Re: [Haskell-cafe] clonetype

Another issue with saying "derive all instances for this newtype" is that not all instances are coercible. type family F a where F Char = Int class C a where f :: a -> F a instance C Char where f _ = 'a' newtype D = D Char There is no way to derive an instance C D. A more explicit and flexible solution would be "deriving synonyms", discussed recently here: https://www.reddit.com/r/haskell/comments/9dx6s9/proposal_data_deriving_syno... Li-yao On 9/24/18 8:47 AM, Oleg Grenrus wrote:

The problem is that "All instances" is hard to pin point. We have open world assumption, so instances can be added later (in the dependency tree). Should they be cloned too? And even of you restrict to "instances visible at clonetype definition", that's IMHO not a good idea either, as it's implicit and volatile set (editing imports changes may change the set).

So use either tagged-trick/pattern or GND.

Haskell's heavy type machinery exists so we can explicitly and exactly say what we need or want. Tagged/phantom-pattern and GND are different, they have different pros & cons. Having `clonetype-fits-all` seems non-trivial to me.

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On 24 Sep 2018, at 15.25, Harendra Kumar mailto:harendra.kumar@gmail.com> wrote:

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That comes close, but Haskell having such a heavy type machinery to do all the esoteric type stuff in the world, but not allowing you to express such a simple day to day programming thing in a simpler manner is not very satisfying.

-harendra

On Mon, 24 Sep 2018 at 16:16, Oleg Grenrus mailto:oleg.grenrus@iki.fi> wrote:

data MyStringTag type MyString = Tagged MyStringTag String

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On 24 Sep 2018, at 13.42, Harendra Kumar mailto:harendra.kumar@gmail.com> wrote:

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Often, we need to create a newtype that is equivalent to a given type for safety reasons. Using type  synonym is useless from type safety perspective. With newtype, we have to add a "deriving" clause to it for deriving the required instances, to make it practically useful.

Does it make sense, and is it possible to have something like a "clonetype" that creates a new type and derives all the instances of the parent type as well? It will be quite helpful in creating equivalent newtype synonyms quickly. Almost always, I do not use a newtype where I should just because of the inconvenience of deriving the instances. Ideally, we should just be able to say something like:

clonetype MyString = String

and we are good to go.  What is the shortest possible way to achieve this with currently available mechanisms, if any?

-harendra _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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