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

Vanessa McHale vanessa.mchale at iohk.io
Thu Jul 19 03:44:16 UTC 2018

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:
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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