Traditionally developers and testers created huge numbers of explicit tests, enumerating interesting cases, perhaps
biased by what they believe to be the current boundary conditions of the function being tested. Or at
least, they were supposed to.
A major step forward was the development of property testing. Property testing requires the user to write a few
functional properties that are used to generate tests, and requires an external library or tool to create test data
for the tests. As such many thousands of tests can be created for a single property. For the purely functional
programming language Haskell there are several such libraries; for example QuickCheck [CH00], SmallCheck
and Lazy SmallCheck [RNL08].
Unfortunately, property testing still requires the user to write explicit tests. Fortunately, we note there are
already many implicit tests present in programs. Developers may throw assertion errors, or the compiler may
silently insert runtime exceptions for incomplete pattern matches.
We attempt to automate the testing process using these implicit tests. Our contributions are in four main
areas: (1) We have developed algorithms to automatically infer appropriate constructors and functions needed
to generate test data without requiring additional programmer work or annotations. (2) To combine the
constructors and functions into test expressions we take advantage of Haskell's lazy evaluation semantics by
applying the techniques of needed narrowing and lazy instantiation to guide generation. (3) We keep the type
of test data at its most general, in order to prevent committing too early to monomorphic types that cause
needless wasted tests. (4) We have developed novel ways of creating Haskell case expressions to inspect elements
inside returned data structures, in order to discover exceptions that may be hidden by laziness, and to make
our test data generation algorithm more expressive.
In order to validate our claims, we have implemented these techniques in Irulan, a fully automatic tool for
generating systematic black-box unit tests for Haskell library code. We have designed Irulan to generate high
coverage test suites and detect common programming errors in the process
