Haskell build systems for non-Haskellers

Posted on October 17, 2019

The Haskell build system war seems to have quieted down a bit, with both cabal and stack seeing wide use. I have my own preference (cards on the table: it’s stack for the moment), but I feel my reasons were not represented well in the flame wars. I figure I should write them down.

It is my impression that many Haskell programmers fall into two groups:

  1. Those who mainly produce libraries for distribution on Hackage, which must work with many different dependencies and versions of GHC.

  2. Those who mainly write Haskell applications that they also host or deploy themselves, and who tend to freeze their environment.

(This is a gross simplification, bear with me.)

I am mostly related to the second group, but with some differences: my main project is a Haskell program whose audience is users who do not much care that it is implemented in Haskell. While I try to make binary packages available, it is still important that people can easily download and compile the source code themselves.

For most C programs, this has historically been straightforward. You obtain the code, run ./configure && make (or the cmake equivalent) and that’s it. This is because most C programs have either no dependencies, or just a few popular ones that are likely to be available via the OS package manager. Further, C is stable enough that it tends not to matter exactly which version of the C compiler you are using.

For both good and ill, Haskell is a more unstable language, and programs tend to have dozens of dependencies. In practice, if I want my users (who may not care about Haskell!) to have a reasonable chance of successfully compiling my program, I must ensure that they use exactly the same compiler and library versions as I do.

This was stack’s major advantage for me. While cabal freeze can be used to pin dependencies for cabal, you cannot also pin the versions of GHC and tools such as happy. While I can certainly document the required versions, providing instructions on how to install them on a large variety of operating systems is no easy task, particularly when the obvious solutions (e.g apt install ghc happy) are likely not to be correct. In contrast, it is very easy for me to just point at stacks installation instructions and let them worry about making it work on whatever strange operating systems people come up with.

There is also a particularly tricky category of users: people who already have some old or defective Haskell setup that they obtained for a course or idly experiment and since forgot about. Debugging build errors due to interactions with these remnants can be quite maddening.

Modern cabal has solved most of these problems well. With Nix-style local builds, most tools and libraries needed will be installed automatically and isolated from anything else going on with the system. It is, however, still on the user to install two programs themselves: cabal and the right version of GHC.

While most package managers should already contain an adequately new version of cabal (or will soon in any case), the situation with GHC is more complicated. Some systems, like Ubuntu LTS, will contain a version that is too old. Others, like Homebrew, may contain one that is too new! This is because it tends to take a couple of months after a GHC release before all of my dependencies have been updated. There is of course ghcup, which makes it easy to obtain a specific version of GHC, but I must then maintain human-readable documentation on how to do so (and my users must manually keep track of when it changes). In contrast, stack build will automatically work the same for my users as for me, which means I will likely notice any problems quickly.

In short, while my fondness for cabal has grown significantly for my own uses, I still find stack superior for the task of giving my source code to users who do not particularly care that it is written in Haskell. My philosophy is that the installation instructions are a crucial part of a program’s user interface.