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Yves Parès

22 Feb 2011 22 Feb '11

5:07 p.m.

Hello, When importing Control.Monad.Error, it shows me that (Either e) is an instance of MonadFix, but not an instance of Monad. Isn't there a problem?

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Daniel Fischer

22 Feb 22 Feb

5:33 p.m.

On Tuesday 22 February 2011 23:07:05, Yves Parès wrote:

Hello,

When importing Control.Monad.Error, it shows me that (Either e) is an instance of MonadFix, but not an instance of Monad. Isn't there a problem?

Prelude Control.Monad.Error> :i Either data Either a b = Left a | Right b -- Defined in Data.Either instance (Eq a, Eq b) => Eq (Either a b) -- Defined in Data.Either instance Monad (Either e) -- Defined in Control.Monad.Instances instance Functor (Either a) -- Defined in Control.Monad.Instances <snip> instance MonadFix (Either e) -- Defined in Control.Monad.Fix <snip> Works right here. With mtl-1.*, there's an (Error e) restriction on the instances.

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Yves Parès

23 Feb 23 Feb

3:58 a.m.

I am using mtl-2. Importing Control.Monad.Error doesn't work, I have to explicitly import Control.Monad.Instances. 2011/2/22 Daniel Fischer :

On Tuesday 22 February 2011 23:07:05, Yves Parès wrote:

...
Hello,

When importing Control.Monad.Error, it shows me that (Either e) is an instance of MonadFix, but not an instance of Monad. Isn't there a problem?

Prelude Control.Monad.Error> :i Either data Either a b = Left a | Right b -- Defined in Data.Either instance (Eq a, Eq b) => Eq (Either a b) -- Defined in Data.Either instance Monad (Either e) -- Defined in Control.Monad.Instances instance Functor (Either a) -- Defined in Control.Monad.Instances <snip> instance MonadFix (Either e) -- Defined in Control.Monad.Fix <snip>

Works right here. With mtl-1.*, there's an (Error e) restriction on the instances.

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Daniel Fischer

7:53 a.m.

On Wednesday 23 February 2011 09:58:56, Yves Parès wrote:

I am using mtl-2. Importing Control.Monad.Error doesn't work, I have to explicitly import Control.Monad.Instances.

Weird. Control.Monad.Error imports Control.Monad.Instances (), so it ought to be visible. What exact versions of mtl, base, GHC are you using?

2011/2/22 Daniel Fischer :

...
On Tuesday 22 February 2011 23:07:05, Yves Parès wrote:

...
Hello,

When importing Control.Monad.Error, it shows me that (Either e) is an instance of MonadFix, but not an instance of Monad. Isn't there a problem?

Prelude Control.Monad.Error> :i Either data Either a b = Left a | Right b -- Defined in Data.Either instance (Eq a, Eq b) => Eq (Either a b) -- Defined in Data.Either instance Monad (Either e) -- Defined in Control.Monad.Instances instance Functor (Either a) -- Defined in Control.Monad.Instances <snip> instance MonadFix (Either e) -- Defined in Control.Monad.Fix <snip>

Works right here. With mtl-1.*, there's an (Error e) restriction on the instances.

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Yves Parès

8:14 a.m.

The latest, I think : GHC 7.0.1, mtl-2.0.1.0, base-4.3.0.0 2011/2/23 Daniel Fischer :

On Wednesday 23 February 2011 09:58:56, Yves Parès wrote:

...
I am using mtl-2. Importing Control.Monad.Error doesn't work, I have to explicitly import Control.Monad.Instances.

Weird. Control.Monad.Error imports Control.Monad.Instances (), so it ought to be visible. What exact versions of mtl, base, GHC are you using?

...
2011/2/22 Daniel Fischer :

...
On Tuesday 22 February 2011 23:07:05, Yves Parès wrote:

...
Hello,

When importing Control.Monad.Error, it shows me that (Either e) is an instance of MonadFix, but not an instance of Monad. Isn't there a problem?

Prelude Control.Monad.Error> :i Either data Either a b = Left a | Right b -- Defined in Data.Either instance (Eq a, Eq b) => Eq (Either a b) -- Defined in Data.Either instance Monad (Either e) -- Defined in Control.Monad.Instances instance Functor (Either a) -- Defined in Control.Monad.Instances <snip> instance MonadFix (Either e) -- Defined in Control.Monad.Fix <snip>

Works right here. With mtl-1.*, there's an (Error e) restriction on the instances.

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Daniel Fischer

8:29 a.m.

On Wednesday 23 February 2011 14:14:46, Yves Parès wrote:

The latest, I think : GHC 7.0.1, mtl-2.0.1.0, base-4.3.0.0

Hmm, that's exactly what I have. Weirder and weirder. For the moment, I'm out of ideas.

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Yves Parès

24 Feb 24 Feb

5:27 a.m.

I have the same problem with instances of Functor. For instance when I import Control.Applicative, to get the instance "Functor ((->) a)" I also have to import Control.Monad.Instances. 2011/2/23 Daniel Fischer :

On Wednesday 23 February 2011 14:14:46, Yves Parès wrote:

...
The latest, I think : GHC 7.0.1, mtl-2.0.1.0, base-4.3.0.0

Hmm, that's exactly what I have. Weirder and weirder. For the moment, I'm out of ideas.

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Yves Parès

26 Feb 26 Feb

6:54 a.m.

When I look at the documentation of Control.Monad.Error [1] or Control.Applicative [2] it is not said that it re-exports Control.Monad.Instances. So maybe this behaviour is normal... [1] http://hackage.haskell.org/packages/archive/mtl/2.0.1.0/doc/html/Control-Mon... [2] http://hackage.haskell.org/packages/archive/base/4.3.0.0/doc/html/Control-Ap... 2011/2/24 Yves Parès

I have the same problem with instances of Functor. For instance when I import Control.Applicative, to get the instance "Functor ((->) a)" I also have to import Control.Monad.Instances.

2011/2/23 Daniel Fischer :

...
On Wednesday 23 February 2011 14:14:46, Yves Parès wrote:

...
The latest, I think : GHC 7.0.1, mtl-2.0.1.0, base-4.3.0.0

Hmm, that's exactly what I have. Weirder and weirder. For the moment, I'm out of ideas.

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Daniel Fischer

7:28 a.m.

On Saturday 26 February 2011 12:54:02, Yves Parès wrote:

When I look at the documentation of Control.Monad.Error [1] or Control.Applicative [2] it is not said that it re-exports Control.Monad.Instances. So maybe this behaviour is normal...

I don't think so. Instances are automatically propagated, they can't be hidden once they're in scope. Functor, Monad, Either and (->) are all available in the Prelude, so unless you have NoImplicitPrelude, the instances ought to be in scope from any transitive dependency. And they are in scope for me when I import Control.Monad.Error or Control.Applicative, so something is amiss if they're not in scope for you.

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Yves Parès

7:49 a.m.

I tried reinstalling GHC 7.0.1 from scratch, the issue remains... 2011/2/26 Daniel Fischer

On Saturday 26 February 2011 12:54:02, Yves Parès wrote:

...
When I look at the documentation of Control.Monad.Error [1] or Control.Applicative [2] it is not said that it re-exports Control.Monad.Instances. So maybe this behaviour is normal...

I don't think so. Instances are automatically propagated, they can't be hidden once they're in scope. Functor, Monad, Either and (->) are all available in the Prelude, so unless you have NoImplicitPrelude, the instances ought to be in scope from any transitive dependency. And they are in scope for me when I import Control.Monad.Error or Control.Applicative, so something is amiss if they're not in scope for you.

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