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lecture_tactics_long_version.html
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<link href="coqdoc.css" rel="stylesheet" type="text/css" />
<title>lecture_tactics_long_version</title>
</head>
<body>
<div id="page">
<div id="header">
</div>
<div id="main">
<h1 class="libtitle">Library lecture_tactics_long_version</h1>
<div class="code">
</div>
<div class="doc">
<a id="lab1"></a><h1 class="section">Lecture 4: Tactics in UniMath</h1>
by Ralph Matthes, IRIT, Université de Toulouse, CNRS, Toulouse INP, UT3, Toulouse, France
<div class="paragraph"> </div>
My employer is the CNRS, my lab is the IRIT, but the above affiliation is obligatory for
any scientific production I release.
<div class="paragraph"> </div>
This is the extended version of a presentation at the
UniMath school 2022 in Cortona, meant for self-study
and for exploring the UniMath library.
<div class="paragraph"> </div>
The material is based on the extended version of the
presentation at the UniMath schools 2017 and 2019 in
Birmingham, but updated to fit with the changes in (the
practice of) UniMath.
<div class="paragraph"> </div>
Works with current UniMath (as of July 15, 2022, but also the Coq platform of April '22).
<div class="paragraph"> </div>
Compiles with the command
<br/>
<span class="inlinecode"><span class="id" title="var">coqc</span> -<span class="id" title="keyword">type</span>-<span class="id" title="tactic">in</span>-<span class="id" title="keyword">type</span> <span class="id" title="var">lecture_tactics_long_version.v</span>
<div class="paragraph"> </div>
</span>when Coq is set up according to the instructions for this school and the associated coqc executable
has priority in the path. However, you do not need to compile this file. The option is crucial, and
also your own developments will need the Coq options configured through the installation instructions,
most notably the present one.
<div class="paragraph"> </div>
Can be transformed into HTML documentation with the command
<br/>
<span class="inlinecode"><span class="id" title="var">coqdoc</span> -<span class="id" title="var">utf8</span> <span class="id" title="var">lecture_tactics_long_version.v</span>
<div class="paragraph"> </div>
</span> (If internal links in the generated lecture_tactics_long_version.html are desired,
compilation with coqc is needed.)
<div class="paragraph"> </div>
In Coq, one can define concepts by directly giving well-typed
terms (see Part 2), but one can also be helped in the construction by the
interactive mode.
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Require</span> <span class="id" title="keyword">Import</span> <span class="id" title="library">UniMath.Foundations.Preamble</span>.<br/>
<br/>
</div>
<div class="doc">
<a id="lab2"></a><h2 class="section">define a concept interactively:</h2>
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Locate</span> <span class="id" title="var">bool</span>. </div>
<div class="doc">
a separate definition - <span class="inlinecode"><span class="id" title="var">Init.Datatypes.bool</span></span> is in the Coq library,
not available for UniMath
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Definition</span> <a id="myfirsttruthvalue" class="idref" href="#myfirsttruthvalue"><span class="id" title="definition">myfirsttruthvalue</span></a>: <span class="id" title="inductive">bool</span>.<br/>
</div>
<div class="doc">
only the identifier and its type given, not the definiens
<div class="paragraph"> </div>
This opens the interactive mode.
<div class="paragraph"> </div>
The <a href="https://github.com/UniMath/UniMath/tree/master/UniMath/README.md#unimath-coding-style">UniMath
style guide</a> asks us to start what follows with <span class="inlinecode"><span class="id" title="keyword">Proof</span>.</span> in a separate line.
In vanilla Coq, this would be optional (it is anyway a "nop").
</div>
<div class="code">
<span class="id" title="keyword">Proof</span>.<br/>
</div>
<div class="doc">
Now we still have to give the term, but we are in interactive mode. If you want to see everything in the currently loaded part of the UniMath library
that *involves* booleans, then do
</div>
<div class="code">
<span class="id" title="keyword">Search</span> <span class="id" title="inductive">bool</span>.<br/>
</div>
<div class="doc">
If you only want to find library elements that *yield* booleans, then try
</div>
<div class="code">
<span class="id" title="keyword">Search</span><span class="id" title="var">Pattern</span> <span class="id" title="inductive">bool</span>.<br/>
</div>
<div class="doc">
<span class="inlinecode"><span class="id" title="var">true</span></span> does not take an argument, and it is already a term we can take as definiens.
</div>
<div class="code">
<span class="id" title="tactic">exact</span> <span class="id" title="constructor">true</span>.<br/>
</div>
<div class="doc">
<span class="inlinecode"><span class="id" title="tactic">exact</span></span> is a tactic which takes the term as argument and informs Coq in the proof mode to
finish the current goal with that term.
<div class="paragraph"> </div>
We see in the response buffer: "No more subgoals."
Hence, there is nothing more to do, except for leaving the proof mode properly.
</div>
<div class="code">
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
</div>
<div class="doc">
<span class="inlinecode"><span class="id" title="keyword">Defined</span>.</span> instructs Coq to complete the whole interactive construction of a term,
verify it and to associate it with the given identifer, here <span class="inlinecode"><span class="id" title="var">myfirsttruthvalue</span></span>.
This may go wrong for different reasons, including implementation errors of the Coq
system - that will not affect trustworthiness of the library.
</div>
<div class="code">
<span class="id" title="keyword">Search</span> <span class="id" title="inductive">bool</span>.<br/>
</div>
<div class="doc">
The new definition appears at the beginning of the list.
</div>
<div class="code">
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#myfirsttruthvalue"><span class="id" title="definition">myfirsttruthvalue</span></a>. </div>
<div class="doc">
or just point to the identifier and hit the
key combination mentioned in Part 2
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">myfirsttruthvalue</span></span> <span class="inlinecode"><span class="id" title="var">relies</span></span> <span class="inlinecode"><span class="id" title="var">on</span></span> <span class="inlinecode"><span class="id" title="var">an</span></span> <span class="inlinecode"><span class="id" title="var">unsafe</span></span> <span class="inlinecode"><span class="id" title="var">universe</span></span> <span class="inlinecode"><span class="id" title="var">hierarchy</span></span> is output to indicate
that we are using Coq with option <span class="inlinecode">-<span class="id" title="keyword">type</span>-<span class="id" title="tactic">in</span>-<span class="id" title="keyword">type</span></span>.
