Upgrading from MTL 1 to MTL 2
From HaskellWiki
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
withliftM
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
withFunctor m
to make it work with mtl-2, or add aFunctor 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 theState
data constructor with thestate
function.
- Not in scope: runState
- You probably imported
State(..)
, which won't work now thatState
is a type synonym. You need to importState
andrunState
. (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 forState
. Otherwise you need to generalize your instance toStateT
. If that's not possible, you may need to introduce anewtype
.