Upgrading from MTL 1 to MTL 2

From HaskellWiki

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

.

Category: Monad