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Mtl-2.3.x-Migration.md

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Migrating to mtl-2.3.x

In mtl-2.3.x, re-exports were removed from the library in order to achieve PVP compliance, as well as disentangle some historical warts in its design. This was a breaking change that maintainers decided was in the mtl's best interest due to the re-exports overlapping with base, as well as generally being confusing and outdated for modern GHC versions. While all changes were detailed in the changelog, navigating the removal of re-exports can be tricky. Below is a detailed review of what needs to be done to use the new mtl.

Removed re-exports

The following base-specific re-exports were cut from many modules:

  • Control.Monad
  • Control.Monad.Fix
  • Data.Monoid

In mtl-2.3, the following export was erroneously removed from mtl-specific modules:

  • ExceptT and related functions from Control.Monad.Except

What do I need to do?

First, update your *.cabal file to point to the new mtl-2.3.1 release (currently on Hackage), or add the following to your cabal.project:

packages: .

source-repository-package
  type: git
  location: https://github.com/haskell/mtl
  tag: 2.3.1

allow-newer:
  *:mtl

Now, if upon recompile you see anything to the effect that particular functions from any of the above re-exports are missing, then you'll need to import the modules explicitly in order to expose the functions.