<div class="paragraph"> </div>
<a id="lab3"></a><h3 class="section">a more compelling example</h3>
</div>
<div class="code">
<span class="id" title="keyword">Definition</span> <a id="mysecondtruthvalue" class="idref" href="#mysecondtruthvalue"><span class="id" title="definition">mysecondtruthvalue</span></a>: <span class="id" title="inductive">bool</span>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="keyword">Search</span> <span class="id" title="inductive">bool</span>.<br/>
<span class="id" title="tactic">apply</span> <span class="id" title="definition">negb</span>.<br/>
</div>
<div class="doc">
applies the function <span class="inlinecode"><span class="id" title="var">negb</span></span> to obtain the required boolean,
thus the system has to ask for its argument
</div>
<div class="code">
<span class="id" title="tactic">exact</span> <a class="idref" href="lecture_tactics_long_version.html#myfirsttruthvalue"><span class="id" title="definition">myfirsttruthvalue</span></a>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#mysecondtruthvalue"><span class="id" title="definition">mysecondtruthvalue</span></a>.<br/>
</div>
<div class="doc">
<br/>
<span class="inlinecode"><span class="id" title="var">mysecondtruthvalue</span> = <span class="id" title="var">negb</span> <span class="id" title="var">myfirsttruthvalue</span><br/>
: <span class="id" title="var">bool</span>
<div class="paragraph"> </div>
</span>
<div class="paragraph"> </div>
the definition is "as is", evaluation can be done subsequently:
</div>
<div class="code">
<span class="id" title="keyword">Eval</span> <span class="id" title="tactic">compute</span> <span class="id" title="tactic">in</span> <a class="idref" href="lecture_tactics_long_version.html#mysecondtruthvalue"><span class="id" title="definition">mysecondtruthvalue</span></a>.<br/>
</div>
<div class="doc">
<br/>
<span class="inlinecode"> = <span class="id" title="var">false</span><br/>
: <span class="id" title="var">bool</span>
<div class="paragraph"> </div>
</span>
<div class="paragraph"> </div>
Again, not much has been gained by the interactive mode.
<div class="paragraph"> </div>
Here, we see a copy of the definition from the Coq library:
</div>
<div class="code">
<span class="id" title="keyword">Definition</span> <a id="andb" class="idref" href="#andb"><span class="id" title="definition">andb</span></a> (<a id="b1:1" class="idref" href="#b1:1"><span class="id" title="binder">b1</span></a> <a id="b2:2" class="idref" href="#b2:2"><span class="id" title="binder">b2</span></a>: <span class="id" title="inductive">bool</span>) : <span class="id" title="inductive">bool</span> := <span class="id" title="keyword">if</span> <a class="idref" href="lecture_tactics_long_version.html#b1:1"><span class="id" title="variable">b1</span></a> <span class="id" title="keyword">then</span> <a class="idref" href="lecture_tactics_long_version.html#b2:2"><span class="id" title="variable">b2</span></a> <span class="id" title="keyword">else</span> <span class="id" title="constructor">false</span>.<br/>
</div>
<div class="doc">
only for illustration purposes - it would be better to define
it according to UniMath style
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Definition</span> <a id="mythirdtruthvalue" class="idref" href="#mythirdtruthvalue"><span class="id" title="definition">mythirdtruthvalue</span></a>: <span class="id" title="inductive">bool</span>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="keyword">Search</span> <span class="id" title="inductive">bool</span>.<br/>
<span class="id" title="tactic">apply</span> <a class="idref" href="lecture_tactics_long_version.html#andb"><span class="id" title="definition">andb</span></a>.<br/>
</div>
<div class="doc">
<span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">andb</span>.</span> applies the function <span class="inlinecode"><span class="id" title="var">andb</span></span> to obtain the required boolean,
thus the system has to ask for its TWO arguments, one by one.
<div class="paragraph"> </div>
This follows the proof pattern of "backward chaining" that tries to
attack goals instead of building up evidence. In the course of action,
more goals can be generated. The proof effort is over when no more
goal remains.
<div class="paragraph"> </div>
UniMath coding style asks you to use proof structuring syntax,
while vanilla Coq would allow you to write formally verified
"spaghetti code".
<div class="paragraph"> </div>
We tell Coq that we start working on the first subgoal.
</div>
<div class="code">
-<br/>
</div>
<div class="doc">
only the "focused" subgoal is now on display
</div>
<div class="code">
<span class="id" title="tactic">apply</span> <a class="idref" href="lecture_tactics_long_version.html#andb"><span class="id" title="definition">andb</span></a>.<br/>
</div>
<div class="doc">
this again spawns two subgoals
<div class="paragraph"> </div>
we tell Coq that we start working on the first subgoal
</div>
<div class="code">
+<br/>
</div>
<div class="doc">
normally, one would not leave the "bullet symbol" isolated in a line
</div>
<div class="code">
<span class="id" title="tactic">exact</span> <a class="idref" href="lecture_tactics_long_version.html#mysecondtruthvalue"><span class="id" title="definition">mysecondtruthvalue</span></a>.<br/>
+ <span class="id" title="tactic">exact</span> <a class="idref" href="lecture_tactics_long_version.html#myfirsttruthvalue"><span class="id" title="definition">myfirsttruthvalue</span></a>.<br/>
</div>
<div class="doc">
The response buffer signals:
<br/>
<span class="inlinecode"><span class="id" title="var">There</span> <span class="id" title="var">are</span> <span class="id" title="var">unfocused</span> <span class="id" title="keyword">goals</span>.
<div class="paragraph"> </div>
</span> ProofGeneral would give more precise instructions as how to proceed.
But we know what we are doing...
</div>
<div class="code">
- <span class="id" title="tactic">exact</span> <span class="id" title="constructor">true</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
</div>
<div class="doc">
The usual "UniMath bullet order" is -, +, *, --, ++, **, ---, +++, ***,
and so on (all the ones shown are being used).
<div class="paragraph"> </div>
Coq does not impose any order, so one can start with, e.g., *****,
if need be for the sake of experimenting with a proof.
<div class="paragraph"> </div>
Reuse of bullets even on one branch is possible by enclosing subproofs
in curly braces {}.
