[Haskell-cafe] On to applicative

[Brandon S Allbery KF8NH]

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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 at kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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