[Tex/LaTex] Theorem environment with numbers on the left

theorems

I am trying to construct a theorem environment which roughly looks like this: 

1.1   Theorem   Statement of the theorem. Here is some extra text so you can see how the
      theorem body text should be indented (aligned with theorem head).

The theorem head should be bold, with no punctuation after. The spacing between thmnumber and thmname should be adjustable. Body text normal.

I want to be able to use the environment for definitions as well (which won't be numbered), so it should be possible to remove the number without affecting the alignment of the theorem head. Example:

      Definition   Statement of the definition. It should be aligned the same way as a 
      theorem. 

Here is some body text in the document. Note that the theorem numbers are not in the
margin. 

1.1   Theorem   Statement of the theorem. Here is some extra text so you can see how the
      theorem body text should be indented (aligned with theorem head).

Best Answer

Here's a way to do it; this will also comply with lists such as enumerate in the statement, which solutions with \hangindent won't.

\documentclass{article}
\usepackage{showframe} % just for the example
\usepackage{amsthm}
\usepackage{enumitem}
\usepackage{xparse}

\usepackage{lipsum}

\newtheoremstyle{fctaylor}% name
  {\topsep}%      Space above
  {\topsep}%      Space below
  {\normalfont}%         Body font
  {}%         Indent amount (empty = no indent, \parindent = para indent)
  {\bfseries}% Thm head font
  {}%        Punctuation after thm head
  {0pt}%     Space after thm head: " " = normal interword space;
  {\makethmhead{#1}{#2}{#3}}

\newlength\fctaylortheoremindent
\AtBeginDocument{\setlength\fctaylortheoremindent{3em}} % <- customize here
\newlength\fctaylorlabelsep
\AtBeginDocument{\setlength\fctaylorlabelsep{1em}} % <- customize here

\makeatletter
\newcommand{\makethmhead}[3]{%
  \gdef\thisthmhead{%
    \makebox[\fctaylortheoremindent][l]{\bfseries#2}%
    {\bfseries#1}%
    \@ifnotempty{#3}{ (#3)}%
    \hspace{\fctaylorlabelsep}%
  }%
}
\makeatother

\newenvironment{fctayloritemize}
 {\list{}{%
    \leftmargin=\fctaylortheoremindent
    \labelwidth=\dimexpr\fctaylortheoremindent-\labelsep\relax
    \itemindent=0pt
  }}
 {\endlist}

\NewDocumentCommand{\newfctaylortheorem}{smomo}{%
  \IfBooleanTF{#1}
   {\newtheorem*{fctaylor@#2}{#4}}
   {\IfNoValueTF{#3}
     {\IfNoValueTF{#5}
       {\newtheorem{fctaylor@#2}{#4}}
       {\newtheorem{fctaylor@#2}{#4}[#5]}}
     {\newtheorem{fctaylor@#2}[fctaylor@#3]{#4}}}%
  \NewDocumentEnvironment{#2}{o}
   {\IfNoValueTF{##1}{\begin{fctaylor@#2}}{\begin{fctaylor@#2}[##1]}%
    \begin{fctayloritemize}\item[\thisthmhead\hfill]}
   {\end{fctayloritemize}\end{fctaylor@#2}}%
}

\theoremstyle{fctaylor}
\newfctaylortheorem{thm}{Theorem}[section]
\newfctaylortheorem*{defn}{Definition}

\begin{document}
\section{One}

\begin{defn}
\lipsum*[2]
\end{defn}

\begin{thm}\label{A}
\lipsum*[2]
\end{thm}

\begin{thm}[Somebody]\label{B}
Something that should show how the text is split across line boundaries
and is correctly indented. And some equivalent conditions:
\begin{enumerate}[label=\upshape(\alph*),ref=(\alph*)]
\item a condition
\item another
\item and another
\end{enumerate}
which show the point made.
\end{thm}

\ref{A} and \ref{B}

\end{document}

enter image description here

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