# Incompatibilities between MTL 1 and MTL 2

Version 2 of the Monad Transformer Library introduced some small incompatibilities relative to version 1, described below. See "Upgrading from MTL 1 to MTL 2" for instructions on how to make code written for version 1 work with version 2.

The interface of mtl-2 is close to that of mtl-1, but with the following differences (illustrated with `Reader`

):

- Instances of
`Applicative`

and`Alternative`

have been added as appropriate, e.g.

```
instance (Applicative m) => Applicative (ReaderT r m) where ...
instance (Alternative m) => Alternative (ReaderT r m) where ...
```

*Rationale:* These classes postdate the MTL, and such instances have been repeatedly defined in various packages. They belong together with the type constructors.

`Functor`

instances for monad transformers no longer require`Monad`

where`Functor`

is sufficient. Unfortunately this is incompatible because`Functor`

is not a superclass of`Monad`

, e.g.

```
instance (Monad m) => Functor (ReaderT r m) where ...
```

- is replaced by

```
instance (Functor m) => Functor (ReaderT r m) where ...
```

*Rationale:* These instances are more general, and are consistent with the instances of other classes.

- Simple monads are now aliases for monad trasformers applied to
`Identity`

, e.g.

```
newtype Reader r a = Reader { runReader :: r -> a }
```

- is replaced by

```
type Reader r = ReaderT r Identity
reader :: (r -> a) -> Reader r a
reader f = ReaderT (Identity . f)
runReader :: Reader r a -> r -> a
runReader m = runIdentity . runReaderT m
```

*Rationale:* This avoids repetition in the interfaces of both
transformers
and mtl-2. It makes transformers more useful on its own, and also saves clients of mtl from defining instances for both `Reader r`

and `ReaderT r`

and ensuring that they are consistent.

- The instance
`Error String`

is restructured to avoid a type synonym instance:

```
instance Error String where
noMsg = ""
strMsg = id
```

is replaced by

```
instance ErrorList a => Error [a] where
strMsg = listMsg
class ErrorList a where
listMsg :: String -> [a]
instance ErrorList Char where
listMsg = id
```

*Rationale:* This makes the instance Haskell 98, so it can be included in the
transformers
package.