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Installing a zprint filter

If you haven't used zprint before and are running on MacOS or Linux, check out the native-image approach. Continue with this article if that doesn't work for you.

Would you be interested in a capability where you could type a few keystrokes, wait about 1.5 seconds, and have a Clojure(script) function pretty-printed from scratch (not just indented) right in your editor?

Quick Start:

The more complete explanation...

What is a Unix filter

In the Unix world (which includes Linux and MacOS for the purposes of this discussion), a filter is a program that reads from standard input, performs some operation on the data, and writes to standard output. Examples are grep, sort, cat, uniq, tail and fmt. All of these process and transform or select text from standard in (stdin) and put the results of these transformations on standard out (stdout).

fmt is an interesting example, because it doesn't actually change the content of the text passed through it, but rather it changes the layout or formatting of the text -- by word wrapping the result. Back in the day, fmt was used to word-wrap email and even today people use it make text fit conveniently into some given number of columns. The markdown source text for this paragraph, for instance, was piped through fmt to make it fit nicely inside of an 80 column editor window.

What would a zprint filter do

In the same mold as fmt, which reformats plain ASCII text, a zprint filter would accept Clojure(script) source on stdin, and produce pretty-printed Clojure(script) source on stdout.

Why would I want a zprint filter

Most editors or IDE's have some way to take a section of the source and pipe it though a filter and replace the source with whatever the filter returns. Thus, I can take one of the paragraphs of this text file and pipe it through fmt with the vi clone editor that I use. Certainly vim and emacs both can take text (which might be Clojure(script) source) and pipe it through a filter and then replace the source with whatever is returned.

So, if you had a zprint filter, by typing a few keystrokes, you could have any individual function pretty-printed by zprint while you were sitting in an editor or IDE looking at the function.

We all know Clojure startup is so slow that this is never going to work

Yes, I thought that Clojure startup would be way too slow, so I spent some significant time exploring Clojurescript specifically so that I could build a zprint formatting filter using Clojurescript. And I did, and it does work in Clojurescript -- you can see the results of my efforts in lein-zprint here

However, I have found that, if I AOT compile all of zprint and the associated libraries (and use just minimal libraries), and use the right Java command line arguments (and caching techniques), that I can get a Clojure zprint filter to startup slightly faster than a Clojurescript zprint filter. And to do so in just under 1 second. Which isn't zero, but which is fast enough to be perfectly useful.

Moreover, as the amount of source to be formatted grows, the Clojure version runs considerably faster than the Clojurescript version.

Performance Results

I have built two Clojurescript zprint filters, one using planck, which uses JavascriptCore on MacOS, and one using lumo, which uses node.js. There are also three different invocations of the Clojure zprint filter, which I will explain later.

The bottom line: If you download an uberjar from github, you can have a filter that will pretty-print your Clojure(script) source code in about 1.5 seconds for a reasonable sized function. There are two ways to set it up to be fast, with the "appcds" approach being about 10% faster than the "bootclasspath" approach. Both are quite simple, and there are complete instructions below on how to do it.


If you want the details, here they are.

The machine is a MacBook Air (13-inch, Mid 2012), 2 GHz Intel Core i7, 8 GB 16000 MHz DDR3. SSD disk, but mostly it runs out of the file cache.

All numbers are an average of at least 7 runs, using whatever kind of cache is available (both Clojurescript filters cache the compiled Javascript, and the zprint-filter appcds has a cache as well).

The table below has a lot of data in it. There are three basic experiments:

  • "Startup Secs" -- how long does it take to format the string "hello world", which measures startup and actually doing something -- but not much.

  • "Format defn" -- how long does it take to format the 1.8 source for the function defn. This is 49 lines long, and it is a reasonable example for a function you might want to format.

  • "Format 3K loc" -- how long does it take to format a source file in zprint itself, which has 3K lines in it. It contains 113 lines of comments. It has a number of of normal sized functions and a couple of very large ones. This is the entire source file, something you could certainly use the filter for, but not the expected use case.

I've included this edge case because it shows how Java really does run faster than Javascript, but it can take some work to get it to start up quickly.

Here are the numbers:

Filter Startup Secs % of basic Startup Format defn (49 loc) % of basic format defn Format 3K loc % of basic format 3K loc
planck (JavascriptCore) 3.100 158% 3.530 146% 13.854 194%
zprint-filter basic 1.955 100% 2.415 100% 7.107 100%
zprint-filter bootclasspath 1.179 60% 1.611 67% 5.914 83%
lumo (node.js) 1.003 51% 2.188 91% 20.442 288%
zprint-filter appcds .996 51% 1.407 58% 5.900 83%

Your machine is probably faster than my 2012 MacBook Air, so you would see even better numbers.

Okay, how do I set this up for myself

The two lines you should be interested in are the zprint-filter bootclasspath and zprint-filter appcds lines. As you might imagine, the bootclasspath is easier to set up, and the appcds runs faster for small functions.

