Ignore in import

Mike Houghton

24 Oct 2015 24 Oct '15

7:55 a.m.

Hi, I’m reading Typeclassopdedia and an exercise is "A good exercise is to implement Functor instances for Either e, ((,) e), and ((->) e).” so I have instance Functor ((,) a) where fmap = undefined which gives compiler error Duplicate instance declarations: instance Functor ((,) a) -- Defined at /Users/mike/haskell/FingerTrees/M.hs:65:10 instance Functor ((,) a) -- Defined in ‘GHC.Base’ so I added import GHC.Base hiding ((,)) but I still get the error. Where am I going wrong? Many thanks Mike

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Imants Cekusins

24 Oct 24 Oct

9:13 a.m.

...

Duplicate instance declarations:

it appears that there is (or was?) no way to hide instance definitions: https://mail.haskell.org/pipermail/haskell-cafe/2009-July/063842.html however looking at the exercise: "... implement Functor instances for Either e, ((,) e), and ((->) e).” does this not suggest 1) instance Functor Either e, ((,) e) and 2) instance Functor Either e, ((->) e) ?

Imants Cekusins

9:16 a.m.

... or: 1) instance Functor Either e ((,) e) 2) instance Functor Either e ((->) e) ?

Mike Houghton

9:35 a.m.

Mmmm I think its functor for each of Either e ((,) e) and ((->) e) ?

...

On 24 Oct 2015, at 14:16, Imants Cekusins wrote:

... or:

1) instance Functor Either e ((,) e) 2) instance Functor Either e ((->) e)

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Imants Cekusins

10:05 a.m.

...

I think its functor for each of ...

sorry, you are right. well maybe it is possible with a non-ghc compiler?

Petr Vápenka

10:16 a.m.

You can try NoImplicitPrelude language extension (this may not work, too) or use newtype wrappers or normal data types with the same shape. Dne 24.10.2015 16:05 napsal uživatel "Imants Cekusins" :

...

...
I think its functor for each of ...

sorry, you are right.

well maybe it is possible with a non-ghc compiler? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Ryan Trinkle

11:25 a.m.

For the purpose of learning the material, it might be easiest to just write: class MyFunctor f where myFmap :: (a -> b) -> f a -> f b and then create instances of that, instead. This way, you won't have any conflicts with existing instances. On Sat, Oct 24, 2015 at 10:16 AM, Petr Vápenka wrote:

...

You can try NoImplicitPrelude language extension (this may not work, too) or use newtype wrappers or normal data types with the same shape. Dne 24.10.2015 16:05 napsal uživatel "Imants Cekusins" :

...
I think its functor for each of ...

sorry, you are right.

well maybe it is possible with a non-ghc compiler? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Mike Houghton

12:13 p.m.

Yes! It’s obvious once its pointed out. Thanks

...

On 24 Oct 2015, at 16:25, Ryan Trinkle wrote:

For the purpose of learning the material, it might be easiest to just write:

class MyFunctor f where myFmap :: (a -> b) -> f a -> f b

and then create instances of that, instead.

This way, you won't have any conflicts with existing instances.

On Sat, Oct 24, 2015 at 10:16 AM, Petr Vápenka mailto:petr.vapenka@gmail.com> wrote: You can try NoImplicitPrelude language extension (this may not work, too) or use newtype wrappers or normal data types with the same shape.

Dne 24.10.2015 16:05 napsal uživatel "Imants Cekusins" mailto:imantc@gmail.com>:

...
I think its functor for each of ...

sorry, you are right.

well maybe it is possible with a non-ghc compiler? _______________________________________________ Beginners mailing list Beginners@haskell.org mailto:Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Imants Cekusins
Mike Houghton
Petr Vápenka
Ryan Trinkle

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