[Haskell-beginners] Functor instance for ordered lists

[Benjamin Edwards]

This is a bit better:
https://dorchard.wordpress.com/2011/10/18/subcategories-in-haskell-exofunctors/

On Mon, 4 Jan 2016 at 14:46 Benjamin Edwards <edwards.benj at gmail.com> wrote:

> It is impossible.
>
> You can make a new functor class using contraint kinds that allows what
> you want. There is probably a package out there already that does!
>
> http://www.cl.cam.ac.uk/~dao29/publ/constraint-families.pdf
>
> Sections 2.2 and 5.1 have the relevant stuff. I realise this is a bit err,
> dense for the beginners list. There are probably better references out
> there.
>
> Ben
>
> On Mon, 4 Jan 2016 at 14:30 martin <martin.drautzburg at web.de> wrote:
>
>> Am 01/04/2016 um 11:45 AM schrieb Imants Cekusins:
>> >> a newtype for ordered lists
>> >
>> > why not:
>> > newtype Ordlist a = Ordlist [a]
>>
>> All nice and dandy, but at first you already need an Ord constraint for
>> your smart constructor
>>
>> -- and a ctor:
>> ordList::(Ord a) => [a]-> OrdList a
>> ordList = OrdList . sort
>>
>> but this is still not the main problem. When you try to define a Functor
>> instance, you'd be tempted to do this (at least
>> I was):
>>
>> instance Functor OrdList where
>>         fmap f (OrdList xs) = OrdList $ sort $ map f xs
>>
>> but you can't do this, because of: "No instance for (Ord b) arising from
>> a use of ‘sort’", where b is the return type of
>> f :: (a->b). This does make sense, the function has to return something
>> which can be sorted.
>>
>> So my question is: is it impossible to write a functor instance for
>> ordered lists? It appears so, because a Functor does
>> not impose any constraints of f. But my knowledge is quite limited and
>> maybe a well-set class constraint can fix things.
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20160104/ce0c140a/attachment.html>