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#mythirdtruthvalue"><span class="id" title="definition">mythirdtruthvalue</span></a>.<br/>
<span class="id" title="keyword">Eval</span> <span class="id" title="tactic">compute</span> <span class="id" title="tactic">in</span> <a class="idref" href="lecture_tactics_long_version.html#mythirdtruthvalue"><span class="id" title="definition">mythirdtruthvalue</span></a>.<br/>
<br/>
</div>
<div class="doc">
You only saw the tactics <span class="inlinecode"><span class="id" title="tactic">exact</span></span> and <span class="inlinecode"><span class="id" title="tactic">apply</span></span> at work, and there was no context.
<div class="paragraph"> </div>
<a id="lab4"></a><h2 class="section">doing Curry-Howard logic</h2>
<div class="paragraph"> </div>
Interactive mode is more wide-spread when it comes to carrying out proofs
(the command <span class="inlinecode"><span class="id" title="keyword">Proof</span>.</span> is reminiscent of that).
<div class="paragraph"> </div>
Disclaimer: this section has a logical flavour, but the "connectives"
are not confined to the world of propositional or predicate logic.
In particular, there is no reference to the sort Prop of Coq.
Prop is not used at all in UniMath!
<div class="paragraph"> </div>
On first reading, it is useful to focus on the logical meaning.
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Locate</span> "->". </div>
<div class="doc">
non-dependent product, can be seen as implication
</div>
<div class="code">
<span class="id" title="keyword">Locate</span> "∅".<br/>
<span class="id" title="keyword">Print</span> <span class="id" title="inductive">empty</span>. </div>
<div class="doc">
an inductive type that has no constructor
</div>
<div class="code">
<span class="id" title="keyword">Locate</span> "¬". </div>
<div class="doc">
we need to refer to the UniMath library more explicitly
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Require</span> <span class="id" title="keyword">Import</span> <span class="id" title="library">UniMath.Foundations.PartA</span>.<br/>
</div>
<div class="doc">
Do not write the import statements in the middle of a vernacular file.
Here, it is done to show the order of appearance, but this is only for
reasons of pedagogy.
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Locate</span> "¬".<br/>
<span class="id" title="keyword">Print</span> <span class="id" title="definition">neg</span>.<br/>
</div>
<div class="doc">
Negation is not a native concept; it is reduced to implication,
as is usual in constructive logic.
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Locate</span> "×".<br/>
<span class="id" title="keyword">Print</span> <span class="id" title="definition">dirprod</span>. </div>
<div class="doc">
non-dependent sum, can be seen as conjunction
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Definition</span> <a id="combinatorS" class="idref" href="#combinatorS"><span class="id" title="definition">combinatorS</span></a> (<a id="A:3" class="idref" href="#A:3"><span class="id" title="binder">A</span></a> <a id="B:4" class="idref" href="#B:4"><span class="id" title="binder">B</span></a> <a id="C:5" class="idref" href="#C:5"><span class="id" title="binder">C</span></a>: <span class="id" title="definition">UU</span>): <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:3"><span class="id" title="variable">A</span></a> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#B:4"><span class="id" title="variable">B</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:5"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:3"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:4"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#A:3"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:5"><span class="id" title="variable">C</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
</div>
<div class="doc">
how to infer an implication?
</div>
<div class="code">
<span class="id" title="tactic">intro</span> <span class="id" title="var">Hyp123</span>.<br/>
<span class="id" title="tactic">set</span> (<span class="id" title="var">Hyp1</span> := <span class="id" title="projection">pr1</span> <span class="id" title="var">Hyp123</span>).<br/>
</div>
<div class="doc">
This is already a bit of "forward chaining" which is a fact-building process.
</div>
<div class="code">
<span class="id" title="tactic">set</span> (<span class="id" title="var">Hyp23</span> := <span class="id" title="projection">pr2</span> <span class="id" title="var">Hyp123</span>).<br/>
<span class="id" title="var">cbn</span> <span class="id" title="tactic">in</span> <span class="id" title="var">Hyp23</span>.<br/>
</div>
<div class="doc">
<span class="inlinecode"><span class="id" title="var">cbn</span></span> simplifies a goal, and <span class="inlinecode"><span class="id" title="var">cbn</span></span> <span class="inlinecode"><span class="id" title="tactic">in</span></span> <span class="inlinecode"><span class="id" title="var">H</span></span> does this for hypothesis <span class="inlinecode"><span class="id" title="var">H</span></span>;
note that <span class="inlinecode"><span class="id" title="tactic">simpl</span></span> has the same high-level description but should better
be avoided in new developments.
