[Haskell-beginners] "Learn You a Haskell" question

[Bob Ippolito]

The definition of a Functor requires that exactly one of the type variables
be free, which is why it's written as `Either a` instead of `Either a b`.
Any fields that are not `b` must be simply passed through as-is by fmap.
There could be a separate functor that would fmap over the Left, but there
isn't (in the base package anyhow).

There's a related Functor for `(,) a` where the Functor fmaps over the snd
of the tuple, and the fst is left as-is.

fmap (+1) ('a', 2) == ('a', 3)
fmap (+1) (Right 2) == Right 3
fmap (+1) (Left 'a') == Left 'a'

Chris Done recently prototyped a fmap explorer that you might find useful:
http://www.reddit.com/r/haskell/comments/2dok9w/functor_explorer/

-bob



On Mon, Aug 18, 2014 at 11:02 AM, Frank <frankdmartinez at gmail.com> wrote:

> Hi,
>     I'm reading "Learn You a Haskell..." and have a question about the
> chapter "Making Our Own Types and Typeclasses". On the 'Functor'/'Either'
> example, I feel completely lost. I don't see why the 'Left x' portion of
> 'Functor (Either a)' is simply 'Left x' and the document is not exactly
> clear. Any clarification would be most appreciated.
>
> Sincerely,
> Frank D. Martinez
>
>
> --
> P.S.: I prefer to be reached on BitMessage at
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