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An R package for lightweight acceptance testing
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chad - A lightweight acceptance test framework

This provides a way to run acceptance tests for/from R, along with some scripting glue for updating them.

Goal: Never waste time writing assert()s again.

Unit tests are great for developers, but a time sink for data scientists.

How it works

The chad function tests itself for idempotency:

f(f(x)) = f(x)

where f is chad and x is your R code. By substitution, we also have

f(u) = u 

where u is a chad script: a combination of code and output.

Any chad script where that does not hold is automatically considered a test failure.

Side note: When f is a computer and u a program and the above holds, u is a quine. See Godel, Escher, Bach for a deep dive on quines.


In addition to R, this package needs a diff program on the search path. On Windows, you may need to install Rtools. For a nicer experience, you can use git and colordiff instead.


Chad scripts are basically R output from the console. Here is anova.chad:

> anova(lm(extra~., sleep))
Analysis of Variance Table

Response: extra
          Df Sum Sq    Mean Sq  F value    Pr(>F)   
group      1 10.482 12.4820000 16.50088 0.0028329 **
ID         9 58.078  6.4531111  8.53085 0.0019014 **
Residuals  9  6.808  0.7564444                      
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

When we run it with chad('anova.chad'), ir reruns the code and generates any output. If any of it had changed, it would display it in standard diff notation:

> chad('inst/tests/chad/sleep.chad')
Chadding	sleep.chad	...	
--- sleep.chad	2015-12-22 16:08:50.886015712 -0800
+++ sleep.chad.out	2015-12-22 16:09:13.942504368 -0800
@@ -6,7 +6,7 @@
 Response: extra
           Df Sum Sq    Mean Sq  F value    Pr(>F)   
-group      1 10.482 12.4820000 16.50088 0.0028329 **
+group      1 12.482 12.4820000 16.50088 0.0028329 **
 ID         9 58.078  6.4531111  8.53085 0.0019014 **
 Residuals  9  6.808  0.7564444                      

The + and minus signs indicated the lines that have changed. Here, a 10.482 has changed to a 12.482. If this was an expected change (perhaps the data set was updated or a bug was fixed), we could update the test at the AAG prompt:

Difference in output; accept as golden? [y/n/git]

Choosing y will update the test to the newer output. The git option will additionally commit the change. Because chad scripts are human readable text, they play nicely with VCSs.

Additionally, you can chad an entire directory with chadRecur():

> chadRecur('.')
Chadding  error.chad	...	FAIL
Chadding	example.chad	...	SUCCESS
          file status  time
1   error.chad      1 0.017
2 example.chad      0 0.026

Following UNIX tradition status 0 is a pass, anything else is a failure.

Package Development

To use chad scripts for package testing, run


from the package root. A test stub and example test will be copied to the 'tests/' folder.

Chad Notebooks

Valid chad scripts are by definition reproducible research and are easy to share.

By setting the MIME type of .chad files to text/html , we can have a web browser render any html content a chad script contains.

It looks better if you start the file with a code tag to style the rest of the document and break lines correctly:

># Example Notebook <pre><code>

You can use the chadPlot() to inline images:

>png(); barplot(1:10); chadPlot()
<img src="data:image/png;base64,iVBORw0KGgoAAAAN...uQmCC"/>

Differences from R CMD check

R has related testing mechanism based around R CMD check output.

  • check requires two files: .R (code) and (output); they can easily get out of sync while developing.
  • Each check test runs in it's own Rsession. This is 'safer' but slower, and no setup/teardown can be shared between tests.
  • check has to hack around the MOTD headers R emits when it is started.
  • check runs R, chad runs inside R
  • check is part of base, chad is an add-on package.

###Dedicated to Chad Whipkey.

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