</div>
<div class="code">
<span class="id" title="tactic">set</span> (<span class="id" title="var">Hyp2</span> := <span class="id" title="projection">pr1</span> <span class="id" title="var">Hyp23</span>).<br/>
<span class="id" title="tactic">set</span> (<span class="id" title="var">Hyp3</span> := <span class="id" title="projection">pr2</span> <span class="id" title="var">Hyp23</span>).<br/>
<span class="id" title="var">cbn</span> <span class="id" title="tactic">in</span> <span class="id" title="var">Hyp3</span>.<br/>
<span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp1</span>.<br/>
<span class="id" title="tactic">apply</span> <span class="id" title="constructor">tpair</span>. </div>
<div class="doc">
could be done with <span class="inlinecode"><span class="id" title="tactic">split</span>.</span> as well
</div>
<div class="code">
- <span class="id" title="tactic">assumption</span>. </div>
<div class="doc">
instruct Coq to look into the current context
<div class="paragraph"> </div>
this could be done with <span class="inlinecode"><span class="id" title="tactic">exact</span></span> <span class="inlinecode"><span class="id" title="var">Hyp3</span>.</span> as well
</div>
<div class="code">
- <span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp2</span>.<br/>
<span class="id" title="tactic">assumption</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS"><span class="id" title="definition">combinatorS</span></a>.<br/>
<span class="id" title="keyword">Eval</span> <span class="id" title="tactic">compute</span> <span class="id" title="tactic">in</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS"><span class="id" title="definition">combinatorS</span></a>.<br/>
<br/>
<span class="id" title="keyword">Local</span> <span class="id" title="keyword">Definition</span> <a id="combinatorS_intro_pattern" class="idref" href="#combinatorS_intro_pattern"><span class="id" title="definition">combinatorS_intro_pattern</span></a> (<a id="A:6" class="idref" href="#A:6"><span class="id" title="binder">A</span></a> <a id="B:7" class="idref" href="#B:7"><span class="id" title="binder">B</span></a> <a id="C:8" class="idref" href="#C:8"><span class="id" title="binder">C</span></a>: <span class="id" title="definition">UU</span>):<br/>
<span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:6"><span class="id" title="variable">A</span></a> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#B:7"><span class="id" title="variable">B</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:8"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:6"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:7"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#A:6"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:8"><span class="id" title="variable">C</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">intros</span> [<span class="id" title="var">Hyp1</span> [<span class="id" title="var">Hyp2</span> <span class="id" title="var">Hyp3</span>]]. </div>
<div class="doc">
deconstruct the hypothesis at the time of introduction;
notice that <span class="inlinecode">×</span> associates to the right;
<span class="inlinecode"><span class="id" title="tactic">intros</span></span> can also introduce multiple hypotheses, see below
</div>
<div class="code">
<span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp1</span>.<br/>
<span class="id" title="tactic">split</span>.<br/>
- <span class="id" title="tactic">assumption</span>.<br/>
- <span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp2</span>.<br/>
<span class="id" title="tactic">assumption</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_intro_pattern"><span class="id" title="definition">combinatorS_intro_pattern</span></a>.<br/>
<br/>
</div>
<div class="doc">
the two definitions are even convertible:
</div>
<div class="code">
<span class="id" title="keyword">Eval</span> <span class="id" title="tactic">compute</span> <span class="id" title="tactic">in</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_intro_pattern"><span class="id" title="definition">combinatorS_intro_pattern</span></a>.<br/>
<br/>
<span class="id" title="keyword">Local</span> <span class="id" title="keyword">Lemma</span> <a id="combinatorS_intro_pattern_is_the_same" class="idref" href="#combinatorS_intro_pattern_is_the_same"><span class="id" title="lemma">combinatorS_intro_pattern_is_the_same</span></a>:<br/>
<a class="idref" href="lecture_tactics_long_version.html#combinatorS"><span class="id" title="definition">combinatorS</span></a> <span class="id" title="notation">=</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_intro_pattern"><span class="id" title="definition">combinatorS_intro_pattern</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">apply</span> <span class="id" title="abbreviation">idpath</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
</div>
<div class="doc">
In late 2017, <span class="inlinecode"><span class="id" title="var">combinatorS_intro_pattern</span></span> would have contained <span class="inlinecode"><span class="id" title="keyword">match</span></span> constructs,
but now, the introduction patterns use less overhead when possible. The UniMath
style guide still does not want them to be used with square brackets.
<div class="paragraph"> </div>
another style to make life easier:
</div>
<div class="code">
<span class="id" title="keyword">Local</span> <span class="id" title="keyword">Definition</span> <a id="combinatorS_destruct" class="idref" href="#combinatorS_destruct"><span class="id" title="definition">combinatorS_destruct</span></a> (<a id="A:9" class="idref" href="#A:9"><span class="id" title="binder">A</span></a> <a id="B:10" class="idref" href="#B:10"><span class="id" title="binder">B</span></a> <a id="C:11" class="idref" href="#C:11"><span class="id" title="binder">C</span></a>: <span class="id" title="definition">UU</span>):<br/>
<span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:9"><span class="id" title="variable">A</span></a> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#B:10"><span class="id" title="variable">B</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:11"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:9"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:10"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#A:9"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:11"><span class="id" title="variable">C</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">intro</span> <span class="id" title="var">Hyp123</span>.<br/>
<span class="id" title="tactic">destruct</span> <span class="id" title="var">Hyp123</span> <span class="id" title="keyword">as</span> [<span class="id" title="var">Hyp1</span> <span class="id" title="var">Hyp23</span>]. </div>
<div class="doc">
deconstruct the hypothesis when needed
</div>
<div class="code">
<span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp1</span>.<br/>
<span class="id" title="tactic">destruct</span> <span class="id" title="var">Hyp23</span> <span class="id" title="keyword">as</span> [<span class="id" title="var">Hyp2</span> <span class="id" title="var">Hyp3</span>]. </div>
<div class="doc">
deconstruct the hypothesis when needed
</div>
<div class="code">
<span class="id" title="tactic">split</span>.<br/>
- <span class="id" title="tactic">assumption</span>.<br/>
- <span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp2</span>.<br/>
<span class="id" title="tactic">assumption</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_destruct"><span class="id" title="definition">combinatorS_destruct</span></a>.<br/>
<br/>
</div>
<div class="doc">
Again, the definition is definitionally equal to the first one:
</div>
<div class="code">
<span class="id" title="keyword">Eval</span> <span class="id" title="tactic">compute</span> <span class="id" title="tactic">in</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_destruct"><span class="id" title="definition">combinatorS_destruct</span></a>.<br/>
<br/>
<span class="id" title="keyword">Local</span> <span class="id" title="keyword">Lemma</span> <a id="combinatorS_destruct_is_the_same" class="idref" href="#combinatorS_destruct_is_the_same"><span class="id" title="lemma">combinatorS_destruct_is_the_same</span></a>: <a class="idref" href="lecture_tactics_long_version.html#combinatorS"><span class="id" title="definition">combinatorS</span></a> <span class="id" title="notation">=</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_destruct"><span class="id" title="definition">combinatorS_destruct</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">apply</span> <span class="id" title="abbreviation">idpath</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
</div>
<div class="doc">
In late 2017, <span class="inlinecode"><span class="id" title="var">combinatorS_destruct</span></span> would also have contained <span class="inlinecode"><span class="id" title="keyword">match</span></span> constructs,
which is why <span class="inlinecode"><span class="id" title="tactic">destruct</span></span> is forbidden in the UniMath style guide. Now, this is fine
in our example.
