[Haskell-cafe] Type System vs Test Driven Development

Jonathan Geddes geddes.jonathan at gmail.com
Wed Jan 5 21:02:37 CET 2011

> The Haskell type system is simply not rich enough to guarantee everything you might need.

That's true, and after giving this a bit more thought, I realized it's
not JUST the type system that I'm talking about here. There are a few
other features that make it hard for me to want to use unit/property

For example, say (for the sake of simplicity and familiarity) that I'm
writing the foldl function. If I were writing this function in any
other language, this would be my process: first I'd write a test to
check that foldl returns the original accumulator when the list is
empty. Then I would write code until the test passed. Then I would
move on to the next property of foldl and write a test for it. Rinse

But in Haskell, I would just write the code:

> foldl _ acc [] = acc

The function is obviously correct for my (missing) test. So I move on
to the next parts of the function:

>foldl _ acc [] = acc
>foldl f acc (x:xs) = foldl f (f acc x) xs

and this is equally obviously correct. I can't think of a test that
would increase my confidence in this code. I might drop into the ghci
repl to manually test it once, but not a full unit test.

I said that writing Haskell code feels like "writing code against a
solid test base." But I think there's more to it than this. Writing
Haskell code feels like writing unit tests and letting the machine
generate the actual code from those tests. Declarative code for the

Despite all this, I suspect that since Haskell is at a higher level of
abstraction than other languages, the tests in Haskell must be at a
correspondingly higher level than the tests in other languages. I can
see that such tests would give great benefits to the development
process. I am convinced that I should try to write such tests. But I
still think that Haskell makes a huge class of tests unnecessary.

> Haskell has some awesome testing tool, and I highly recommend getting
> acquainted with them.

I will certainly take your advice here. Like I said, I use TDD in
other languages but mysteriously don't feel its absence in Haskell. I
probably need to get into better habits.


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