Building the Spock tutorial example fails

Question

I wanted to get going with Haskell a little bit and therefore took a look at the Spock framework. To start clean, I uninstalled everything Haskell related from my Arch Linux machine and installed ghcup, Cabal and Stack using the install scripts from their respective websites.

Now I want to follow Spock's Tutorial. Trying to install Spock globally with cabal install Spock as suggested gives me an error (abbreviated):

src/Web/Spock/Internal/Wire.hs:43:1: error:
    Could not find module ‘Web.Routing.AbstractRouter’
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
43 | import Web.Routing.AbstractRouter
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cabal: Failed to build Spock-0.9.0.1. See the build log above for details.

I already found a question on reddit on the topic, but the solution does not apply because I'm not trying to use a specific version of the libraries as implied.

So I try to follow along and build only locally. But when I reach the point where it says stack build --fast --pedantic, the build plan can not be constructed and Stack suggests to add another dependency, stm-containers. Doing so, I am presented with two additional suggestions for focus and primitive. When I add these, the plan fails again, this time without a simple solution:

In the dependencies for primitive-0.6.4.0:
    base-4.13.0.0 from stack configuration does not match >=4.5 && <4.13  (latest matching version is 4.12.0.0)
needed due to Spock-example-0.1.0.0 -> primitive-0.6.4.0

I can do a little thing with Haskell, but with the build system(s), I am way out of my comfort zone. Help and hints appreciated. Oh, and all versions of course are the latest by the time of this post.

Answer 1

Due to incompatible versions of dependencies, Spock won't build with GHC 8.8 and above. A similar problem is described in Spock issue #149, though I'm not fully sure it is exactly the same incompatibility. The error you got from Stack hints at that, as base-4.13.0.0 is the version of base that is bundled with GHC 8.8. cabal-install failed in a more obscure way because, upon noting the incompatibility, it tries to solve the dependencies using older versions of Spock, eventually picking 0.9.0.1, attempting and, thanks to a missing version upper bound for the reroute dependency, failing to build it.

(Shortly after this answer was posted, the missing upper bound was retrofitted to the old Spock version, so attempting to reproduce the problem now will lead to an easier to understand failure.)

Casting the tutorial aside, the most straightforward way to use Spock given those complications is probably through cabal-install 3+. Begin by using ghcup to switch to GHC 8.6.5:

$ ghcup install 8.6.5
$ ghcup set 8.6.5

Then, create a blank project with cabal-install:

$ mkdir myproject
$ cd myproject
$ cabal init

Add Spock to the build-depends section of myproject.cabal:

  build-depends:       base >=4.12 && <4.13
                     , Spock == 0.13.*

Finally, you can run:

$ cabal build

Which will install Spock and its dependencies before building the project. (Note that you generally don't need to use cabal install to install libraries with cabal-install 3.)

It is presumably possible to make it work with Stack as well, by changing to the lts-14.27 resolver (the latest one that uses GHC 8.6.5), tracking down all dependency versions that need to be overriden (as you had began to do) and manually adding them to the extra-deps of stack.yaml.