Restate statements before declaration

Question

TLDR

How to throw all dirty but necessary "lemmas/proofs" in the appendix, and restate them in the main text?

---

I had a few attempts, and it seemed that my requests are weird.. so I would explain a bit before I ask the question.

I'm writing a mathematical paper. I view writing math arguments as writing codes: statements are "functions" and proofs are the flesh. For completeness, I want to include as much proof as I want. However, since the structure of argument is not linear, doing so must by large decreases the readability. Therefore, I want to hide the dirty things in the appendix, and quote the def/thm/proof in my main text.

Ideally, a pseudo tex code should look as follows.

Section 1
#call{main-theorem}
  % nothing input here.
#endcall
.
.
Section 10
.
.
Appendix
#theorem[callable]{main-theorem}
2-1=1.
#end-theorem

To achieve this, @Bernard has pointed to thmtools for me in [1]. To achieve what I really want, I got an error, but since it's not the main topic there, I make a new question here.

Minimal example

Below is a (almost) working minimal example.

\documentclass{article}
\usepackage{thmtools, thm-restate}
\declaretheorem{theorem}
\begin{document}
%\firsteuclid*
%%% Uncommenting the above causes an error:
%%% > ! Undefined control sequence.
%%% > l.7 \firsteuclid
\begin{restatable}[Euclid]{theorem}{firsteuclid}
\label{thm:euclid}
$$1+1 = 2.$$
\end{restatable}
\firsteuclid* % This, however, works fine.
\end{document}

It compiles correctly. However, I hope that I can first call \firsteuclid* before it has been stated. An attempt fails. You can replicate that by uncommenting the commented line.

One work-around, of course, is to accept its limit, and declare statements anyway as in [2]. But I also hope that the dirty codes can all be grouped together in the source file. It will make my life much easier in the future, when I want to use them in my next paper.. again, the analogy holds: you write good codes, and use them again and again.

Question

How to throw all dirty but necessary "codes" in the appendix, and restate them in the main text?

Reference

• [1] Quoting from the same document?

• [2] using a restatable before it is stated

• [3] Also related.. my another attempt Delay presenting a section

Answer 1

Here's a proof of concept.

\documentclass{article}
\usepackage{amsthm}
\ExplSyntaxOn
\NewDocumentCommand{\theoremstatement}{m m}
 {% #1 = label, #2 = theorem type
  \use:e
   {
    \exp_not:N \begin{#2}
    \prop_item:Nn \g_student_theorems_statements_prop { #1 }
    \exp_not:N \end{#2}
   }
  \prop_gput:Nnx \g_student_theorems_numbers_prop { #1 } { \thethm }
 }
\NewDocumentEnvironment{delayedtheorem}{m m +b}
 % #1 = label, #2 = theorem type, #3 = theorem statement
 {
  \cs_set:Npx \thethm { \prop_item:Nn \g_student_theorems_numbers_prop { #1 } }
  \begin{#2}#3\end{#2}
  \iow_now:Nn \g_student_theorems_statements_iow
   {
    \studentpropgput { g_student_theorems_statements_prop } { #1 } { #3 }
   }
 }{}
\cs_new_eq:NN \studentpropgput \prop_gput:cnn
\iow_new:N \g_student_theorems_statements_iow
\prop_new:N \g_student_theorems_statements_prop
\prop_new:N \g_student_theorems_numbers_prop
\AtBeginDocument
 {
  \file_if_exist_input:n { \c_sys_jobname_str.thm }
  \iow_open:Nn \g_student_theorems_statements_iow { \c_sys_jobname_str.thm }
 }
\ExplSyntaxOff
\newtheorem{thm}{Theorem}[section]
\newtheorem{lem}[thm]{Lemma}
\begin{document}
\section{First}
\begin{thm}
This is a normal theorem.
\end{thm}
\begin{proof}
Which has a proof.
\end{proof}
\theoremstatement{A}{thm}
\section{Second}
\theoremstatement{B}{lem}
\appendix
\section{Statements and proofs}
\begin{delayedtheorem}{A}{thm}
This is a theorem whose proof is in the appendix.
\end{delayedtheorem}
\begin{proof}
And its proof.
\end{proof}
\begin{delayedtheorem}{B}{lem}[With name]
A boring lemma.
\end{delayedtheorem}
\begin{proof}
Even more boring proof.
\end{proof}
\end{document}

thmtools

The above code doesn't work properly with thmtools, but it can be adjusted.

\documentclass{article}
\usepackage{amsthm}
\usepackage{thmtools}
\ExplSyntaxOn
\NewDocumentCommand{\theoremstatement}{m m}
 {% #1 = label, #2 = theorem type
  \use:e
   {
    \exp_not:N \begin{#2}
    \prop_item:Nn \g_student_theorems_statements_prop { #1 }
    \exp_not:N \end{#2}
   }
  \prop_gput:Nnx \g_student_theorems_numbers_prop { #1 } { \use:c {the#2} }
 }
\NewDocumentEnvironment{delayedtheorem}{m m +b}
 % #1 = label, #2 = theorem type, #3 = theorem statement
 {
  \cs_set:cpx {the#2} { \prop_item:Nn \g_student_theorems_numbers_prop { #1 } }
  \begin{#2}#3\end{#2}
  \iow_now:Nn \g_student_theorems_statements_iow
   {
    \studentpropgput { g_student_theorems_statements_prop } { #1 } { #3 }
   }
 }{}
\cs_new_eq:NN \studentpropgput \prop_gput:cnn
\iow_new:N \g_student_theorems_statements_iow
\prop_new:N \g_student_theorems_statements_prop
\prop_new:N \g_student_theorems_numbers_prop
\AtBeginDocument
 {
  \file_if_exist_input:n { \c_sys_jobname_str.thm }
  \iow_open:Nn \g_student_theorems_statements_iow { \c_sys_jobname_str.thm }
 }
\ExplSyntaxOff
\declaretheorem[within=section]{theorem}
\declaretheorem[sibling=theorem]{lemma}
\begin{document}
\section{First}
\begin{theorem}
This is a normal theorem.
\end{theorem}
\begin{proof}
Which has a proof.
\end{proof}
\theoremstatement{A}{theorem}
\section{Second}
\theoremstatement{B}{lemma}
\appendix
\section{Statements and proofs}
\begin{delayedtheorem}{A}{theorem}
This is a theorem whose proof is in the appendix.
\end{delayedtheorem}
\begin{proof}
And its proof.
\end{proof}
\begin{delayedtheorem}{B}{lemma}[With name]
A boring lemma.
\end{delayedtheorem}
\begin{proof}
Even more boring proof.
\end{proof}
\end{document}