Haskell Testing Desiderata

A few days ago I wrote about testing in Haskell. It’s not quite the whole story, though, and I wanted to dig into what I’d like to see.

Current solutions

I currently use Intero in Emacs, and switch between ghcid and stack build --file-watch --test --fast. None of them is quite perfect, as I’ll describe.



We are aiming for subsecond response times. In a big project (such as my current betterteam work project, we have about 21000 lines of Haskell, we just don’t have time to wait for the compiler to reload almost everything. Intero and ghcid work well here, stack build does not.


By responsivity, I mean that when anything that could affect the semantics of the code changes, the relevant tests are fired. This is really important, because you need to be able to trust that your test-runner is telling you the full, up-to-date truth. Without this, you need to keep the possibility in your mind that your tools have started to lag the truth of the situation.

Unfortunately, the ghcid solution I mentioned yesterday doesn’t handle this case quite perfectly: if I have a large project with sub-projects set up through stack and change something in a subproject, ghcid will not notice the problem. (It has improved recently, in that it watches stack.yaml and the appropriate cabal file, but widespread damage detection is a little outside its remit.)

Intero has a similar problem, and while I find it really useful, I frequently have to restart it so it will notice added packages in the stack.yaml and cabal file, or subproject changes.

In contrast, stack build --file-watch --test --fast, my other go-to, handles this flawlessly.


We shouldn’t do more work than is necessary. This also sounds pretty obvious, but the file-based nature of Haskell code militates against us here: if module A depends on module B, and we changed a line of code in module B that module A does not in fact use, it’s difficult to tell that not only does module A not need to be recompiled, but its tests do not need to be rerun. Barring the introduction of something like Unison’s addressing-by-hash-of-toplevel-value approach, we’re going to have to cop this slowdown, and just try to keep our files small & coherent enough that a minimum of pointless work needs to be done.

As far as just loading the code goes, the interpreter approaches do ok (unless you have significant Template Haskell, in which case heaven help you.) The compilation approach is usually pretty slow, at least if we keep the subsecond goal in mind.

I haven’t seen anything capable of only running the tests that have been affected. The closest is Hspec’s –rerun flag, which keeps a running tally of failing tests and only runs them until they all pass. This helps a lot, especially if you haven’t put the work in to make your tests fast (we have a fair number of integration tests that interact with the database, and currently it takes 180s to run everything).

(Quick plug: if you use the stack build --file-watch --test --fast approach, –rerun doesn’t work out of the box, as it relies on environment variables. My little hspec-stack-rerun project works around this: with any luck hspec’s upcoming support for config files will make it unnecessary soon.)

Fantasyland project

In the world I see when I shut my eyes, I have a process watching for file changes that can compute a parsimonious set of code to be reloaded and tests to be run. That would talk to a running ghci process (probably something similar to ghcid or intero), instructing it first which files to reload, and then which tests to run. It might even be aware of subprojects to the extent that it would only run failing tests in the lowest level subproject until it passes, to let you stay drilled in, then run whatever tests have been possibly compromised at higher levels until everything passes. I want my testrunner to be Tenzing Norgay, helping me through the rough bits and making up for my laziness, sloppiness and impatience.

Then I open my eyes, sigh, and hit M-x intero-restart.