sandfix

sandfix fixes a cabal sandbox created on another machine by adjusting the global paths and the package dependencies in the sandbox's package database. It has only very basic dependencies, and assumes a ghc installation.

We'll call the machine the sandbox was created on the host, the machine you want to port it to the target.

There are a couple of requirements your target machine needs to satisfy for the sandbox to work:

• It should have the same version of ghc and the same architecture as the host. (check with ghc --info | grep 'Target platform')

• The set of global packages on the host should be a subset of the target's global packages. (check with ghc-pkg list --global)

The second requirement is pretty weird, it is caused by the fact that all sandboxes depend transitively on packages outside of the sandbox, in the global DB. An example is base or array.

This can be a problem for example if the sandbox is created on a box with the Haskell Platform which preinstalls all kinds of junk to the global package DB, and later you try to use it on a machine that only has a basic ghc installation. This will break, as the sandbox will assume that all those packages are in your global DB. The solution is to create sandboxes with a minimal ghc installation. Alternatively you can also install those packages in your global DB, sandfix will tell you about these.

Usage

For cabal sandboxes:

$ runhaskell src/SandFix.hs PATH_TO_SANDBOX [--package-db=...]

or with custom package databases:

$ runhaskell src/SandFix.hs PATH_TO_SANDBOX PKGDIR_NAME [--package-db=...]

where PKGDIR_NAME is the relative path of the package database under PATH_SANDBOX.

Package database stack can be specified in the same way as for cabal. The default is equivalent to --package-db=global.

To turn on more verbose output, use -v or --verbose.

Troubleshooting

• If you cabal add-source a package to the sandbox on the host watch out for this message when you use cabal functions:

  Warning: The package list for the local repo 'PATH_TO_SANDBOX' is missing.
  The repo is invalid.
  

  Solution: Delete PATH_TO_SANDBOX/packages/00-index.tar and reinitialize the sandbox with cabal sandbox init.

  Reason: For some reason cabal "encodes" the global path of the add-source'd package into the tar, which it later tries to open.

  If you want to dig in:

  The global path writing: https://github.com/haskell/cabal/blob/5c70361b362e41b3c13d48b58b46224d42f401dc/cabal-install/Distribution/Client/Sandbox/Index.hs#L92

  The "encoding" function: https://github.com/haskell/cabal/blob/5c70361b362e41b3c13d48b58b46224d42f401dc/cabal-install/Distribution/Client/Utils.hs#L130 Yeah who needs UTF when we can invent our own, better encoding.

  The code that actually breaks: https://github.com/haskell/cabal/blob/5c70361b362e41b3c13d48b58b46224d42f401dc/cabal-install/Distribution/Client/IndexUtils.hs#L458