3

I'm trying to install aeson on GHC 9.2.1. I first ran cabal install --allow-newer --lib aeson, which failed when it got to building attoparsec. It turns out this problem is already fixed in their Git repo, but hasn't landed in a release on Hackage yet. I then did these steps to build a local version with the fix:

git clone https://github.com/haskell/attoparsec.git
cd attoparsec
cabal install --allow-newer --lib .
cd ..

That succeeded, but then when I did cabal install --allow-newer --lib aeson again, it tried to build attoparsec from Hackage again, and so failed again. How can I make cabal use what I just built and installed instead?

Joseph Sible-Reinstate Monica
  • 45,431
  • 5
  • 48
  • 98

1 Answers1

8

There are two main methods.

One is to create a cabal project that includes both the cloned version of attoparsec and the local packages that you are working on (packages that might depend on aeson). It could be as simple as

packages: attoparsec yourpackage

In fact, you don't even need to clone the repo, you could use the source-repository-package field instead.

Because local packages always take precedence over external ones, the repo version will be chosen when resolving dependencies.

This approach works well but has the disadvantage that if you use the patched attoparsec in many different projects, you'll have to reference and re-compile it each time.

Another approach is to create a local no-index package repository in your machine, give it priority over stardard Hackage, and put attoparsec's sdist tarball there.

You need to declare the extra package repository using the repository field of your global cabal config (the path to the config is displayed in the last line of cabal help). By default, there's only Hackage:

repository hackage.haskell.org
   url: http://hackage.haskell.org/

To give the local package repository priority over Hackage, you need to use the active-repositories field, either in your global Cabal config or in a cabal.project file.

The advantage of this method is that you don't need to create a Cabal project, and that the patched version of attoparsec will only be compiled once (as if it came from Hackage).

danidiaz
  • 26,936
  • 4
  • 45
  • 95
  • 1
    Related: https://stackoverflow.com/questions/65680185/difference-between-source-repository-package-and-local-no-index-repositories – danidiaz Oct 29 '21 at 20:20
  • 1
    I went with the second method and it worked for me, thanks. One caveat that might be worth mentioning: you need to delete noindex.cache from the local repository every time you add something to it. – Joseph Sible-Reinstate Monica Oct 29 '21 at 20:34