<div class="paragraph"> </div>
The (hitherto) preferred idiom:
</div>
<div class="code">
<span class="id" title="keyword">Definition</span> <a id="combinatorS_induction" class="idref" href="#combinatorS_induction"><span class="id" title="definition">combinatorS_induction</span></a> (<a id="A:12" class="idref" href="#A:12"><span class="id" title="binder">A</span></a> <a id="B:13" class="idref" href="#B:13"><span class="id" title="binder">B</span></a> <a id="C:14" class="idref" href="#C:14"><span class="id" title="binder">C</span></a>: <span class="id" title="definition">UU</span>): <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:12"><span class="id" title="variable">A</span></a> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#B:13"><span class="id" title="variable">B</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:14"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:12"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:13"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#A:12"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:14"><span class="id" title="variable">C</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">intro</span> <span class="id" title="var">Hyp123</span>.<br/>
<span class="id" title="tactic">induction</span> <span class="id" title="var">Hyp123</span> <span class="id" title="keyword">as</span> [<span class="id" title="var">Hyp1</span> <span class="id" title="var">Hyp23</span>].<br/>
<span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp1</span>.<br/>
<span class="id" title="tactic">induction</span> <span class="id" title="var">Hyp23</span> <span class="id" title="keyword">as</span> [<span class="id" title="var">Hyp2</span> <span class="id" title="var">Hyp3</span>].<br/>
<span class="id" title="tactic">split</span>.<br/>
- <span class="id" title="tactic">assumption</span>.<br/>
- <span class="id" title="tactic">apply</span> <span class="id" title="var">Hyp2</span>.<br/>
<span class="id" title="tactic">assumption</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_induction"><span class="id" title="definition">combinatorS_induction</span></a>.<br/>
<span class="id" title="keyword">Eval</span> <span class="id" title="tactic">compute</span> <span class="id" title="tactic">in</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_induction"><span class="id" title="definition">combinatorS_induction</span></a>.<br/>
</div>
<div class="doc">
the comfort for the user does not change the normal form of the constructed proof
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Definition</span> <a id="combinatorS_curried" class="idref" href="#combinatorS_curried"><span class="id" title="definition">combinatorS_curried</span></a> (<a id="A:15" class="idref" href="#A:15"><span class="id" title="binder">A</span></a> <a id="B:16" class="idref" href="#B:16"><span class="id" title="binder">B</span></a> <a id="C:17" class="idref" href="#C:17"><span class="id" title="binder">C</span></a>: <span class="id" title="definition">UU</span>): <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:15"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:16"><span class="id" title="variable">B</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:17"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">→</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:15"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:16"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#A:15"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:17"><span class="id" title="variable">C</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
</div>
<div class="doc">
use <span class="inlinecode"><span class="id" title="tactic">intro</span></span> three times or rather <span class="inlinecode"><span class="id" title="tactic">intros</span></span> once; reasonable coding style
gives names to all hypotheses that are not already present
in the goal formula, see also the next definition
</div>
<div class="code">
<span class="id" title="tactic">intros</span> <span class="id" title="var">H1</span> <span class="id" title="var">H2</span> <span class="id" title="var">H3</span>.<br/>
<span class="id" title="tactic">apply</span> <span class="id" title="var">H1</span>.<br/>
- <span class="id" title="tactic">assumption</span>.<br/>
- <span class="id" title="tactic">set</span> (<span class="id" title="var">proofofB</span> := <span class="id" title="var">H2</span> <span class="id" title="var">H3</span>).<br/>
</div>
<div class="doc">
set up abbreviations that can make use of the current context;
will be considered as an extra element of the context:
</div>
<div class="code">
<span class="id" title="tactic">assumption</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
<span class="id" title="keyword">Print</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_curried"><span class="id" title="definition">combinatorS_curried</span></a>.<br/>
</div>
<div class="doc">
We see that <span class="inlinecode"><span class="id" title="tactic">set</span></span> gives rise to <span class="inlinecode"><span class="id" title="keyword">let</span></span>-expressions that are known
from functional programming languages, in other words: the use of
<span class="inlinecode"><span class="id" title="tactic">set</span></span> is not a "macro" facility to ease typing.
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="keyword">let</span></span>-bindings disappear when computing the normal form of a term:
</div>
<div class="code">
<span class="id" title="keyword">Eval</span> <span class="id" title="tactic">compute</span> <span class="id" title="tactic">in</span> <a class="idref" href="lecture_tactics_long_version.html#combinatorS_curried"><span class="id" title="definition">combinatorS_curried</span></a>.<br/>
<br/>
</div>
<div class="doc">
<span class="inlinecode"><span class="id" title="tactic">set</span></span> can only be used if the term of the desired type is provided,
but we can also work interactively as follows:
</div>
<div class="code">
<span class="id" title="keyword">Definition</span> <a id="combinatorS_curried_with_assert" class="idref" href="#combinatorS_curried_with_assert"><span class="id" title="definition">combinatorS_curried_with_assert</span></a> (<a id="A:18" class="idref" href="#A:18"><span class="id" title="binder">A</span></a> <a id="B:19" class="idref" href="#B:19"><span class="id" title="binder">B</span></a> <a id="C:20" class="idref" href="#C:20"><span class="id" title="binder">C</span></a>: <span class="id" title="definition">UU</span>):<br/>
<span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:18"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:19"><span class="id" title="variable">B</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:20"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">→</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:18"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:19"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#A:18"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:20"><span class="id" title="variable">C</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">intros</span> <span class="id" title="var">H1</span> <span class="id" title="var">H2</span> <span class="id" title="var">H3</span>.<br/>
</div>
<div class="doc">
we can momentarily forget about our goal and build up knowledge:
</div>
<div class="code">
<span class="id" title="tactic">assert</span> (<span class="id" title="var">proofofB</span> : <span class="id" title="var">B</span>).<br/>
</div>
<div class="doc">
the current goal <span class="inlinecode"><span class="id" title="var">C</span></span> becomes the second sub-goal, and the new current goal is <span class="inlinecode"><span class="id" title="var">B</span></span>
<div class="paragraph"> </div>
It is not wise to handle this situation by "bullets" since many assertions
can appear in a linearly thought argument. It would pretend a tree structure
although it would rather be a comb. The proof of the assertion should
be packaged by enclosing it in curly braces like so:
</div>
<div class="code">
{ <span class="id" title="tactic">apply</span> <span class="id" title="var">H2</span>.<br/>
<span class="id" title="tactic">assumption</span>.<br/>
}<br/>
</div>
<div class="doc">
Now, <span class="inlinecode"><span class="id" title="var">proofofB</span></span> is in the context with type <span class="inlinecode"><span class="id" title="var">B</span></span>.
