Spaces in newenvironment

Question

I observed an unwanted behavior of my self-made environment. The following code sometimes creates spaces after the environment head and sometimes doesn't.

\documentclass[a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage{ifthen}

\newenvironment{Theorem}[1][]{%
    \medskip%
    \par%
    \noindent%
    \textbf{Theorem.}%
    \ifthenelse{ \equal{#1}{} }{}{ (#1)}%
    \it%
    }%
    {%
    \smallskip\par%
    }

\newenvironment{Theorem2}{%
    \medskip%
    \par%
    \noindent%
    \textbf{Theorem.}%
    \it%
    }%
    {%
    \smallskip\par%
    }

\begin{document}

\begin{Theorem}
That is a cool Theorem
\end{Theorem}

\begin{Theorem}[of some famous guy]
    That is a cool Theorem
\end{Theorem}

\begin{Theorem2}
another cool theorem
\end{Theorem2}

\begin{Theorem2}%
another cool theorem
\end{Theorem2}

\end{document}

The result is:

Is there a way of forcing LaTeX to ignore all normal spaces in the definition of an environment such that I can only add spaces by \space?

And can I add something to the definition of the environment such that it ignores all spaces and pars that appear directly after the call of the environment such that

\begin{Theorem2}
some text
\end{Theorem2}

produces the same as

\begin{Theorem2}%
some text
\end{Theorem2}

?

Answer 1

Yes! Use \ignorespaces at the end of the first argument to \newenvironment

\documentclass{article}
\newenvironment{Theorem2}{%
    \medskip%
    \par%
    \noindent%
    \textbf{Theorem.}%
    \itshape\ignorespaces%
    }%
    {%
    \smallskip\par%
    }
\begin{document}
  \begin{Theorem2}
  some text
  \end{Theorem2}
\end{document}

\ignorespaces is a primitive that ignores the next spaces until some text is to be output. When you do e.g. \newenvironment{thm}{}{} then a macro \thm is made, which in turn is the last macro executed by the \begin{thm} command.

Answer 2

Yes, there is: use xparse and exploit that in the scope of \ExplSyntaxOn spaces are ignored; one can use ~ to insert a real space.

\documentclass[a4paper]{article}

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentEnvironment{Theorem}{o}
 {
  \par\addvspace{\medskipamount}
  \noindent
  \normalfont
  \textbf{Theorem.~}
  \IfValueT{#1}{(#1)~}
  \itshape\ignorespaces
 }
 {
  \par\addvspace{\smallskipamount}
 }
\ExplSyntaxOff

\begin{document}

Some text before the theorem to see the vertical spacing
in context.

\begin{Theorem}
That is a cool Theorem
\end{Theorem}

Some text after and before the theorem to see the vertical spacing
in context.

\begin{Theorem}[of some famous guy]
    That is a cool Theorem
\end{Theorem}

Some text after the theorem to see the vertical spacing
in context.

\end{document}

I made some adjustments: \medskip and \smallskip are wrong in this context. However, \smallskipamount is definitely too small as the image shows. With \addvspace the vertical spaces don't accumulate; with your code two consecutive theorems would be separated by a small skip plus a medium skip.

On the other hand, \newtheorem* from amsthm is much simpler.

\documentclass[a4paper]{article}

\usepackage{amsthm}

\newtheorem*{Theorem}{Theorem}

\begin{document}

Some text before the theorem to see the vertical spacing
in context.

\begin{Theorem}
That is a cool Theorem
\end{Theorem}

Some text after and before the theorem to see the vertical spacing
in context.

\begin{Theorem}[of some famous guy]
    That is a cool Theorem
\end{Theorem}

Some text after the theorem to see the vertical spacing
in context.

\end{document}