+++
date = "2022-10-03"
draft = false
path = "/blog/cabal-test-dev-trick"
tags = ["haskell"]
title = "The cabal 'test-dev' trick"
+++
I found something mysterious while working on a large Haskell codebase: there
was a Cabal target called `test:test-dev`, containing `test` and `src` in
`source-dirs`. For a while, I didn't realise what it's for, and it seemed like
the kind of thing that someone goes and creates for some old use case. Now I
have put this into at least three other codebases because it fixes my problems.
What changed?
I learned about some limitations of cabal, GHC, and GHCi:
* Cabal will only build modules in one package (and target!) at a time.
In practice this means that your test suite can only start building after
your entire library is built. This is silly!
To illustrate the problem, say you have some module deep in your module tree,
`Instrumentation` that depends on `hs-opentelemetry`, and you have
`hs-opentelemetry` in your workspace. Then if you change something in
`hs-opentelemetry`, Cabal will build that package in its entirety before
starting on building yours, even though most of your package doesn't need any
of `hs-opentelemetry`.
Cross-package parallelism has motivated some [intrepid hackers at
Tweag][haskell-bazel] to make Bazel work well for Haskell builds. This work
allows you to build things in parallel, ignoring package boundaries for
modules that don't require the entire dependency package to be ready yet.
However, it also loses the fine grained recompilation avoidance that's always
improving inside `ghc-make`. It turns out, we can hack cabal to do nearly the
same thing.
* GHCi doesn't know how to have interpreted modules from multiple packages.
This is consequential for a couple of reasons:
* No `:load` support if you want to work on dependencies and grab everything
in scope including private items
* The debugger doesn't work across packages! This one really hurts in the
occasions when you absolutely need the debugger. While figuring out a bug
in my HSpec integration for OpenTelemetry, I needed to point the GHCi
debugger at HSpec, and this was the (only) way.
* `:reload` does not work across packages. This alone is enough to recommend
this trick with abandon.
[haskell-bazel]: https://www.tweag.io/blog/2022-06-23-haskell-module/
What if you could Not Have Those Problems? It turns out, they are all solved by
Putting More Code in a Target, although sometimes this needs to be done with
a little more persuasion of the build system.
If you think you have too much code in one package, you are probably mistaken.
Most of GHC, representing a Lot of Haskell, is one package. Avoiding the
limitations of the build infrastructure around multiple packages improves build
times and developer productivity.
## How do I `test-dev`?
Include both `src` and `test` in `hs-source-dirs` (`source-dirs` in hpack), and
then use `cabal run test:test-dev` (yes, `run`; cabal's shiny new test
runner doesn't let you control the entry point, which is no fun), or
`cabal repl test:test-dev`.
You will need to put all your dependencies for `src/` *and* your dependencies
for `test/` in the dependencies for `test-dev`.
## But what if a dependency is causing cross-target sadness?
In the spirit of [the yolo method of setting up HLS][yolo-method] in which you
unceremoniously temporarily stuff some third party project into a cabal
workspace with your code, you can also just lie to cabal that the dependency is
part of your package.
[yolo-method]: ../nix-hls-for-deps#yolo-method
Simply tell cabal "it's *my* package now" by putting its source directory into
`hs-source-dirs` and remove the dependency (since it's your package now).
Cabal will apparently not think much of it!
Let's use hspec as an example. First clone hspec below your project directory
(nested git repos are no problem as long as they remain untracked):
```
$ git clone https://github.com/hspec/hspec
```
Then do something like this in your cabal file:
```
test-suite test-dev
-- ...
hs-source-dirs:
src
test
hspec/hspec
hspec/hspec-core
build-depends:
-- your app depends PLUS (your test depends MINUS hspec/hspec-core) PLUS hspec/hspec-core's depends
```
Finally you can `cabal run test:test-dev`, or `cabal repl test:test-dev`, and
it will build your code *and* the hspec code all as one imaginary package,
allowing you to use the full GHCi feature set and compile faster, especially if
you're actively working on hspec.
In this way, you can blur the line between dependencies and your own code,
working on them as one, while also keeping them separate the rest of the time.
### Bonus section: It also makes HLS work better
You can also put `test-dev` in place of the other targets in `hie.yaml` in all
the paths it includes, which will improve HLS's overall performance and
usability, although perhaps at the cost of more startup time. This is because
HLS has the same problem as cabal where it seemingly doesn't build dependent
targets in parallel with their dependencies.
More frustratingly, if a dependency fails to compile, it will sometimes take
down all the dependent code with it, even if the dependent code doesn't all
actually import the dependency.
