Upgrading from MTL 1 to MTL 2

Version 2 of the Monad Transformer Library introduced some incompatibilities relative to version 1. This page provides instructions for upgrading code written for MTL 1 to work with MTL 2.

Many packages written for earlier versions of MTL build unchanged with version 2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with version 1 without some CPP directives. Here are the fixes for common messages:

Illegal instance declaration for Applicative (State a)

This usually means that the instance you're defining is now defined in the transformers package. You may wish to check it's the same, so you can just delete it.

Could not deduce (Functor m) from the context (Monad m)
arising from a use of fmap

This will be a situation where you're using a monad constructed with a transformer as a Functor. You can replace fmap with liftM and the code will build with old and new versions of mtl.

Could not deduce (Functor m) from the context (Monad m)
arising from the superclasses of an instance declaration

This will be another situation where you're using a monad constructed with a transformer as a Functor. You can replace the Monad m with Functor m to make it work with mtl-2, or add a Functor m constraint to make it work with old and new versions of mtl.

Not in scope: data constructor State

The State type is now a type synonym. You could replace the State data constructor with the state function.

Not in scope: runState

You probably imported State(..), which won't work now that State is a type synonym. You need to import State and runState. (That will work with old versions of mtl too.)

Illegal instance declaration for Myclass (State Foo)

If you have a matching instance for StateT, you can delete the instance for State. Otherwise you need to generalize your instance to StateT. If that's not possible, you may need to introduce a newtype.