[Haskell-beginners] Functor instance
sasa bogicevic
brutallesale at gmail.com
Mon Jan 9 16:26:12 UTC 2017
Good explanation! Thanks I understand it now
Sasa Bogicevic
{
phone: +381606006200
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> 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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