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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: <a class="moz-txt-link-freetext" href="https://github.com/haskell-backpack/backpack-str">https://github.com/haskell-backpack/backpack-str</a><br>
<br>
As you can see (particularly here:
<a class="moz-txt-link-freetext" href="https://github.com/haskell-backpack/backpack-str/blob/master/str-sig/Str.hsig#L237">https://github.com/haskell-backpack/backpack-str/blob/master/str-sig/Str.hsig#L237</a>),
one can let the *type* vary in a way that typeclasses do not allow.<br>
<br>
<div class="moz-cite-prefix">On 07/18/2018 10:19 PM, Clinton Mead
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAFDVgwE8G1=i+sCtsCdjS9XgYG3+kOvWiHXLjOF6786XBfR1Vg@mail.gmail.com">
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<div dir="ltr">Hi All
<div><br>
</div>
<div>I read though Edward Z. Yang's <a
href="http://blog.ezyang.com/2016/10/try-backpack-ghc-backpack/"
moz-do-not-send="true">blog post</a> about Backpack but I
can't understand what it does above and beyond classes and
instances.</div>
<div><br>
</div>
<div>It seems to me we could just replace a "signature" with a
"class" and a "unit" with an "instance" and achieve the same
thing. </div>
<div><br>
</div>
<div>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". </div>
<div><br>
</div>
<div>Edward Kmett's <a
href="https://www.reddit.com/r/haskell/comments/8a5w1n/new_package_unpackedcontainers/"
moz-do-not-send="true">unpacked-containers</a> gets some
speed improvements using Backpack, but is Backpack just
optimisation thing, or are there ways you can organise code
with <span
style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">Backpack</span>
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?</div>
<div><br>
</div>
<div>Thanks,</div>
<div>Clinton</div>
</div>
<br>
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