io.aviso/pretty¶

Sometimes, neatness counts¶

If you are trying to puzzle out a stack trace, pick a critical line of text out of a long stream of console output, or compare two streams of binary data, a little bit of formatting can go a long way.

That’s what the io.aviso/pretty library is for. It adds support for pretty output where it counts:

Readable output for exceptions
ANSI font and background color support
Hex dump of binary data
Hex dump of binary deltas
Formatting data into columns

Here’s an example of pretty at work:

License¶

Pretty is released under the terms of the Apache Software License 2.0.

ANSI Formatting¶

The io.aviso.ansi namespace defines a number of functions and constants for producing ANSI escape codes.

For each of the supported colors (black, red, green, yellow, blue, magenta, cyan, and white) there will be four functions and four constants:

color - function to set text color
color-bg - function to set background color
bold-color - function to set enable bold text and the text color
bold-color-bg - function to enable bold text and the background color
color-font - constant that enables the text color
color-bg-font - constant that enables the color as background
bold-color-font - constant that enables the text color in bold
bold-color-bg-font - constant that enables the bold color as background

The functions are passed a string and wrap the string with ANSI codes to enable an ANSI graphic representation for the text, with a reset after the text.

Note that the exact color interpretation of the ANSI codes varies significantly between platforms and applications, and is frequently configurable, often using themes. You may need to adjust your application’s settings to get an optimum display.

In addition there are functions bold, inverse, and italic and constants bold-font, inverse-font, italic-font, and reset-font.

The above example could also be written as:

(println (str "The following text will be " bold-red-font "bold and red" reset-font "."))

Formatted Exceptions¶

Pretty’s main focus is on formatting of exceptions for readability, addressing one of Clojure’s core weaknesses.

Rationale¶

Exceptions in Clojure are extremely painful for many reasons:

They are often nested (wrapped and rethrown)
Stack frames reference the JVM class for Clojure functions, leaving the user to de-mangle the name back to the Clojure name
Stack traces are output for every exception, which clogs output without providing useful detail
Stack traces are often truncated, requiring the user to manually re-assemble the stack trace from several pieces
Many stack frames represent implementation details of Clojure that are not relevant

This is addressed by the io.aviso.exception/write-exception function; it take an exception and writes it to the console, *out*.

This is best explained by example; here’s a SQLException wrapped inside two RuntimeExceptions, and printed normally:

java.lang.RuntimeException: Request handling exception
  at user$make_exception.invoke(user.clj:30)
  at user$eval1322.invoke(NO_SOURCE_FILE:1)
  at clojure.lang.Compiler.eval(Compiler.java:6619)
  at clojure.lang.Compiler.eval(Compiler.java:6582)
  at clojure.core$eval.invoke(core.clj:2852)
  at clojure.main$repl$read_eval_print__6588$fn__6591.invoke(main.clj:259)
  at clojure.main$repl$read_eval_print__6588.invoke(main.clj:259)
  at clojure.main$repl$fn__6597.invoke(main.clj:277)
  at clojure.main$repl.doInvoke(main.clj:277)
  at clojure.lang.RestFn.invoke(RestFn.java:1096)
  at clojure.tools.nrepl.middleware.interruptible_eval$evaluate$fn__808.invoke(interruptible_eval.clj:56)
  at clojure.lang.AFn.applyToHelper(AFn.java:159)
  at clojure.lang.AFn.applyTo(AFn.java:151)
  at clojure.core$apply.invoke(core.clj:617)
  at clojure.core$with_bindings_STAR_.doInvoke(core.clj:1788)
  at clojure.lang.RestFn.invoke(RestFn.java:425)
  at clojure.tools.nrepl.middleware.interruptible_eval$evaluate.invoke(interruptible_eval.clj:41)
  at clojure.tools.nrepl.middleware.interruptible_eval$interruptible_eval$fn__849$fn__852.invoke(interruptible_eval.clj:171)
  at clojure.core$comp$fn__4154.invoke(core.clj:2330)
  at clojure.tools.nrepl.middleware.interruptible_eval$run_next$fn__842.invoke(interruptible_eval.clj:138)
  at clojure.lang.AFn.run(AFn.java:24)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
  at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.RuntimeException: Failure updating row
  at user$update_row.invoke(user.clj:22)
  ... 24 more
Caused by: java.sql.SQLException: Database failure
SELECT FOO, BAR, BAZ
FROM GNIP
failed with ABC123
  at user$jdbc_update.invoke(user.clj:6)
  at user$make_jdbc_update_worker$reify__214.do_work(user.clj:17)
  ... 25 more

