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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Categories defined using propositional equality are very hard to work with. I’m not surprised things went sideways.<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"><span style="mso-fareast-language:EN-US">Here’s a full construction of the Comma Category in Agda
<a href="https://agda.github.io/agda-categories/Categories.Category.Construction.Comma.html">
https://agda.github.io/agda-categories/Categories.Category.Construction.Comma.html</a><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"><span style="mso-fareast-language:EN-US">In Setoid-Enriched categories, we get to define what equivalent arrows are. In particular, the proofs are not part of the definition, and that makes life much simpler.<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"><span style="mso-fareast-language:EN-US">Recommendation: if you’re going to insist on using propositional equality, do yourself a favour, and use –with-K. That will already help. Eventually, you’ll likely have to assume function extensionality
as well.<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"><span style="mso-fareast-language:EN-US">Jacques<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Agda <agda-bounces@lists.chalmers.se>
<b>On Behalf Of </b>Daniel Lee<br>
<b>Sent:</b> December 14, 2020 8:40 PM<br>
<b>To:</b> agda@lists.chalmers.se<br>
<b>Subject:</b> [Agda] Tips for working around proof relevance<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi everyone,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I'm an undergraduate in CS, trying to use this winter break to learn category theory via Categories in Context (and hopefully topology and HoTT). I thought it would be cool to formalize in Agda at the same time. <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Here's the link to the Agda development for reference: </span><a href="https://github.com/dan-iel-lee/categories-in-agda">https://github.com/dan-iel-lee/categories-in-agda</a> . Relevant files
are probably "Core" and "Examples/Comma".<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I've run into a roadblock, specifically with proving that the comma category is a category, but which I imagine could be an issue in the future as well.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I've defined the arrows in the category as:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">Σ[ (h , k) </span><span style="font-family:"Cambria Math",serif">∈</span><span style="font-family:"Courier New""> d
</span><span style="font-family:"Cambria Math",serif">⟶</span><span style="font-family:"Courier New"">ᴰ d' × e
</span><span style="font-family:"Cambria Math",serif">⟶</span><span style="font-family:"Courier New"">ᴱ e' ] f'
</span><span style="font-family:"Cambria Math",serif">∘</span><span style="font-family:"Courier New""> (</span><span style="font-family:"Cambria Math",serif">ℱ</span><span style="font-family:"Courier New""> h) ≡ (</span><span style="font-family:"Cambria Math",serif">𝒢</span><span style="font-family:"Courier New"">
k) </span><span style="font-family:"Cambria Math",serif">∘</span><span style="font-family:"Courier New""> f</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Where h and k are the component arrows, and the body of the Sigma is the commutativity condition. Defining composition and identity are relatively straightforward, but proving the left/right
unit and associativity properties become difficult because it requires proving that the identities are identified. And because there are a lot of steps, compositions,... involved, the Goal is super hard to read and I don't know where to start in trying to
prove it. I managed to work around it by enforcing that all the hom-sets are sets (in the sense that their identification types are all identical). Is there a nice way to prove this without using proof irrelevance though?</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">All help is very very appreciated. Thanks :)))</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Best,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Daniel</span><o:p></o:p></p>
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