So fmap is undefined for Either? Why make Either a functor and not define fmap?

Michael


Prelude> let l = Left 3
Prelude> let r = Right "foo"
Prelude> :t l
l :: Either Integer b
Prelude> :t r
r :: Either a [Char]
Prelude> fmap (*2) l

<interactive>:1:0:
    No instance for (Functor (Either Integer))
      arising from a use of `fmap' at <interactive>:1:0-10
    Possible fix:
      add an instance declaration for (Functor (Either Integer))
    In the expression: fmap (* 2) l
    In the definition of `it': it = fmap (* 2) l
Prelude>




--- On Sat, 8/28/10, Brandon S Allbery KF8NH <allbery@ece.cmu.edu> wrote:

From: Brandon S Allbery KF8NH <allbery@ece.cmu.edu>
Subject: Re: [Haskell-cafe] On to applicative
To: haskell-cafe@haskell.org
Date: Saturday, August 28, 2010, 9:06 PM

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On 8/28/10 20:43 , michael rice wrote:
> I'm looking at a discussion of Either (as functor) here:
>
> http://learnyouahaskell.com/making-our-own-types-and-typeclasses#the-functor-typeclass
>
> instance Functor (Either a) where 
>     fmap f (Right x) = Right (f x) 
>     fmap f (Left x) = Left x
>
> And this line in Data.Either
>
> Functor (Either a)
>
> but no fmap defined here.
>
> How come?

Historical accident, to wit:  Haskell 98 minimally defined Either in the
Prelude, so in practice we get the basic definitions (Either itself and its
Functor and Monad instances) from the Prelude and other utility functions
from Data.Either.

- --
brandon s. allbery     [linux,solaris,freebsd,perl]      allbery@kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery@ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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