</div>
<div class="code">
<span class="id" title="tactic">apply</span> <span class="id" title="var">H1</span>.<br/>
- <span class="id" title="tactic">assumption</span>.<br/>
- <span class="id" title="tactic">assumption</span>.<br/>
<span class="id" title="keyword">Defined</span>.<br/>
<br/>
</div>
<div class="doc">
the wildcard <span class="inlinecode">?</span> for <span class="inlinecode"><span class="id" title="tactic">intros</span></span>
</div>
<div class="code">
<span class="id" title="keyword">Definition</span> <a id="combinatorS_curried_variant" class="idref" href="#combinatorS_curried_variant"><span class="id" title="definition">combinatorS_curried_variant</span></a> (<a id="A:21" class="idref" href="#A:21"><span class="id" title="binder">A</span></a> <a id="B:22" class="idref" href="#B:22"><span class="id" title="binder">B</span></a> <a id="C:23" class="idref" href="#C:23"><span class="id" title="binder">C</span></a>: <span class="id" title="definition">UU</span>):<br/>
<span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:21"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:22"><span class="id" title="variable">B</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#C:23"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">→</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:21"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#B:22"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">→</span> <span class="id" title="keyword">∀</span> <a id="H7:24" class="idref" href="#H7:24"><span class="id" title="binder">H7</span></a>: <a class="idref" href="lecture_tactics_long_version.html#A:21"><span class="id" title="variable">A</span></a>, <a class="idref" href="lecture_tactics_long_version.html#C:23"><span class="id" title="variable">C</span></a>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">intros</span> <span class="id" title="var">H1</span> <span class="id" title="var">H2</span> ?.<br/>
</div>
<div class="doc">
a question mark instructs Coq to use the corresponding identifier
from the goal formula
</div>
<div class="code">
<span class="id" title="tactic">exact</span> (<span class="id" title="var">H1</span> <span class="id" title="var">H7</span> (<span class="id" title="var">H2</span> <span class="id" title="var">H7</span>)).<br/>
<span class="id" title="keyword">Defined</span>.<br/>
</div>
<div class="doc">
the wildcard <span class="inlinecode"><span class="id" title="var">_</span></span> for <span class="inlinecode"><span class="id" title="tactic">intros</span></span> forgets the respective hypothesis
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Locate</span> "⨿". </div>
<div class="doc">
this symbol is typed as \amalg when the recommended extension
packages for VSCode are loaded
</div>
<div class="code">
<span class="id" title="keyword">Print</span> <span class="id" title="inductive">coprod</span>. </div>
<div class="doc">
defined in UniMath preamble as inductive type,
can be seen as disjunction
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Locate</span> "∏".<br/>
<br/>
<span class="id" title="keyword">Locate</span> "=". </div>
<div class="doc">
the identity type of UniMath
</div>
<div class="code">
<span class="id" title="keyword">Print</span> <span class="id" title="inductive">paths</span>.<br/>
<br/>
</div>
<div class="doc">
A word of warning for those who read "Coq in a Hurry": <span class="inlinecode"><span class="id" title="keyword">SearchRewrite</span></span>
does not find equations w.r.t. this notion, only w.r.t. Coq's built-in
propositional equality.
</div>
<div class="code">
<span class="id" title="keyword">Search</span><span class="id" title="var">Pattern</span> (<span class="id" title="inductive">paths</span> <span class="id" title="var">_</span> <span class="id" title="var">_</span>).<br/>
</div>
<div class="doc">
Among the search results is <span class="inlinecode"><span class="id" title="var">pathsinv0r</span></span> that has <span class="inlinecode"><span class="id" title="var">idpath</span></span> in its conclusion.
</div>
<div class="code">
<span class="id" title="keyword">SearchRewrite</span> <span class="id" title="abbreviation">idpath</span>.<br/>
</div>
<div class="doc">
No result!
<div class="paragraph"> </div>
<a id="lab5"></a><h3 class="section">How to decompose formulas</h3>
<div class="paragraph"> </div>
In "Coq in a Hurry", Yves Bertot gives recipes for decomposing the usual logical
connectives. Crucially, one has to distinguish between decomposition of the goal
or decomposition of a hypothesis in the context.
<div class="paragraph"> </div>
Here, we do it alike.
<div class="paragraph"> </div>
<a id="lab6"></a><h4 class="section">Decomposition of goal formulas:</h4>
<div class="paragraph"> </div>
A1,...,An -> B: tactic <span class="inlinecode"><span class="id" title="tactic">intro</span></span> or <span class="inlinecode"><span class="id" title="tactic">intros</span></span>
<div class="paragraph"> </div>
<span class="inlinecode">¬</span> <span class="inlinecode"><span class="id" title="var">A</span></span>: idem (negation is defined through implication)
<div class="paragraph"> </div>
Π-type: idem (implication is a special case of product)
<div class="paragraph"> </div>
<span class="inlinecode">×</span>: <span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">dirprodpair</span></span>, less specifically <span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">tpair</span></span> or <span class="inlinecode"><span class="id" title="tactic">split</span></span>
<div class="paragraph"> </div>
Σ-type: <span class="inlinecode"><span class="id" title="var">use</span></span> <span class="inlinecode"><span class="id" title="var">tpair</span></span> or <span class="inlinecode"><span class="id" title="tactic">∃</span></span> or <span class="inlinecode"><span class="id" title="tactic">split</span></span> <span class="inlinecode"><span class="id" title="keyword">with</span></span>, see explanations below
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">A</span></span> <span class="inlinecode">⨿</span> <span class="inlinecode"><span class="id" title="var">B</span></span>: <span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">ii1</span></span> or <span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">ii2</span></span>, but this constitutes a choice
of which way to go
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">A</span></span> <span class="inlinecode">=</span> <span class="inlinecode"><span class="id" title="var">B</span></span>: <span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">idpath</span></span>, however this only works when the expressions
are convertible
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">nat</span></span>: <span class="inlinecode"><span class="id" title="tactic">exact</span></span> <span class="inlinecode">1000</span>, for example (a logical reading is not
useful for this type); beware that UniMath knows only 27 numerals,
<span class="inlinecode"><span class="id" title="keyword">Goal</span></span> <span class="inlinecode"><span class="id" title="var">nat</span>.</span> <span class="inlinecode"><span class="id" title="var">Fail</span></span> <span class="inlinecode"><span class="id" title="tactic">exact</span></span> <span class="inlinecode">2022.</span> leads to
<br/>
<span class="inlinecode"><span class="id" title="var">The</span> <span class="id" title="var">command</span> <span class="id" title="var">has</span> <span class="id" title="var">indeed</span> <span class="id" title="var">failed</span> <span class="id" title="keyword">with</span> <span class="id" title="var">message</span>: <span class="id" title="var">No</span> <span class="id" title="var">interpretation</span> <span class="id" title="keyword">for</span> <span class="id" title="var">number</span> "2022".
