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<p>Hi,</p>
<p>following up with Thorsten's command about the word "dependent
type theory", I would like to add a few observations I had in this
discussion:<br>
</p>
<ul>
<li>I think the word "type theory" itself is unclear in this
context. At least several of the emails seem to use different
but related ideas of what that means:</li>
<ul>
<li>It could mean "something where everything has a type" (i.e.,
not the usual ZF). Then HOL would be type theory. (Thorsten's
email quoted below makes sense in that setting because HOL
avoids the problem described there.)<br>
</li>
<li>It could mean the above with dependent types (but not
necessary the Curry-Howard thing from the next item)<br>
</li>
<li>It could mean "a system where we use the same language for
propositions and proofs via Curry-Howard isomorphism" (I
believe this will then also need dependent types since
otherwise the proof terms are not powerful enough)<br>
</li>
<li>It could mean a system with strong normalization (so that
everything terminates), at least some of the answers seem to
see this as part of the meaning of "type theory".</li>
</ul>
</ul>
<p>Of course, there are many interaction between the different
concepts, but if we talk about the costs or benefits of adopting
type theory, it becomes quite important which aspect of it we are
adopting. (E.g., if we have, say, a good reason why we need the
second point, and a big cost for getting the four point, then it
is important not to just say "type theory has the following good
reason and the following cost".)<br>
</p>
<p>Maybe when discussing *why* type theory, we should prefix our
answers by what we mean by type theory (e.g., one of the above).
Otherwise it will be very confusing (at least to me).</p>
<p>Another question is also the context in which we want to use it:</p>
<ul>
<li>Programming (with all the associated things like verification,
type checking, etc.)</li>
<li>Math<br>
</li>
</ul>
<p>These have different requirements, so making explicit which
domain we are thinking of in our answer might make things clearer
as well.</p>
<p>Just my personal thoughts, but I hope they may help to add some
clarity to the discussion.<br>
</p>
<p>Best wishes,<br>
Dominique.</p>
<p><br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 3/4/20 11:42 AM, Thorsten Altenkirch
wrote:<br>
</div>
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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">First
of all I don’t like the word “dependent type theory”.
Dependent types are one important feature of modern Type
Theory but hardly the only one.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal">To me the most important feature of Type
Theory is the support of abstraction in Mathematics and
computer science. Using types instead of sets means that you
can hide implementation choices which is essential if you want
to build towers of abstraction. Set theory fails here badly.
Just as a very simple example: in set theory you have the
notion of union, so for example
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="text-indent:36.0pt">{0,1} \cup
{0,1,2,3} = {0,1,2,3}<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:36.0pt"><o:p> </o:p></p>
<p class="MsoNormal">However, if we change the representation of
the first set and use lets say {true,false} we get a different
result:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="text-indent:36.0pt">{true , false}
\cup {0,1,2,3} = {true,false,0,1,2,3}<o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">This
means that \cup exposes implementation details because the
results are not equivalent upto renaming. In Type Theory we
have the notion of sum, sometimes called disjoint union,
which is well behaved<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span lang="DE">{0,1}
+ {0,1,2,3} = {in1 0,in1 1,in2 0,in2 1,in2 2,in2 3}<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span lang="DE"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span lang="DE">{true
, false} + {0,1,2,3} = {in1 true,in1 false ,in2 0,in2 1,in2
2,in2 3}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="DE"><o:p> </o:p></span></p>
<p class="MsoNormal">Unlike \cup, + doesn’t reveal any
implementation details it is a purely structural operation.
Having only structural operations means that everything you do
is stable under equivalence, that is you can replace one
object with another one that behaves the same. This is the
essence of Voevodsky’s univalence principle.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">There are other nice aspects of Type
Theory. From a constructive point of view (which should come
naturally to a computer scientists) the proporsitions as types
explanation provides a very natural way to obtain “logic for
free” and paedagogically helpful since it reduces logical
reasoning to programming.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">There are performance issues with
implementations of Type Theory, however, in my experience
(mainly agda) the execution of functions at compile time isn’t
one of them. In my experience the main problem is to deal with
a loss of sharing when handling equational constraints which
can blow up the time needed for type checking. I think this is
an engineering problem and there are some suggestions how to
fix this.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thorsten<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:36.0pt"><o:p> </o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<div style="border:none;border-top:solid #B5C4DF
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal" style="margin-left:36.0pt"><b><span
style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black"><a class="moz-txt-link-rfc2396E" href="mailto:coq-club-request@inria.fr">"coq-club-request@inria.fr"</a>
<a class="moz-txt-link-rfc2396E" href="mailto:coq-club-request@inria.fr"><coq-club-request@inria.fr></a> on behalf of Jason Gross
<a class="moz-txt-link-rfc2396E" href="mailto:jasongross9@gmail.com"><jasongross9@gmail.com></a><br>
<b>Reply to: </b><a class="moz-txt-link-rfc2396E" href="mailto:coq-club@inria.fr">"coq-club@inria.fr"</a>
<a class="moz-txt-link-rfc2396E" href="mailto:coq-club@inria.fr"><coq-club@inria.fr></a><br>
<b>Date: </b>Tuesday, 3 March 2020 at 19:44<br>
<b>To: </b>coq-club <a class="moz-txt-link-rfc2396E" href="mailto:coq-club@inria.fr"><coq-club@inria.fr></a>, agda-list
<a class="moz-txt-link-rfc2396E" href="mailto:agda@lists.chalmers.se"><agda@lists.chalmers.se></a>,
<a class="moz-txt-link-rfc2396E" href="mailto:coq+miscellaneous@discoursemail.com">"coq+miscellaneous@discoursemail.com"</a>
<a class="moz-txt-link-rfc2396E" href="mailto:coq+miscellaneous@discoursemail.com"><coq+miscellaneous@discoursemail.com></a>, lean-user
<a class="moz-txt-link-rfc2396E" href="mailto:lean-user@googlegroups.com"><lean-user@googlegroups.com></a><br>
<b>Subject: </b>[Coq-Club] Why dependent type theory?<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal" style="margin-left:36.0pt">I'm in the
process of writing my thesis on proof assistant performance
bottlenecks (with a focus on Coq), and there's a large class
of performance bottlenecks that come from (mis)using the
power of dependent types. So in writing the introduction, I
want to provide some justification for the design decision
of using dependent types, rather than, say, set theory or
classical logic (as in, e.g., Isabelle/HOL). And the only
reasons I can come up with are "it's fun" and "lots of
people do it" <o:p></o:p></p>
<div>
<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal" style="margin-left:36.0pt">So I'm
asking these mailing lists: why do we base proof
assistants on dependent type theory? What are the
trade-offs involved?<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="margin-left:36.0pt">I'm
interested both in explanations and arguments given on
list, as well as in references to papers that discuss
these sorts of choices.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal" style="margin-left:36.0pt">Thanks,<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="margin-left:36.0pt">Jason<o:p></o:p></p>
</div>
</div>
</div>
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