[Haskell-cafe] Backpack vs plain old classes and instances

[Clinton Mead]

Hi Vanessa

Sorry, I still can't understand what backpack is doing beyond classes and
instances.

map-classes
<http://hackage.haskell.org/package/map-classes-0.1.0.0/docs/Control-Class-Impl-Map.html>
gives
a common interface for key-value maps (i.e. arrays, maps, hashmaps, etc),
just using classes and interfaces. Indeed this seems to be what classes
were designed to do, for example, the IArray class gives a common interface
over int index immutable arrays.

I'm missing something obviously. How can the type "vary" in a way type
classes does not allow? Couldn't the code you referenced just be a three
parameter type class?

Thanks,
Clinton

On Thu, Jul 19, 2018 at 1:44 PM, Vanessa McHale <vanessa.mchale at iohk.io>
wrote:

> A module signature can contain data types, and the units can contain the
> concrete data types. This allows e.g. a common interface for all string
> packages: https://github.com/haskell-backpack/backpack-str
>
> As you can see (particularly here: https://github.com/haskell-
> backpack/backpack-str/blob/master/str-sig/Str.hsig#L237), one can let the
> *type* vary in a way that typeclasses do not allow.
>
>
> On 07/18/2018 10:19 PM, Clinton Mead wrote:
>
> Hi All
>
> I read though Edward Z. Yang's blog post
> <http://blog.ezyang.com/2016/10/try-backpack-ghc-backpack/> about
> Backpack but I can't understand what it does above and beyond classes and
> instances.
>
> It seems to me we could just replace a "signature" with a "class" and a
> "unit" with an "instance" and achieve the same thing.
>
> Of course there are issues with orphans instances if we don't own either
> the class or the associated data, but it would seem to me that "orphan
> units" are no less problematic than "orphan instances".
>
> Edward Kmett's unpacked-containers
> <https://www.reddit.com/r/haskell/comments/8a5w1n/new_package_unpackedcontainers/>
>  gets some speed improvements using Backpack, but is Backpack just
> optimisation thing, or are there ways you can organise code with Backpack
> that you couldn't do without just classes and instances? For example, does
> Backpack solve the issue of packages requiring huge dependency lists to
> implement instances that most of their users will not use?
>
> Thanks,
> Clinton
>
>
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