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Incompatibilities between MTL 1 and MTL 2

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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]]

Current revision

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.

Retrieved from "http://www.haskell.org/haskellwiki/Incompatibilities_between_MTL_1_and_MTL_2"

Category: Monad