Using the KL Tool to Develop and Debug Extensions

Fabric Engine version 2.3.1
Copyright (c) 2010-2016, Fabric Software Inc. All rights reserved.

The KL Tool

Distributed with Fabric Engine is a command-line executable kl (kl.exe on Windows) under the bin/ folder in the Fabric Engine distribution. It is referred to as the KL tool.

The KL tool enables running KL programs from the command line and reporting results. The KL tool is used throughout the internal unit test suite at Fabric Software. In addition, the KL tool can be very useful in the development of your own custom KL extensions.

When a Fabric Engine environment is set up by sourcing at the root level of the Fabric Engine distribution, or by double-clicking prompt.bat on Windows, the KL tool should be available on your PATH and can be run on the command line as the command kl. You can verify that it is working by running kl --help, which should show you all the options available.

Compiling and Evaluating KL Source Files with the KL Tool

The KL tool should be run providing the name of a KL source file to compile and run. The provided KL source file is normally a small text file containing an entry operator (see below) and some code to generate output. For example, if the file MyTest.kl contained KL source code, you would run it in the KL tool with the command:

kl MyTest.kl

The entry() Operator

The KL tool will look for an operator called entry taking zero arguments within the code. This operator will be called by the KL tool after it has compiled the source code.

operator entry() {
  report("Hello World");

** Output:

Hello World


Testing Extensions

Often, a simple KL source file can be written that loads and performs various tests on your extension. Simply by including the require MyExtension; statement at the beginning of the file will cause the functionality of your entire extension to included. The require statement will also validate that your extension does not include any syntactic errors. If the compilation of the extension as well as the code in the KL source file that requires it is successful, then the entry operator will be invoked.

In order to test your extension, the KL source file will will typically construct objects and structs defined in your extension, and test the functions by invoking them and reporting the results.


It is better to avoid reporting entire data structures, as these can change without breaking the behavior of the code. Instead, consider invoking methods on your types and check the results. Avoid testing internal methods that are not intended to be exposed to users.Your extension should define methods that are expected to be used in other extensions, or in operators in Splice. These methods define the interface to your extension, and should be tested thoroughly.It is good practice to use access modifiers such as public and private to control the external API to your objects and structs; see Structure, Object and Interface Access Contols for more details.

** Example: A Simple Test in KL

require Math;

operator entry() {
  Mat33 mat33 = Quat(Euler(0.0, HALF_PI, 0.0, RotationOrder('zyx'))).toMat33();

** Output:



Output to a Text File

The output of the KL tool can be sent to a text file by using the > shell operator:

kl MyTest.kl > MyTest.out

This can be useful when the test generates a lot of output, or when you want to search through the output in a text editor. It is also useful for generating the file with correct output for unit test cases.

Setting up Unit Tests that Use the KL Tool

Unit testing systems are easy to setup using the KL tool. Unit tests typically compare a reference output against the current output to determine if the behavior of your code has changed.

If you are using Linux or OS X, or a mingw shell on Windows, you can use the cmp and diff utilities to create simple scripts to perform tests. A sample such script might be:


USAGE="Usage: $0 file1.kl file2.kl ... fileN.kl"

if [ "$#" == "0" ]; then
  echo "$USAGE"
  exit 1

while (( "$#" )); do


  kl "$KL" >"$RES"
  if ! cmp "$OUT" "$RES"; then
    echo "FAIL $KL"
    echo "diff:"
    diff -u "$OUT" "$RES"
    # rm "$RES"
    exit 1
    echo "PASS $KL"
    rm "$RES"



This script expects, for a given KL source file test.kl, for there to be a corresponding file test.out with the correct output. If this script is placed in a file such as, you can then run:

./ test.kl

to see whether the output of test.kl matches the contents of test.out.

More complex scripts can be written for testing. The following example Python code runs the KL tool and compares the output against a reference file. The reference file will be generated if it does not exist, or the --update True option is set.

# Import system modules
import sys, string, os
import argparse
import subprocess

# Parse the commandline args.
parser = argparse.ArgumentParser()
parser.add_argument('klFile', help = "The kl File to use in the test")
parser.add_argument('--update', required=False, help = "Force the update of the reference file")
args = parser.parse_args()

klFile = args.klFile
cmdstring = "kl.exe " + klFile

# Call the KL tool piping output to the output buffer.
proc = subprocess.Popen(cmdstring,stdout=subprocess.PIPE)
output = ""
while True:
  line = proc.stdout.readline()
  if line != '':
    output += line.rstrip()
referencefile = os.path.splitext(klFile)[0]+'.txt'
if not os.path.exists(referencefile) or args.update == 'True':
    with open(referencefile, 'w') as f:
    print "Reference Created"
  referenceTxt = str(open( referencefile ).read())
  if referenceTxt == output:
    print "Test:" + klFile + " Passed"
    print "Test:" + klFile + " Failed"

Place the python code above in a python file called (for example) You would then run it passing a unit test KL source file to test against.

python test.kl

To update test results, run the tester with the –update argument.

​ python test.kl –update

Setting up a unit testing framework for your custom extensions and running them regularly will help catch changes that might break your APIs.

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