Re: [Haskell-cafe] Why does the transformers/mtl 'Error' class exist?
On Sat, Apr 17, 2010 at 2:52 PM, Henning Thielemann
schlepp...@henning-thielemann.de wrote:
Ryan Ingram schrieb:
It's used in the implementation of fail for those monads.
class Monad m where
...
fail :: String - m a
fail = error -- default implementation
which is then used to desugar do-notation when pattern matching fails:
do
Left x - something
return x
=
something = \v - case v of { Left x - return x ; _ - fail
Pattern match failure ... }
You can argue about whether fail belongs in Monad (and many people
have), but that's why it is how it is.
I also prefered to not support the fail method in this way and wrote:
http://hackage.haskell.org/packages/archive/explicit-exception/0.1.4/doc/html/Control-Monad-Exception-Synchronous.html
I find that having pattern match desugar to fail, and supporting
fail, can lead to extremely concise, clear code.
For example, an excerpt from some type checking/inference code I wrote:
data Equal a b = (a ~ b) = Refl
data Typ a where
TInt :: Typ Int
TBool :: Typ Bool
TList :: Typ a - Typ [a]
TArrow :: Typ a - Typ b - Typ (a - b)
eqT :: Typ a - Typ b - Maybe (Equal a b)
eqT TInt TInt = return Refl
eqT TBool TBool = return Refl
eqT (TList a) (TList b) = do
Refl - eqT a b
return Refl
eqT (TArrow a1 a2) (TArrow b1 b2) = do
Refl - eqT a1 b1
Refl - eqT a2 b2
return Refl
eqT _ _ = fail not equal
This relies heavily on the pattern match desugaring to case, which
brings the type equality (a ~ b) into scope, and fail returning
Nothing. For example, the list case:
desugaring eqT (TList TInt) (TList TInt), with all operations on the
Maybe monad inlined and simplified
we have
ta = TList TInt :: Typ a, tb :: TList TInt :: Typ b, for some a, b
eqT = case ta of
...
(TList ta1) -
-- now we have a ~ [a1] for some a1
case tb of
...
(TList tb1) -
-- now we have b ~ [b1] for some b1
case eqT ta1 tb1 of
Just Refl -
-- now we have a1 ~ b1, which gives us [a1] ~ [b1], which gives us a ~ b
Just Refl
_ - Nothing
-- ryan
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Re: [Haskell-cafe] Why does the transformers/mtl 'Error' class exist?
Ryan Ingram schrieb:
It's used in the implementation of fail for those monads.
class Monad m where
...
fail :: String - m a
fail = error -- default implementation
which is then used to desugar do-notation when pattern matching fails:
do
Left x - something
return x
=
something = \v - case v of { Left x - return x ; _ - fail
Pattern match failure ... }
You can argue about whether fail belongs in Monad (and many people
have), but that's why it is how it is.
I also prefered to not support the fail method in this way and wrote:
http://hackage.haskell.org/packages/archive/explicit-exception/0.1.4/doc/html/Control-Monad-Exception-Synchronous.html
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[Haskell-cafe] Why does the transformers/mtl 'Error' class exist?
Both the transformers[1] and mtl[2] define a class named 'Error', for use with MonadError and ErrorT. This class is required for the instance of Monad (and therefore MonadTrans, MonadIO, etc). However, I can't figure out why this class exists. Its presence means that instead of something like: - data NameError = ErrorFoo | ErrorBar validateName :: Monad m = Text - m (Either Error Text) validateName x = runErrorT $ do when (some condition) $ throwError ErrorFoo when (other condition) $ throwError ErrorBar return x - I have to define this, which is more verbose, no more useful, and adds a fake class to the Haddock docs (with a warning not to use it): - data Error = ErrorFoo | ErrorBar instance Error NameError where strMsg = error -- validateName ... - Is there any good reason why the 'Error' class can't just be removed? [1] http://hackage.haskell.org/packages/archive/transformers/0.2.0.0/doc/html/Control-Monad-Trans-Error.html [2] http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Monad-Error-Class.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why does the transformers/mtl 'Error' class exist?
It's used in the implementation of fail for those monads.
class Monad m where
...
fail :: String - m a
fail = error -- default implementation
which is then used to desugar do-notation when pattern matching fails:
do
Left x - something
return x
=
something = \v - case v of { Left x - return x ; _ - fail
Pattern match failure ... }
You can argue about whether fail belongs in Monad (and many people
have), but that's why it is how it is.
-- ryan
On Thu, Apr 15, 2010 at 7:18 PM, John Millikin jmilli...@gmail.com wrote:
Both the transformers[1] and mtl[2] define a class named 'Error', for
use with MonadError and ErrorT. This class is required for the
instance of Monad (and therefore MonadTrans, MonadIO, etc). However, I
can't figure out why this class exists. Its presence means that
instead of something like:
-
data NameError = ErrorFoo | ErrorBar
validateName :: Monad m = Text - m (Either Error Text)
validateName x = runErrorT $ do
when (some condition) $ throwError ErrorFoo
when (other condition) $ throwError ErrorBar
return x
-
I have to define this, which is more verbose, no more useful, and adds
a fake class to the Haddock docs (with a warning not to use it):
-
data Error = ErrorFoo | ErrorBar
instance Error NameError where
strMsg = error
-- validateName ...
-
Is there any good reason why the 'Error' class can't just be removed?
[1]
http://hackage.haskell.org/packages/archive/transformers/0.2.0.0/doc/html/Control-Monad-Trans-Error.html
[2]
http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Monad-Error-Class.html
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