Correct way to remove spaces when adding hooks to environment

Question

Please see addendum before reading the question. I have not deleted or made major edits to it so as not to invalidate @cfr's answer.

I'm trying to add some hooks to a theorem environment, and I noticed they add unwanted spaces next to the surrounding and body text. Here's a simple first try that defines the environment between \ExplSyntaxOn and \ExplSyntaxOff to avoid any accidental spaces:

\documentclass{article}
\usepackage{amsthm}
\NewHook{mythm/prehead}
\NewHook{mythm/posthead}
\NewHook{mythm/prefoot}
\NewHook{mythm/postfoot}
\newtheorem{theorem}{Theorem}
\ExplSyntaxOn
\NewDocumentEnvironment{mytheorem}{ O{} }
  {
    \UseHook{mythm/prehead}
    \begin{theorem}[#1]
    \UseHook{mythm/posthead}
  }
  {
    \UseHook{mythm/prefoot}
    \end{theorem}
    \UseHook{mythm/postfoot}
  }
\ExplSyntaxOff
\AddToHook{mythm/prehead}{PREHEAD}
\AddToHook{mythm/posthead}{POSTHEAD}
\AddToHook{mythm/prefoot}{PREFOOT}
\AddToHook{mythm/postfoot}{POSTFOOT}
\begin{document}
before
\begin{mytheorem}
body
\end{mytheorem}
after
\end{document}

Around the code added to each hook there are unwanted spaces. I'll note that this is the same output as if I'd defined

\NewDocumentEnvironment{mytheorem}{ O{} }
  {
    PREHEAD
    \begin{theorem}[#1]
    POSTHEAD
  }
  {
    PREFOOT
    \end{theorem}
    POSTFOOT
  }

so the \UseHooks aren't the real issue.

After playing around with \unskip, \ignorespaces, and \ignorespacesafterend, I found a combination that removes the spaces:

\documentclass{article}
\usepackage{amsthm}
\NewHook{mythm/prehead}
\NewHook{mythm/posthead}
\NewHook{mythm/prefoot}
\NewHook{mythm/postfoot}
\newtheorem{theorem}{Theorem}
\ExplSyntaxOn
\NewDocumentEnvironment{mytheorem}{ O{} }
  {
    \unskip % \ignorespaces does not work here
    \UseHook{mythm/prehead}
    \begin{theorem}[#1]
    \UseHook{mythm/posthead}
    \ignorespaces % \unskip does not work here
  }
  {
    \unskip % \ignorespaces does not work here
    \UseHook{mythm/prefoot}
    \end{theorem}
    \UseHook{mythm/postfoot}
    \ignorespacesafterend % neither \unskip nor \ignorespaces work here
  }
\ExplSyntaxOff
\AddToHook{mythm/prehead}{PREHEAD}
\AddToHook{mythm/posthead}{POSTHEAD}
\AddToHook{mythm/prefoot}{PREFOOT}
\AddToHook{mythm/postfoot}{POSTFOOT}
\begin{document}
before
\begin{mytheorem}
body
\end{mytheorem}
after
\end{document}

Is this the correct way to go about removing spaces from around an environment? After reading this thread, I understand better what the commands do, but I'd like to know if there are any obvious downsides or incompatibilities (e.g. theorems won't ever be in vertical mode, right?).

Addendum

As @cfr points out in the answer below, there should be spaces before the prehead and postfoot hooks because there are explicit spaces (from the linebreaks) in the input. So, my question should only apply to the appropriateness of \ignorespaces to remove the space after the posthead hook and \unskip before the prefoot hook. Apologies for my silly mistake.

Comparison with thmtools

The idea for these hooks is coming from the thmtools package. It only removes space after the posthead hook, not before the prefoot. Here is an example.

\documentclass{article}
\usepackage{amsthm,thmtools}
\declaretheorem{theorem}
\addtotheorempreheadhook{PREHEAD}
\addtotheorempostheadhook{POSTHEAD}
\addtotheoremprefoothook{PREFOOT}
\addtotheorempostfoothook{POSTFOOT}
\begin{document}
before
\begin{theorem}
body
\end{theorem}
after
\end{document}

 

Answer 1

Note that this answer contains NO working code.

Caveat emptor ...

---

First, as you seem to realise, the spacing is not introduced by your use of hooks. To see this, we can use simply

ABC\UseHook{mythm/prehead}\UseHook{mythm/posthead}\UseHook{mythm/prefoot}\UseHook{mythm/postfoot}XYZ

It is evident from this that none of the hooks are adding any space.

Second, your proposed 'solution' will create many untoward effects. At best, you will get appalling typographical output. (Try \hrule before mytheorem or a sectioning command or a list or ...).

Third, All Kinds of Stuff will break if it stumbles into the vicinity of mytheorem. So while killing all vertical spacing above and below mytheorem will create very ugly documents, this is unlikely to be much of a problem because you're unlikely to get any output at all.

For instance,

\documentclass[a5paper]{article}% does NOT compile without error
\usepackage{amsthm}
\usepackage{kantlipsum}
\NewHook{mythm/prehead}
\NewHook{mythm/posthead}
\NewHook{mythm/prefoot}
\NewHook{mythm/postfoot}
\newtheorem{theorem}{Theorem}
\ExplSyntaxOn
\NewDocumentEnvironment{mytheorem}{ O{} }
  {
    \unskip % \ignorespaces does not work here
    \UseHook{mythm/prehead}
    \begin{theorem}[#1]
    \UseHook{mythm/posthead}
    \ignorespaces % \unskip does not work here
  }
  {
    \unskip % \ignorespaces does not work here
    \UseHook{mythm/prefoot}
    \end{theorem}
    \UseHook{mythm/postfoot}
    \ignorespacesafterend % neither \unskip nor \ignorespaces work here
  }
\ExplSyntaxOff
\AddToHook{mythm/prehead}{PREHEAD}
\AddToHook{mythm/posthead}{POSTHEAD}
\AddToHook{mythm/prefoot}{PREFOOT}
\AddToHook{mythm/postfoot}{POSTFOOT}
\begin{document}
\begin{mytheorem}
\begin{mytheorem}
\kant[1]
\end{mytheorem}
\begin{verbatim}
\macro 
\end{verbatim}
\begin{mytheorem}
\kant[2]
\end{mytheorem}
\end{document}

Perhaps you don't use verbatim?

\begin{mytheorem}
\kant[1]
\end{mytheorem}
\begin{center}
  I'm in the centre.
\end{center}
\begin{mytheorem}
\kant[2]
\end{mytheorem}

Or center? (Or flushleft or ...?)

\begin{mytheorem}
\kant[1]
\end{mytheorem}
[
  x^2
]
\begin{mytheorem}
\kant[2]
\end{mytheorem}

Or display-style maths?

Or ... ?