<div class="paragraph"> </div>
</span>
<div class="paragraph"> </div>
<a id="lab7"></a><h4 class="section">Decomposition of formula of hypothesis <span class="inlinecode"><span class="id" title="var">H</span></span>:</h4>
<div class="paragraph"> </div>
<span class="inlinecode">∅</span>: <span class="inlinecode"><span class="id" title="tactic">induction</span></span> <span class="inlinecode"><span class="id" title="var">H</span></span>
<div class="paragraph"> </div>
This terminates a goal. (It corresponds to ex falso quodlibet.)
<div class="paragraph"> </div>
There is naturally no recipe for getting rid of <span class="inlinecode">∅</span> in the conclusion.
But <span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">fromempty</span></span> allows to replace any goal by <span class="inlinecode">∅</span>.
<div class="paragraph"> </div>
A1,...,An -> B: <span class="inlinecode"><span class="id" title="tactic">apply</span></span> <span class="inlinecode"><span class="id" title="var">H</span></span>, but the formula has to fit with the goal
<div class="paragraph"> </div>
<span class="inlinecode">×</span>: <span class="inlinecode"><span class="id" title="tactic">induction</span></span> <span class="inlinecode"><span class="id" title="var">H</span></span> <span class="inlinecode"><span class="id" title="keyword">as</span></span> <span class="inlinecode">[<span class="id" title="var">H1</span></span> <span class="inlinecode"><span class="id" title="var">H2</span>]</span>
<div class="paragraph"> </div>
As seen above, this introduces names of hypotheses for the two components.
<div class="paragraph"> </div>
Σ-type: idem, but rather more asymmetric as <span class="inlinecode"><span class="id" title="tactic">induction</span></span> <span class="inlinecode"><span class="id" title="var">H</span></span> <span class="inlinecode"><span class="id" title="keyword">as</span></span> <span class="inlinecode">[<span class="id" title="var">x</span></span> <span class="inlinecode"><span class="id" title="var">H'</span>]</span>
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">A</span></span> <span class="inlinecode">⨿</span> <span class="inlinecode"><span class="id" title="var">B</span></span>: <span class="inlinecode"><span class="id" title="tactic">induction</span></span> <span class="inlinecode"><span class="id" title="var">H</span></span> <span class="inlinecode"><span class="id" title="keyword">as</span></span> <span class="inlinecode">[<span class="id" title="var">H1</span></span> <span class="inlinecode">|</span> <span class="inlinecode"><span class="id" title="var">H2</span>]</span>
<div class="paragraph"> </div>
This introduces names for the hypotheses in the two branches.
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">A</span></span> <span class="inlinecode">=</span> <span class="inlinecode"><span class="id" title="var">B</span></span>: <span class="inlinecode"><span class="id" title="tactic">induction</span></span> <span class="inlinecode"><span class="id" title="var">H</span></span>
<div class="paragraph"> </div>
The supposedly equal <span class="inlinecode"><span class="id" title="var">A</span></span> and <span class="inlinecode"><span class="id" title="var">B</span></span> become the same <span class="inlinecode"><span class="id" title="var">A</span></span> in the goal.
<div class="paragraph"> </div>
This is the least intuitive rule for the non-expert in type theory.
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">nat</span></span>: <span class="inlinecode"><span class="id" title="tactic">induction</span></span> <span class="inlinecode"><span class="id" title="var">n</span></span> <span class="inlinecode"><span class="id" title="keyword">as</span></span> <span class="inlinecode">[</span> <span class="inlinecode">|</span> <span class="inlinecode"><span class="id" title="var">n</span></span> <span class="inlinecode"><span class="id" title="var">IH</span>]</span>
<div class="paragraph"> </div>
Here, we assume that the hypothesis has the name <span class="inlinecode"><span class="id" title="var">n</span></span> which
is more idiomatic than <span class="inlinecode"><span class="id" title="var">H</span></span>, and there is no extra name in
the base case, while in the step case, the preceding number
is now given the name <span class="inlinecode"><span class="id" title="var">n</span></span> and the induction hypothesis is
named <span class="inlinecode"><span class="id" title="var">IH</span></span>.
<div class="paragraph"> </div>
<a id="lab8"></a><h2 class="section">Handling unfinished proofs</h2>
<div class="paragraph"> </div>
In the middle of a proof effort - not in the UniMath library - you can use
<span class="inlinecode"><span class="id" title="var">admit</span></span> to abandon the current goal.
</div>
<div class="code">
<span class="id" title="keyword">Local</span> <span class="id" title="keyword">Lemma</span> <a id="badex1" class="idref" href="#badex1"><span class="id" title="lemma">badex1</span></a> (<a id="A:25" class="idref" href="#A:25"><span class="id" title="binder">A</span></a>: <span class="id" title="definition">UU</span>): <span class="id" title="notation">∅</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:25"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#A:25"><span class="id" title="variable">A</span></a><span class="id" title="notation">)</span>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">split</span>.<br/>
- </div>
<div class="doc">
seems difficult in the current context
</div>
<div class="code">
<span class="id" title="var">admit</span>.<br/>
<br/>
</div>
<div class="doc">
we continue with decent proof work:
</div>
<div class="code">
- <span class="id" title="tactic">intro</span> <span class="id" title="var">H</span>.<br/>
<span class="id" title="tactic">assumption</span>.<br/>
<span class="id" title="var">Admitted</span>.<br/>
<br/>
</div>
<div class="doc">
This is strictly forbidden to commit to UniMath! <span class="inlinecode"><span class="id" title="var">admit</span></span> allows to pursue the other goals,
while <span class="inlinecode"><span class="id" title="var">Admitted</span>.</span> makes the lemma available for further proofs. A warning is shown that
<span class="inlinecode"><span class="id" title="var">badex1</span></span> has been assumed as axiom.
