# Incompatibilities between MTL 1 and MTL 2

### From HaskellWiki

(Difference between revisions)

(Introductory paragraph) |
(Added content from the Haskell Platform trac wiki) |
||

Line 3: | Line 3: | ||

See "[[Upgrading from MTL 1 to MTL 2]]" for instructions on how |
See "[[Upgrading from MTL 1 to MTL 2]]" for instructions on how |
||

to make code written for version 1 work with version 2. |
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 <hask>Reader</hask>): |
||

+ | * Instances of <hask>Applicative</hask> and <hask>Alternative</hask> have been added as appropriate, e.g. |
||

+ | <haskell> |
||

+ | instance (Applicative m) => Applicative (ReaderT r m) where ... |
||

+ | instance (Alternative m) => Alternative (ReaderT r m) where ... |
||

+ | </haskell> |
||

+ | |||

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

+ | * <hask>Functor</hask> instances for monad transformers no longer require <hask>Monad</hask> where <hask>Functor</hask> is sufficient. Unfortunately this is incompatible because <hask>Functor</hask> is not a superclass of <hask>Monad</hask>, e.g. |
||

+ | <haskell> |
||

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

+ | </haskell> |
||

+ | :is replaced by |
||

+ | <haskell> |
||

+ | instance (Functor m) => Functor (ReaderT r m) where ... |
||

+ | </haskell> |
||

+ | |||

+ | ''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 <hask>Identity</hask>, e.g. |
||

+ | <haskell> |
||

+ | newtype Reader r a = Reader { runReader :: r -> a } |
||

+ | </haskell> |
||

+ | :is replaced by |
||

+ | <haskell> |
||

+ | 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 |
||

+ | </haskell> |
||

+ | ''Rationale:'' This avoids repetition in the interfaces of both |
||

+ | [http://hackage.haskell.org/package/transformers transformers] |
||

+ | and mtl-2. It makes transformers more useful on its own, and also saves clients of mtl from defining instances for both <hask>Reader r</hask> and <hask>ReaderT r</hask> and ensuring that they are consistent. |
||

+ | * The instance <hask>Error String</hask> is restructured to avoid a type synonym instance: |
||

+ | <haskell> |
||

+ | instance Error String where |
||

+ | noMsg = "" |
||

+ | strMsg = id |
||

+ | </haskell> |
||

+ | is replaced by |
||

+ | <haskell> |
||

+ | instance ErrorList a => Error [a] where |
||

+ | strMsg = listMsg |
||

+ | |||

+ | class ErrorList a where |
||

+ | listMsg :: String -> [a] |
||

+ | |||

+ | instance ErrorList Char where |
||

+ | listMsg = id |
||

+ | </haskell> |
||

+ | ''Rationale:'' This makes the instance Haskell 98, so it can be included in the |
||

+ | [http://hackage.haskell.org/package/transformers transformers] |
||

+ | package. |
||

[[Category:Monad]] |
[[Category:Monad]] |

## Latest revision as of 12:05, 16 November 2010

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 withReader

- Instances of andApplicativehave been added as appropriate, e.g.Alternative

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.

- instances for monad transformers no longer requireFunctorwhereMonadis sufficient. Unfortunately this is incompatible becauseFunctoris not a superclass ofFunctor, e.g.Monad

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 , e.g.Identity

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

Reader r

ReaderT r

- The instance is restructured to avoid a type synonym instance:Error String

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.