The Test Monad

Lots of people have written clever things about writing monads - their mathematical structure, properties etc. I wrote a Test Monad for my FilePath library, which I've since refined for Hoogle. The inspiration for this comes from Lennart Augustsson, who gave a talk at fun in the afternoon on ways to abuse Haskell syntax for other purposes.

For the testing of the parser in Hoogle, I want to write a list of strings, versus the data structures they should produce:

"/info" === defaultQuery{flags = [Flag "info" ""]}
"/count=10" === defaultQuery{flags = [Flag "count" "10"]}

In Haskell, the most syntactically pleasant way of writing a list of things is using do notation in a monad. This means that each (===) operation should be in a monad, and the (>>) bind operation should execute one test, then the next. We also need some way of "executing" all these tests. Fortunately this is all quite easy:

data Test a = Pass

instance Monad Test where
    a >> b = a `seq` b

instance Show (Test a) where
    show x = x `seq` "All tests passed"

pass :: Test ()
pass = Pass

The basic type is Test, which has only one value, being Pass. To represent failure, simply call error, which is failure in all Haskell programs and allows you to give a useful error message. The helper function pass is provided to pin down the type, so you don't get ambiguity errors. The Monad instance simply ensures that all the tests are evaluated, so that if any crash then the whole program will crash. The Show instance demands that all the tests passed, then gives a message stating that.

We can then layer our own test combinators on top, for example for parsec:

parseTest f input output =
    case f input of
        Left x -> err "Parse failed" (show x)
        Right x -> if x == output then pass else
                   err "Parse not equal" (show x)
    where
        err pre post = error $ pre ++ ":\n  " ++
                               input ++ "\n  " ++
                               show output ++ "\n  " ++
                               post

This parseTest function takes a parsec parser, an input and an output. If the parse fails, or doesn't produce the correct answer, an error is raised. If the test passes, then the function calls pass. It's then simple enough to define each test set as:

parse_Query = do
    let (===) = parseTest parseQuery

Here (===) is defined differently for each do block. By evaluating one of these test blocks in an interpreter, the show method will automatically be called, executing all the tests.

I've found this "monad" lets you express tests very succinctly, and execute them easily. I moved to this test format from stand-alone files listing sample inputs and expected results, and its proved much easier to maintain and more useful.