[Haskell-beginners] Functor instance

[David McBride]

There is also a BiFunctor in base which allows you to fmap over the
last two variables in your type.

bimap :: (a -> b) -> (c -> d) -> p a c -> p b d

And it conveniently has methods to map over only one at a time, if you want.

first :: (a -> b) -> p a c -> p b c
second :: (b -> c) -> p a b -> p a c

This makes sense for types like Either a b, where sometimes you might
want to fmap over one or both values.

However, going beyond two would cause the number of options for
mapping over such a type to blow up.  At that point it is better to
just use your own simple function to map over your own type.

On Mon, Jan 9, 2017 at 11:26 AM, sasa bogicevic <brutallesale at gmail.com> wrote:
> Good explanation! Thanks I understand it now
>
> Sasa Bogicevic
> {
>     phone: +381606006200
> }
>
>> On Jan 9, 2017, at 17:00, Francesco Ariis <fa-ml at ariis.it> wrote:
>>
>>> On Mon, Jan 09, 2017 at 04:41:39PM +0100, sasa bogicevic wrote:
>>> Hi all,
>>> Can someone explain to me why exactly when defining the Functor
>>> instance for type that has polimorphic parameter constraint I
>>> am not allowed to put that parameter in definition. So basically:
>>>
>>> data Four a b c d = Four a b c d
>>>
>>> instance Functor (Four a b c) where  <-- why can't I specify also
>>> param d here ??
>>>    fmap f (Four a b c d) = Four a b c (f d)
>>
>> Hello Sasa,
>>    think for a moment about functors you already know: Maybe a, [a],
>> (Either e) a, etc. In every case there is an `a` and something preceding
>> a (we can call it `f`).
>> Now let's look at the class-definition of functor.
>>
>>    class Functor f where -- etc. etc.
>>
>> Here you have it, `f`; so for the instance you should only place
>> the `f`-part in there, like
>>
>>    instance Functor Maybe where        -- not Maybe a!
>>    instance Functor (Four a b c) where -- without the a too!
>>
>> It makes sense as `f` will stay the same and `a` will be mapped over
>> (and change).
>> Indeed you can very this with ghci (using :k Functor, :k Maybe etc.),
>> but after a while it sinks in.
>>
>> Does this help?
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