doctest-extract lets you write test examples and QuickCheck properties
in Haddock comments and extracts them to test modules.
It means that the user sees your tests in the documentation
and knows that the examples and properties are machine-tested,
or at least, she can run the tests herself.
I found the barrier to write tests much lower
when I do not need to write new test modules
but just add some lines to the Haddock comments.
I do not need to think of test names or filling test data structures.
The test identifier is the module name and the line number and
if a test fails I can easily jump to the failing code.
Differences to the original GHCi-based implementation of doctest:
Pros:
Package versions for tests are consistent with tested library
Tests run much faster, especially QuickCheck property tests
No dependency on GHCi or GHC-as-library
The tested package need not be ready for compilation.
Our simple parser requires only clearly recognizable Haskell comments.
QuickCheck properties do not cause confusing type error messages
when actually only identifiers are missing.
You can inspect extracted modules
doctest collects tests from the transitive hull of imports
of the specified modules.
This might help you to keep the list of modules short.
doctest-extract only processes the specified modules
and thus allows you to focus on a module for development of tests.
With option --verbose test source path and line number are formatted
such that Emacs allows you to click and jump to the test definition.
Report success only of real tests.
doctest reports successful imports and
definition of helper types and functions as successful tests.
This makes it hard to monitor the number of real tests,
e.g. whether some tests have been dropped by accident.
Cons:
Cannot test for output of IO functions
or error messages from partial functions.
All free variables in QuickCheck properties
must be all-quantified using lambda.
(Could be even seen as an advantage for the reader of your doctests.)
No support for a single-line let as an example.
The Test module does not automatically import modules
that the tested module imports.
Thus, you usually have to add a setup section with required imports.
You need tools additional to Cabal, e.g. make and a Makefile,
in order generate test modules.
