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
 +
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 ...
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instance (Alternative m) => Alternative (ReaderT r m) where ...
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</haskell>
 +
 +
''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>
 +
instance (Monad m) => Functor (ReaderT r m) where ...
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</haskell>
 +
: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>
 +
 +
''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]
 +
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>
 +
instance Error String where
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    noMsg  = ""
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    strMsg = id
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</haskell>
 +
  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]
 +
 +
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
 +
[http://hackage.haskell.org/package/transformers transformers]
 +
package.
 +
 +
[[Category:Monad]]

Latest revision as of 15:19, 6 February 2021

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.