Evaluating buffer

The simplest way to reload Clojure code is just re-evaluating an entire buffer.

It works for simple cases but fails to account for dependencies. If something depends on your buffer, it won’t see these changes.

The second pitfall is removing/renaming vars or functions. If you had:

(def a 1)

(def b (+ a 1))

and then change it to just

(def b (+ a 1))

it will still compile! New code is evaluated “on top” of the old one, without unloading the old one first. The definition of a will persist in the namespace and let b compile.

It might be really hard to spot these errors during long development sessions.

Comparison: (require ... :reload-all)

Clojure has :reload and :reload-all options for require. They do track upstream dependencies, but that’s about it.

In our original example, if we do

(require 'a :reload-all)

it will load both b and c. This is excessive (b or c might not have changed), doesn’t keep track of downstream dependencies (if we reload b, it will not trigger a, only c) and it also “evals on top”, same as with buffer eval.

Comparison: tools.namespace

tools.namespace is a tool originally written by Stuart Sierra to work around the same problems. It’s a fantastic tool and the main inspiration for clj-reload. I’ve been using it for years and loving it, until I realized I wanted more.

So the main proposition of both tools.namespace and clj-reload is the same: they will track file modification times and reload namespaces in the correct topological order.

This is how clj-reload is different:

• tools.namespace reloads every namespace it can find. clj-reload only reloads the ones that were already loaded. This allows you to have broken/experimental/auxiliary files lie around without breaking your workflow TNS-65

• First reload in tools.namespace always reloads everything. In clj-reload, even the very first reload only reloads files that were actually changed TNS-62

• clj-reload supports namespaces split across multiple files (like core_deftype.clj, core_defprint.clj in Clojure) TNS-64

• clj-reload can see dependencies in top-level standalone require and use forms TNS-64

• clj-reload supports load and unload hooks per namespace. Since tools.namespace doesn’t report which namespaces it’s going to reload, you always have to rebuild the entire state TNS-63

• clj-reload can specify exclusions during configuration, without polluting the source code of those namespaces.

• clj-reload can keep individual vars around and restore previous values after reload. E.g. defonce doesn’t really work with tools.namespace, but it does with clj-reload.

• clj-reload has 2× smaller codebase and 0 runtime dependencies.

• clj-reload doesn’t support ClojureScript. Patches welcome.

ClojureScript?

clj-reload doesn’t support ClojureScript. Patches are welcome.

License

Copyright © 2024 Nikita Prokopov

Licensed under MIT.