<div class="paragraph"> </div>
An alternative to interrupt work on a proof:
</div>
<div class="code">
<span class="id" title="keyword">Lemma</span> <a id="badex2" class="idref" href="#badex2"><span class="id" title="lemma">badex2</span></a> (<a id="A:26" class="idref" href="#A:26"><span class="id" title="binder">A</span></a>: <span class="id" title="definition">UU</span>): <span class="id" title="notation">∅</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:26"><span class="id" title="variable">A</span></a> <span class="id" title="notation">→</span> <a class="idref" href="lecture_tactics_long_version.html#A:26"><span class="id" title="variable">A</span></a><span class="id" title="notation">)</span>.<br/>
<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="tactic">split</span>.<br/>
-<br/>
<span class="id" title="keyword">Abort</span>.<br/>
</div>
<div class="doc">
<span class="inlinecode"><span class="id" title="var">badex2</span></span> is not in the symbol table.
<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="keyword">Abort</span>.</span> is a way of documenting a problem with proving a result.
At least, Coq can check the partial proof up to the <span class="inlinecode"><span class="id" title="keyword">Abort</span>.</span> command.
<div class="paragraph"> </div>
<a id="lab9"></a><h2 class="section">Working with holes in proofs</h2>
<div class="paragraph"> </div>
Our previous proofs were particularly clear because the goal formulas
and all hypotheses were fully given by the system.
</div>
<div class="code">
<br/>
<span class="id" title="keyword">Print</span> <span class="id" title="definition">pathscomp0</span>.<br/>
</div>
<div class="doc">
This is the UniMath proof of transitivity of equality.
<div class="paragraph"> </div>
The salient feature of transitivity is that the intermediate
expression cannot be deduced from the equation to be proven.
</div>
<div class="code">
<span class="id" title="keyword">Lemma</span> <a id="badex3" class="idref" href="#badex3"><span class="id" title="lemma">badex3</span></a> (<a id="A:27" class="idref" href="#A:27"><span class="id" title="binder">A</span></a> <a id="B:28" class="idref" href="#B:28"><span class="id" title="binder">B</span></a> <a id="C:29" class="idref" href="#C:29"><span class="id" title="binder">C</span></a> <a id="D:30" class="idref" href="#D:30"><span class="id" title="binder">D</span></a>: <span class="id" title="definition">UU</span>) : <span class="id" title="notation">(</span><span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:27"><span class="id" title="variable">A</span></a> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#B:28"><span class="id" title="variable">B</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#C:29"><span class="id" title="variable">C</span></a> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#D:30"><span class="id" title="variable">D</span></a><span class="id" title="notation">)</span><span class="id" title="notation">)</span> <span class="id" title="notation">=</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#A:27"><span class="id" title="variable">A</span></a> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><a class="idref" href="lecture_tactics_long_version.html#B:28"><span class="id" title="variable">B</span></a> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#C:29"><span class="id" title="variable">C</span></a><span class="id" title="notation">)</span> <span class="id" title="notation">×</span> <a class="idref" href="lecture_tactics_long_version.html#D:30"><span class="id" title="variable">D</span></a><span class="id" title="notation">)</span>.<br/>
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Notice that the outermost parentheses are needed here.
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<span class="id" title="keyword">Proof</span>.<br/>
<span class="id" title="var">Fail</span> <span class="id" title="tactic">apply</span> <span class="id" title="definition">pathscomp0</span>.<br/>
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<span class="inlinecode"><span class="id" title="var">The</span> <span class="id" title="var">command</span> <span class="id" title="var">has</span> <span class="id" title="var">indeed</span> <span class="id" title="var">failed</span> <span class="id" title="keyword">with</span> <span class="id" title="var">message</span>:<br/>
<span class="id" title="var">Cannot</span> <span class="id" title="var">infer</span> <span class="id" title="var">the</span> <span class="id" title="var">implicit</span> <span class="id" title="var">parameter</span> <span class="id" title="var">b</span> <span class="id" title="keyword">of</span> <span class="id" title="var">pathscomp0</span> <span class="id" title="var">whose</span> <span class="id" title="keyword">type</span> <span class="id" title="keyword">is</span><br/>
"Type" <span class="id" title="tactic">in</span> <span class="id" title="var">environment</span>:<br/>
<span class="id" title="var">A</span>, <span class="id" title="var">B</span>, <span class="id" title="var">C</span>, <span class="id" title="var">D</span> : <span class="id" title="var">UU</span>
<div class="paragraph"> </div>
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<div class="paragraph"> </div>
<span class="inlinecode"><span class="id" title="var">Fail</span></span> announces failure and therefore allows to continue with
the interpretation of the vernacular file.
<div class="paragraph"> </div>
We need to help Coq with the argument <span class="inlinecode"><span class="id" title="var">b</span></span> to <span class="inlinecode"><span class="id" title="var">pathscomp0</span></span>.
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<span class="id" title="tactic">apply</span> (<span class="id" title="definition">pathscomp0</span> (<span class="id" title="var">b</span> := <span class="id" title="var">A</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><span class="id" title="var">B</span> <span class="id" title="notation">×</span> <span class="id" title="notation">(</span><span class="id" title="var">C</span> <span class="id" title="notation">×</span> <span class="id" title="var">D</span><span class="id" title="notation">))</span>)).<br/>
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is this not just associativity with third argument <span class="inlinecode"><span class="id" title="var">C</span></span> <span class="inlinecode">×</span> <span class="inlinecode"><span class="id" title="var">D</span></span>?
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<span class="id" title="keyword">SearchPattern</span> (<span class="id" title="var">_</span> <span class="id" title="notation">×</span> <span class="id" title="var">_</span> <span class="id" title="notation">=</span> <span class="id" title="var">_</span> <span class="id" title="notation">×</span> <span class="id" title="var">_</span>).<br/>
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