Re: [Haskell-cafe] Problem with monad transformer stack
Hmm, it seems like MonadState can be derived even with a non-concrete
type, for instance:
--
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Monad.Error
import Control.Monad.State
import Data.Typeable
data SomeError =
Error1
| Error2
| ErrorFail
deriving (Eq, Show, Typeable)
data MyData a = MyData [a]
instance Error SomeError where
noMsg = ErrorFail
newtype MyMonad a b =
MyMonad ((StateT (MyData a) (Either SomeError) b))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
--
This compiles without errors. So it looks to me like the real problem
was the implicit dependency between the type 'a' in MyData and the
return type 'b' of the monad, which the deriving mechanism couldn't
enforce if 'b' was 'a'. I'm finding it hard to get a good conceptual
understanding of what's really going on here.
Mike
On 10/3/10 7:03 PM, Christopher Done wrote:
On 4 October 2010 03:40, Michael Vaniermvanie...@gmail.com wrote:
newtype MyMonad a =
MyMonad ((StateT (MyData a) (Either SomeError) a))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
I think it's the `a'. I think it needs to be a concrete type. E.g. the
following is OK:
newtype MyMonad a =
MyMonad ((StateT (MyData ()) (Either SomeError) a))
deriving (Monad,
MonadState (MyData ()),
MonadError SomeError,
Typeable)
But
newtype MyMonad a =
MyMonad ((StateT (MyData ()) (Either SomeError) [a]))
deriving (Monad,
MonadState (MyData ()),
MonadError SomeError,
Typeable)
is not. This reminds me of the restriction that impredicative types
remove, but I don't think it's related.
These error messages mean nothing to me. What's going on? Can the more
specific code be made to work? This is with ghc 6.12.3.
It seems like eta-reducing `X' or `x' is enough, but Foo x,, i.e. a
parametrized type with a type variable isn't enough. I think that's
what's going on, but I don't know why.
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[Haskell-cafe] Problem with monad transformer stack
I'm having a problem with a simple monad transformer stack that has me
stumped. Here's the sample code:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Monad.Error
import Control.Monad.State
import Data.Typeable
data SomeError =
Error1
| Error2
| ErrorFail
deriving (Eq, Show, Typeable)
data MyData a = MyData [a]
instance Error SomeError where
noMsg = ErrorFail
{- This works: -}
{-
newtype StateError e s a =
StateError ((StateT s (Either e) a))
deriving (Monad,
MonadState s,
MonadError e,
Typeable)
type MyMonad a = StateError SomeError (MyData a) a
-}
{- This doesn't work: -}
newtype MyMonad a =
MyMonad ((StateT (MyData a) (Either SomeError) a))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
--
Basically, the more abstracted (commented-out) version works, but the
more specific one gives this error:
Weird.hs:33:12:
Can't make a derived instance of `Monad MyMonad'
(even with cunning newtype deriving):
cannot eta-reduce the representation type enough
In the newtype declaration for `MyMonad'
Weird.hs:34:12:
Cannot eta-reduce to an instance of form
instance (...) = MonadState (MyData a) MyMonad
In the newtype declaration for `MyMonad'
Weird.hs:35:12:
Can't make a derived instance of `MonadError SomeError MyMonad'
(even with cunning newtype deriving):
cannot eta-reduce the representation type enough
In the newtype declaration for `MyMonad'
These error messages mean nothing to me. What's going on? Can the more
specific code be made to work? This is with ghc 6.12.3.
Thanks,
Mike
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Re: [Haskell-cafe] Problem with monad transformer stack
On 4 October 2010 03:40, Michael Vanier mvanie...@gmail.com wrote: newtype MyMonad a = MyMonad ((StateT (MyData a) (Either SomeError) a)) deriving (Monad, MonadState (MyData a), MonadError SomeError, Typeable) I think it's the `a'. I think it needs to be a concrete type. E.g. the following is OK: newtype MyMonad a = MyMonad ((StateT (MyData ()) (Either SomeError) a)) deriving (Monad, MonadState (MyData ()), MonadError SomeError, Typeable) But newtype MyMonad a = MyMonad ((StateT (MyData ()) (Either SomeError) [a])) deriving (Monad, MonadState (MyData ()), MonadError SomeError, Typeable) is not. This reminds me of the restriction that impredicative types remove, but I don't think it's related. These error messages mean nothing to me. What's going on? Can the more specific code be made to work? This is with ghc 6.12.3. It seems like eta-reducing `X' or `x' is enough, but Foo x,, i.e. a parametrized type with a type variable isn't enough. I think that's what's going on, but I don't know why. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problem with monad transformer stack
On Sun, Oct 3, 2010 at 9:40 PM, Michael Vanier mvanie...@gmail.com wrote:
{- This doesn't work: -}
newtype MyMonad a =
MyMonad ((StateT (MyData a) (Either SomeError) a))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
This simply isn't allowed by the generalised newtype derivation machinery,
because the type variable a appears in one of the classes you're deriving.
In fact, I'm not sure how you're hoping for your type to actually work as a
monad. If you try using (=) on your type synonym that currently appears to
typecheck, you'll find that the only value that can inhabit the state
parameter is bottom. Try writing out and using a definition of (=) by hand
to understand your confusion.
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Re: [Haskell-cafe] Problem with monad transformer stack
On 10/3/10 7:06 PM, Bryan O'Sullivan wrote:
On Sun, Oct 3, 2010 at 9:40 PM, Michael Vanier mvanie...@gmail.com
mailto:mvanie...@gmail.com wrote:
{- This doesn't work: -}
newtype MyMonad a =
MyMonad ((StateT (MyData a) (Either SomeError) a))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
This simply isn't allowed by the generalised newtype derivation
machinery, because the type variable a appears in one of the classes
you're deriving.
In fact, I'm not sure how you're hoping for your type to actually work
as a monad. If you try using (=) on your type synonym that currently
appears to typecheck, you'll find that the only value that can inhabit
the state parameter is bottom. Try writing out and using a definition
of (=) by hand to understand your confusion.
I disagree with your second point. I have this in working code:
--
newtype StateErrorIO e s a =
StateErrorIO { runS :: (StateT s (ErrorT e IO) a) }
deriving (Monad,
MonadIO,
MonadState s,
MonadError e,
Typeable)
--
I can assure you that it works on non-bottom types.
As for the first point, that makes sense. So if I do this:
--
newtype MyMonadS s a =
MyMonad ((StateT s (Either SomeError) a))
deriving (Monad,
MonadState s,
MonadError SomeError,
Typeable)
type MyMonad a = MyMonadS (MyData a) a
--
it type checks. And yeah, writing out the instances by hand is the best
way to understand what's going on.
Mike
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