I have provided an uberjar in the releases area, which you need to download. It doesn't have a .jar extension because I find that various programs get all excited about .jar files. Why upset them needlessly?

First, you need to find a directory which is on your path (or, if you don't have one, you need to create one). I use ~/bin, but it doesn't matter. You are going to create a shell script.

appcds version

This approach should give you sub-second startup for the zprint-filter!

It uses a "commercial" feature of Java that, unless you have paid for a license, you are not supposed to use in production. However, developers can use this feature for free, and using it while editing source is clearly not "production". I have actually talked to people at Oracle and have been assured that developers can use this feature while doing development.

This option was added in Java 1.8.0_40. If you don't have a version of Java 1.8 later than _40, don't bother with this approach, go on to the bootclasspath version below.

Find out what you have this way:

java -version

This approach (using the same zprint-filter downloaded from Github), creates a cache of information in a single file that will speed up startup. Not clear exactly what is in the cache, but it does help, more than any other approach.

There are two ways to set this up.

The automated (easy) way

You can use a script released along with the zprint-filter uberjar to create another script which will run the zprint-filter in a way that it will startup in less than a second.

  1. Download zprint-filter-0.5.3 from releases

  2. Download the install script appcds from releases

  3. Put both of these files in a directory where they can live for a while. Once you run the appcds script, the zprint-filter uberjar can't be moved.

  4. Make the appcds script executable:

chmod +x appcds
  1. Decide what you would like to have your new shell script be named. You might want this to be a short name, because you may be typing it to pretty print a function in your editor. So the number of keystrokes it takes to run it probably matters. You can rename it later if you want. For now, call it za (for "zprint-filter appcds"), which is what some of the examples later assume.

  2. Run the install script:

./appcds zprint-filter-0.5.3 za

It will run pretty quickly when it works, and will spit out less than a page of status and statistics. When it completes, it outputs some text showing you how to test the result.

  1. You can put the za script anywhere, but it will only be useful if it resides in a directory on your PATH. You can name it anything you want.

This output script will accept Clojure(script) source on stdin and send the pretty-printed version to stdout. See below for how to use it.

The manual way

Here are the steps to set this version up manually:

  1. Put the zprint-filter uberjar in some directory on your path. It doesn't have to be there, but these instructions assume it will be. Feel free to move it (but you'll have to understand what is going on a bit to do so).

  2. Create a file for testing called helloworld.clj with the single line "hello world" in it.

  3. Choose a filename for the new class cache. This example assumes zprint.filter.cache

  4. Create the list of classes used on startup -- type this command:

java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:off \
        -XX:DumpLoadedClassList=zprint.filter.classlist \
    -cp zprint-filter-0.5.3 \
        zprint.main  < helloworld.clj > /dev/null

or, for easier copying and pasting:

java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:off -XX:DumpLoadedClassList=zprint.filter.classlist -cp zprint-filter-0.5.3 zprint.main < helloworld.clj > /dev/null
  1. Build the cache now that you have the list of classes used -- type this command. Note that this assumes it is running with a shell that will replace "pwd" with the path.
java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:dump \
        -XX:SharedClassListFile=zprint.filter.classlist \
        -XX:SharedArchiveFile=`pwd`/zprint.filter.cache \
    -cp `pwd`/zprint-filter-0.5.3 \
        zprint.main < helloworld.clj 

or, for easier copying and pasting:

java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:dump -XX:SharedClassListFile=zprint.filter.classlist -XX:SharedArchiveFile=`pwd`/zprint.filter.cache -cp `pwd`/zprint-filter-0.5.3 zprint.main < helloworld.clj 

This will output a bunch of statistics about building the cache.

  1. Create a file named za (for zprint-filter appcds) with the following contents:
java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:on \
        -XX:SharedArchiveFile=`pwd`/zprint.filter.cache \
    -cp `pwd`/zprint-filter-0.5.3 zprint.main

or, for easier copying and pasting:

java -XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:on -XX:SharedArchiveFile=`pwd`/zprint.filter.cache -cp `pwd`/zprint-filter-0.5.3 zprint.main
  1. Make that file executable:
chmod +x za
  1. Try it out:
za < helloworld.clj

you should see "hello world" come out on your terminal.

  1. Try it from somewhere else. Change your directory to somewhere other than the directory with the za file in it and type:
za < wherever-the-helloworld-file-is/helloworld.clj

You should see the "hello world" come out on the terminal.

If you see this instead:

Error: Could not find or load main class zprint.main

something is wrong with the path to the archive file and the zprint-filter file. These files need to be specified with absolute paths, and that was why there is a pwd in the commands above. The za script you have created needs to have absolute paths to the class cache and the uberjar or it won't work.

For what it is worth, this should startup about twice as fast as just running the uberjar "normally":

java -jar zprint-filter <helloworld.clj

and about 10% faster than the bootclasspath approach, described above.

bootclasspath version

  1. Put the zprint-filter uberjar in some directory on your path. It doesn't actually have to be there -- only the shell script you are creating does -- but these instructions assume it will be. Feel free to put it wherever you want (but you will have to modify these instructions a bit to do so).

