Does \input really just include the file?

Question

I have an issue with the \input command.

As it is stated here, "it's equivalent to typing all the commands from filename.tex right into the current file where the \input line is."

I have the feeling that this is not exactly the case, at least in some situations, and I would like some clarifications.

Consider this MVCE (say, in a file main.tex):

\documentclass{article}
\usepackage{mypackage}
\begin{document}
\begin{myenvir}{Hello}
\mycommand{World}
\end{myenvir}
\end{document}

Now the mypackage.sty file (stored in same folder):

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mypackage}[version 0.1]

\newenvironment{myenvir}[1]
{%
    \newcommand{\mycommand}[1]
    { test:command arg=##1 environment arg=#1}
    \begin{center}
}
{\end{center}} 
\endinput

When compiling main.tex, this works fine, and produces.

test:command arg=World environment arg=Hello

But now, lets say for some reason (*), I want to store the command definition in a separate file. So I just change the sty file to:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mypackage}[version 0.1]

\newenvironment{myenvir}[1]
{%
    \input{fcommand}
    \begin{center}
}
{\end{center}} 
\endinput

And store into a file fcommand.tex these two lines:

\newcommand{\mycommand}[1]
{ test:command arg=##1 environment arg=#1}

Shouldn't that give strictly the same result?

But when compiling main.tex I get this error:

You can't use `macro parameter character # in horizontal mode

and the output (pdf) file holds:

test:command arg=1 environment arg=World

Question: what is going on ? A far as I understand, there seems to be a problem with argument substitution when code is in a separate file. Thus my assumption that \input is not truly equivalent to having the code in same file.

I have also checked this related question, where the answer states to add \unskip. So I tried this:

\input{fcommand}\unskip%

and changed the command file content:

\newcommand{\mycommand}[1]
{ test:command arg=##1 environment arg=#1}\endinput

But still got the same error.

Other related question: https://tex.stackexchange.com/a/319512/11083

(*) On the reason to try this, this question is actually a follow up on another question I am currently dealing with.

Answer 1

The problem is that when TeX reads the environment command it detects where all the #1 are, but the \input is not processed. When the environment is executed (that is, the underlying \myenvir command is expanded), all the #1 seen previously are replaced by the argument Hello. So using

\begin{myenvir}{Hello}
\mycommand{World}
\end{myenvir}

is the same as (minus the comments)

% \begin{myenvir}{Hello}
\input{fcommand}
\begin{center}
% environment contents
\mycommand{World}
% \end{myenvir}
\end{center}

The Hello was lost because it wasn't used anywhere. Now the \input is expanded:

% \begin{myenvir}{Hello}
\newcommand{\mycommand}[1]
{ test:command arg=##1 environment arg=#1}
\begin{center}
% environment contents
\mycommand{World}
% \end{myenvir}
\end{center}

Then, after the definition is done, the expansion of \mycommand yields:

% \begin{myenvir}{Hello}
% \newcommand{\mycommand}[1] %%% Already defined
% { test:command arg=##1 environment arg=#1}
\begin{center}
% environment contents
test:command arg=#1 environment arg=World
% \end{myenvir}
\end{center}

Ans now you're trying to write #1 in the middle of the text, which raises the error:

! You can't use `macro parameter character #' in horizontal mode.
l.40 test:command arg=#
                       1 environment arg=World