[Haskell-cafe] On to applicative

michael rice nowgate at yahoo.com
Sat Aug 28 22:15:06 EDT 2010

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


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

    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

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

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

Hash: SHA1

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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