Good explanation! Thanks I understand it now Sasa Bogicevic { phone: +381606006200 }

On Jan 9, 2017, at 17:00, Francesco Ariis 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? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

I will look at bimap too, sounds usefull, thanks! Sasa Bogicevic { phone: +381606006200 }

On Jan 9, 2017, at 17:40, David McBride wrote:

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 wrote: Good explanation! Thanks I understand it now

Sasa Bogicevic { phone: +381606006200 }

...

...

On Jan 9, 2017, at 17:00, Francesco Ariis 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? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

---

Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

---

Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners