assert-explainer

assert-explainer copied to clipboard

#+TITLE: Assert Explainer (or: can we have py.test in Haskell?)

Assert Explainer is a library & GHC source plugin to help writing assertions. In particular, it is to help you understand why an assertion has failed.

How many times have you written some kind of unit test assertion like

#+BEGIN_SRC haskell assert (length xs == 4) #+END_SRC

And got:

#+BEGIN_SRC exception: Assertion failed! #+END_SRC

This sucks! Why did the assertion fail? When things have gone wrong, it's too late to find out way - the information has gone.

With AssertExplainer, you simply write =Bool=-valued expressions, and the plugin will take care of the rest.

First:

#+BEGIN_SRC haskell {-# OPTIONS -fplugin=AssertExplainer #-} #+END_SRC

Then write your assertion. The above would be simply:

#+BEGIN_SRC haskell assert (length xs == 4) #+END_SRC

No need for lots of special =assertEqual= etc functions.

When the assertion fails, you will get much more context:

#+BEGIN_SRC ✘ Assertion failed! length xs == 4 /= True (at Test.hs:18:12-25)

I found the following sub-expressions: - length xs = 3 - xs = [1,2,3] #+END_SRC

Ultimately, my goal is to write something more akin to [[https://docs.pytest.org/en/latest/example/reportingdemo.html#tbreportdemo][py.test's assertion magic]]. This is just a proof of concept right now, but maybe we'll get there!