# Difference between revisions of "Incompatibilities between MTL 1 and MTL 2"

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

− | The interface of mtl-2 is close to that of mtl-1, but with the following differences (illustrated with <hask>Reader</hask>): |
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− | * Instances of <hask>Applicative</hask> and <hask>Alternative</hask> have been added as appropriate, e.g. |
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− | <haskell> |
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− | instance (Applicative m) => Applicative (ReaderT r m) where ... |
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− | instance (Alternative m) => Alternative (ReaderT r m) where ... |
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− | </haskell> |
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− | |||

− | ''Rationale:'' These classes postdate the MTL, and such instances have been repeatedly defined in various packages. They belong together with the type constructors. |
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− | * <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. |
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− | <haskell> |
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− | instance (Monad m) => Functor (ReaderT r m) where ... |
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− | </haskell> |
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− | :is replaced by |
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− | <haskell> |
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− | instance (Functor m) => Functor (ReaderT r m) where ... |
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− | </haskell> |
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− | |||

− | ''Rationale:'' These instances are more general, and are consistent with the instances of other classes. |
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− | * Simple monads are now aliases for monad trasformers applied to <hask>Identity</hask>, e.g. |
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− | <haskell> |
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− | newtype Reader r a = Reader { runReader :: r -> a } |
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− | </haskell> |
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− | :is replaced by |
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− | <haskell> |
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− | type Reader r = ReaderT r Identity |
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− | |||

− | reader :: (r -> a) -> Reader r a |
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− | reader f = ReaderT (Identity . f) |
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− | |||

− | runReader :: Reader r a -> r -> a |
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− | runReader m = runIdentity . runReaderT m |
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− | </haskell> |
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− | ''Rationale:'' This avoids repetition in the interfaces of both |
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− | [http://hackage.haskell.org/package/transformers transformers] |
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− | 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. |
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− | * The instance <hask>Error String</hask> is restructured to avoid a type synonym instance: |
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− | <haskell> |
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− | instance Error String where |
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− | noMsg = "" |
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− | strMsg = id |
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− | </haskell> |
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− | is replaced by |
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− | <haskell> |
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− | instance ErrorList a => Error [a] where |
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− | strMsg = listMsg |
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− | |||

− | class ErrorList a where |
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− | listMsg :: String -> [a] |
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− | |||

− | instance ErrorList Char where |
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− | listMsg = id |
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− | </haskell> |
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− | ''Rationale:'' This makes the instance Haskell 98, so it can be included in the |
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− | [http://hackage.haskell.org/package/transformers transformers] |
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− | package. |
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− | |||

− | [[Category:Monad]] |