You can set this up in `hie.yaml` something like so:
```yaml
cradle:
cabal:
- path: "src"
component: "test:test-dev"
- path: "test"
component: "test:test-dev"
```
---
## Are split packages a good idea in some contexts?
Here's a [blog post by Matt Parsons on compile times][parsons-compile-times],
which I generally agree with based on my experience working in large Haskell
codebases.
[parsons-compile-times]: https://www.parsonsmatt.org/2019/11/27/keeping_compilation_fast.html#module-parallelism
Some highlights include:
Given a package `B`, depending on `A`:
> By combining `A` and `B` into a single package, we sped up compile
times for a *complete* build of the application by 10%. A clean build of the
new `AB` package was 15% faster to build all told, and incremental builds were
improved too.
He suggests "if you're not willing to GitHub it, then it should probably stay
in the main package".
I agree with this heuristic: you can cache the entire thing as a package (in
fact, Nix will automatically do it for you) if it's on GitHub/Hackage, and it
probably doesn't change that often if you are willing to do that, so the slight
increase in annoyingness of developing it is probably fine.
That said, you can have your cake and eat it too! It's easily possible to [stuff
dependencies into your workspace while working on them][yolo-method], or even
artificially integrate them into your own package while working on them via the
`test-dev` trick.
Using these methods, you can get exactly the same developer experience while working
on libraries as if they are fully part of the codebase by telling cabal that
they *are* fully part of the codebase only while developing on them, then have
them be separate while releasing.
---
## The elephant in the room: "Wait, GHC fixed this?"
My post describes a workaround for the absence of a GHC feature called
"multiple home units", which released in GHC 9.4, and is supposed to solve
these problems. However, [Cabal does not yet support it][cabal-mhu], which is
rather a roadblock. Also, [GHCi has limited functionality under multiple home
units][ghci-mhu].
You can read more about the ongoing work on multiple home units [at the
Well-Typed blog post on the subject][wt-multiple-home-units].
[wt-multiple-home-units]: https://well-typed.com/blog/2022/01/multiple-home-units/
[cabal-mhu]: https://github.com/haskell/cabal/issues/8238
[ghci-mhu]: https://gitlab.haskell.org/ghc/ghc/-/issues/20889
That post points out that Stack does the `test-dev` hack described in this
article if you do something like `stack repl exe:myexample lib:myexample`, but
I don't use Stack.
The `test-dev` trick will continue working into the future, but I look forward
to the day that it is no longer necessary.
### Limitations
This hack has some unfortunate limitations, many of which are discussed in the
posts about multiple home units above:
- It doesn't reuse object code from the build of `lib:sample` or `sample:test:test`.
- You need to set `default-extensions` to the union of all of the
`default-extensions` that each of the directories are normally built with. I
didn't actually notice this since I typically use a copious number.
- Other GHC flags need to be compatible.
- It breaks [-XPackageImports].
- Module name conflicts probably don't do anything good.
- I doubt that [cabal `mixins`][cabal-mixins] work with this.
[cabal-mixins]: https://cabal.readthedocs.io/en/3.4/cabal-package.html#pkg-field-mixins
[-XPackageImports]: https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/package_qualified_imports.html?highlight=packageimports#extension-PackageImports
## Bonus section: `ghcid` on tests
You know what fixing GHCi on tests in the presence of app modifications does?
That's right, you can run your test suite with ghcid on every file save. This
is seriously awesome since it uses interpreted mode for those sweet GHCi reload
times.
This section was inspired by [Matt Parsons' blog post on ghcid][parsons-ghcid],
which incidentally uses this trick because Stack does it under the hood!
[parsons-ghcid]: https://www.parsonsmatt.org/2018/05/19/ghcid_for_the_win.html
Back to our regularly scheduled bonus content:
First, create a file `test/Main.hs` something like so (this circumvents the
`cabal test` runner which doesn't let you have a custom entry point):
```haskell
module Main where
import Spec (spec)
import Hspec
main :: IO ()
main = testMain
-- avoids overlapping names with Spec.main
testMain :: IO ()
testMain = hspec spec
```
Then you can ghcid your tests with:
```
$ ghcid --command 'cabal repl test:test-dev --ghc-options="-osuf dyn_o -hisuf dyn_hi"' --test testMain
```
When you save any file, ghcid will pilot the GHCi session to reload and rerun
the tests.
#### Acknowledgements
Thanks to Hazel Weakly, Matt Parsons, and Chris Zehner for their valuable
feedback and discussion on drafts of this post.