On a good day, the exception messages will include all the details you need to resolve the problem … even though Clojure encourages you to use the ex-info to create an exception, which puts important data into properties of the exception, which are not normally printed.

Meanwhile, you will have to mentally scan and parse the above text explosion, to parse out file names and line numbers, and to work backwards from mangled Java names to Clojure names.

It’s one more bit of cognitive load you just don’t need in your day.

Instead, here’s the equivalent, using a hooked version of Clojure’s clojure.repl/pst, modified to use write-exception.

As you can see, this lets you focus in on the exact cause and location of your problem.

write-exception flips around the traditional order, providing a chronologically sequential view:

The stack trace leading to the root exception comes first, and is ordered outermost frame to innermost frame.
The exception stack comes after the stack trace, and is ordered root exception (innermost) to outermost, reflecting how the stack has unwound, and the root exception was wrapped in new exceptions and rethrown.

The stack trace is carefully formatted for readability, with the left-most column identifying Clojure functions or Java class and method, and the right columns presenting the file name and line number.

The stack frames themselves are filtered to remove details that are not relevant. This filtering is via an optional function, so you can define filters that make sense for your code. For example, the default filter omits frames in the clojure.lang package (they are reduced to ellipses), and truncates the stack trace when when it reaches clojure.main/repl/read-eval-print.

Repeating stack frames are also identified and reduced to a single line (that identifies the number of frames). This allows your infinite loop that terminates with a StackOverflowException to be reported in just a few lines, not thousands.

The inverted (from Java norms) ordering has several benefits:

Chronological order is maintained, whereas a Java stack trace is in reverse chronological order.
The most relevant details are at (or near) the bottom not the top; this means less “scrolling back to see what happened”.

The related function, format-exception, produces the same output, but returns it as a string.

For both format-exception and write-exception, output of the stack trace is optional, or can be limited to a certain number of stack frames.

io.aviso.repl¶

This namespace includes a function, install-pretty-exceptions, which hooks into all the common ways that exceptions are output in Clojure and redirects them to use write-exception.

When exceptions occur, they are printed out without a stack trace or properties. The clojure.repl/pst function is overridden to fully print the exception (with properties and stack trace).

In addition, clojure.stacktrace/print-stack-trace and clojure.stacktrace/print-cause-trace are overwritten; these are used by clojure.test. Both do the same thing: print out the full exception (again, with properties and stack trace).

You may not need to invoke this directly, as pretty can also act as a Leiningen Plugin.

io.aviso.logging¶

This namespace includes functions to change clojure.tools.logging to use Pretty to output exceptions, and to add a default Thread.UncaughtExceptionHandler that uses clojure.tools.logging.

REPL¶

Pretty includes some useful REPL utilities in the io.aviso.repl namespace.

Primarily, the install-pretty-exceptions function overrides several internal Clojure functions to enable formatted exceptions. This function is normally invoked for you when Pretty is used as a Leiningen plugin.

The remaining functions exist to help you make things pretty that don’t originate in the REPL. You will often see output in logs: EDN data perhaps, or often, raw exceptions.

