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
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 <hask>Reader</hask>):
* Instances of <hask>Applicative</hask> and <hask>Alternative</hask> 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.
* <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.
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 <hask>Identity</hask>, 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
[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:
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
[http://hackage.haskell.org/package/transformers transformers]

Revision as of 14:29, 6 February 2021