  2. Change your current directory to the directory on your path.

  3. Create a file for testing called helloworld.clj with the single line "hello world" in it.

  4. Create a file called zb (for zprint-filter bootclasspath) containing the following single line:

java -Xbootclasspath/a:`pwd`/zprint-filter-0.5.3 zprint.main

Note that the switch you are using is: -Xbootclasspath/a:, and immediately after that (with no spaces) comes the fully specified file path to zprint-filter.

  1. Make that file executable:

chmod +x zb

  1. Then, try it out:

./zb < helloworld.clj

you should see "hello world" come out on your terminal.

  1. Try it from somewhere else. Change your directory to somewhere other than the directory with the zb file in it, and try it on some Clojure(script) source (called "x.clj" in the following example):

zb <x.clj

You should see the source to x.clj come out on your terminal.

Now, how do I use it

There are two aspects to using this effectively in your editing environment, and a third that can help:

  1. How to move to the start of a function definition? [nice to have]

  2. How to select the text to send to the zprint-filter? [required]

  3. How to actually send the text to the zprint-filter? [required]

You may find one command that will do both #2 and #3 (see vim, below). But they might be separate commands or keystrokes that you might want to make a macro or user defined command in your editor.


In vim, if your cursor is at the top level left parenthesis of a function definition, you can type !a(za if you have created the filter script to be called za. This will pipe everything to the balanced right parenthesis through za, formatting the entire function.

There are other ways to mark all of the text between parentheses in vim.

You can also pass an options map to the zprint-filter, though you have to enclose it in single quotes. For instance, you could type this: !a(za '{:vector {:wrap? false}}' and have the zprint-filter no pack things into vectors.


Emacs seems to have several ways to move to the top of a function definition and to then mark a region bounded by parentheses, and your personal configuration may have even more. I'll leave it to you to find the best way to do that in your environment.

To pipe a region through a filter and replace the region with the returned text use C-u M-| za RET, where za is the filter script that you created above.

Of course you don't have to limit what you send to just the characters of a function definition. You can send multiple function defintions, or you can include ;!zprint {...} formatting directives. You can even send a whole buffer through the filter. What you cannot do is send an unbalanced set of delimiters (parentheses, braces, brackets) through the filter. If you do, it will return an error and the unmodified original text.

Sublime Text 2 or 3

In Sublime Text, you can use the External Command plugin to send either the entire file or your current selection to this executable.

Once you've installed External Command (manually or via Package Control), place the following in your sublime-keymap file (where we assume za is the name of the filter script you created, and KYBRD SHORTCUT is what you want to use to do this operation when editing):

{ "keys": ["KYBRD SHORTCUT"], "command": "filter_through_command", "args": { "cmdline": "za" } }

At this point, you can use your keyboard shortcut to send the entire file to your executable.

If you have selected text, it will send your current selection to the executable and replace only that selection with the output. You can also use Selection > Expand Selection to Brackets (defaults to Ctrl-Shift-m) to select between parentheses and repeat it once to include the parentheses themseleves. If you are deep within a function, you may need to repeat it several times to get to the top level. There may be other ways to get to the top level.

Other Editors

I would suggest that you check the documentation for your editor or IDE for how to send text to the fmt filter, as the answer to that will tell you how to send text to the zprint-filter.

You then want to figure out how to send all of the items between balanced parentheses to an external program.

Then put these together.

If you get this to work in an editor not yet listed here, please file an issue and tell me how to you did it, and I'll update the instructions here.

Configuring the filter

You can configure the filter in two (and only two) ways:

  1. Using the ~/.zprintrc file.

  2. Placing an options-map on the command line as the only argument.

You cannot configure the filter from environment-variables or from Java properties.

The filter will recognize and respond to ;!zprint { < options-map > } directives in a source file -- but only if it actually encounters them. So if you pipe a whole file through the zprint-filter, it will encounter any ;!zprint directives you have included. If you pipe only a function definition through the zprint-filter, it won't see any of the ;!zprint directives. If you have !zprint directives located contiguous to function definitions, it is fairly easy to include them in the text you send to the zprint-filter script.

Different files for different styles

You can create multiple files, each with a different format directive. For instance, the options map {:style :spec} is really helpful for formatting clojure.spec files, so you might have:

File zb: java -Xbootclasspath/a:zprint-filter zprint.main

File zs: java -Xbootclasspath/a:zprint-filter zprint.main '{:style :spec}'

and you would use zs when formatting spec files, and zb when formatting other files. You can do the same with the AppCDS cache file approach, and each shell script can use the same cache.

You might have a file zj for {:style :justified}, so for certain functions you might format them justified.

You can also edit the appcds version (which we called za above) to do the same things.

You can have as many different shell scripts as you want, that each do something different.

If you have used ;!zprint {< option >} in your source files, if you don't pipe those comments though commands through the filter, it won't see them and format things the way you want. But with different shell scripts you can get fine grained control of the filter.

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