If you have a REPL running, you can use the following functions to get a better view of that data:

copy¶

The copy function will return the current contents of the system clipboard as a string. This requires that AWT is running. On OS X, you will see a window for your application start when you first invoke this function

The pretty-print and format-exception functions can be invoked with no arguments, in which case the call to copy happens automatically.

Consult the API documentation for more details.

pretty-print¶

This will pretty-print the contents of the clipboard; the clipboard text is parsed as EDN.

format-exception¶

This will parse a normal Java stack trace and format it for readability.

paste¶

This will copy a string back on to the clipboard, so it can be pasted into another window.

(use 'io.aviso.repl)

(-> (copy) format-exception paste)

Leiningen Plugin¶

pretty can act as a plugin to Leiningen.

To enable pretty exception reporting automatically, add pretty to both the :plugins and the :dependencies lists of your project.clj.

(defproject ...
 :plugins [[io.aviso/pretty "1.0.0"]]
 :dependencies [...
                [io.aviso/pretty "1.0.0"]]
 ...)

Adjust the version number for the current version, “0.1.26”.

This adds middleware to enable pretty exception reporting when running a REPL, tests, or anything else that starts code in the project.

Another option is to add the following to your ~/.lein/profiles.clj:

:pretty {
  :plugins [[io.aviso/pretty "1.0.0"]]
  :dependencies [[io.aviso/pretty "1.0.0"]]
}

This creates an opt-in profile that adds and enables pretty exception reporting.

You can then enable pretty in any project, even one that does not normally have pretty as a dependency, as follows:

lein with-profiles +pretty run

or:

lein with-profiles +pretty do clean, test, install

You may also want to add the following to your ~/.bash_profile:

alias pretty="lein with-profile +pretty"

At which point, you can use the command pretty instead of lein.

Binary Output¶

The io.aviso.binary namespace provides support output of binary data.

Binary data is represented using the protocol BinaryData; this protocol is extended on byte arrays, on String, and on nil. BinaryData is simply a randomly accessible collection of bytes, with a known length.

(write-binary "Choose immutability and see where it takes you.")

0000: 43 68 6F 6F 73 65 20 69 6D 6D 75 74 61 62 69 6C 69 74 79 20 61 6E 64 20 73 65 65 20 77 68 65 72
0020: 65 20 69 74 20 74 61 6B 65 73 20 79 6F 75 2E

write-binary can write to a java.io.Writer (defaulting to *out*) or a StringBuilder (or other things, as defined by the StringWriter protocol). The full version explicitly specifies where to write to, as well as options:

Alternately, format-binary will return the formatted binary output string.

You can also compare two binary data values with write-binary-delta:

If the two data are of different lengths, the shorter one is padded with -- to make up the difference.

As with write-binary, there’s a format-binary-delta, and a three-argument version of write-binary-delta for specifying a StringWriter target.

Columnar Output¶

The io.aviso.columns namespace is what’s used by the exceptions namespace to format the exceptions, properties, and stack traces.

The format-columns function is provided with a number of column definitions, each of which describes the width and justification of a column. Some column definitions are just a string to be written for that column, such as a column separator. format-columns returns a function that accepts a StringWriter (such as *out*) and the column values.

write-rows takes the function provided by format-columns, plus a set of functions to extract column values, plus a seq of rows. In most cases, the rows are maps, and the extraction functions are keywords (isn’t Clojure magical that way?).

Here’s an example, from the exception namespace:

(defn- write-stack-trace
  [writer exception]
  (let [elements (->> exception expand-stack-trace (map preformat-stack-frame))
        formatter (c/format-columns [:right (c/max-value-length elements :formatted-name)]
                                    "  " (:source *fonts*)
                                    [:right (c/max-value-length elements :file)]
                                    2
                                    [:right (->> elements (map :line) (map str) c/max-length)]
                                    (:reset *fonts*))]
    (c/write-rows writer formatter [:formatted-name
                                    :file
                                    #(if (:line %) ": ")
                                    :line]
                  elements)))