How to reuse a command inside another environment

Question

For my own needs, I am writing a package holding some environments definitions. I also have some commands that I want to be available only inside these environments. So I define these commands inside the environment definition.

My .sty files looks like this

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mypackage}[version 0.1]
\newenvironment{myenvirA}[1]
{
   \newcommand{\mycommandA}[1]
    { %
    %... do something
    }%
   % here, the "before" part of code 
}
{
   % here, the "after" part of code 
}


\newenvironment{myenvirB}[1]
{
   \newcommand{\mycommandB}[1]
    { %
    %.. do something
    }%
}
{
}

Now, say I want to make \mycommandA available inside myenvirB blocs. Sure, I can copy/paste, but that is bad programming (error prone).

Question: Does Latex provide a way to handle this case ?

If not, can I put that command definition in a file and \input it? (so I only have to define it once). Will that work once the package is installed in my Latex tree ?

Sorry if unclear, please point me on other question if this appears as a dupe. Couldn't find any, although this one is possibly related (?). But it is unclear to me if the answers apply to my question.

Edit forgot some information: the command has to have access to the environment arguments. Thus the idea given by Phelype Oleinik can't work (unless some trick I am not aware of ?).

Here is an MVCE:

main.tex:

\documentclass{article}
\usepackage{mypackage}
\begin{document}
\begin{envirA}{4}
\mycommandA{3}
\end{envirA}    
\begin{envirB}{4}
\mycommandB{3}
\end{envirB}  
\end{document}

mypackage.sty:

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

\newenvironment{envirA}[1]
{
    \newcommand{\mycommandA}[1]
    { 
        \draw (1,0) -- (##1,#1);
    }       
    \begin{tikzpicture}
    \draw (0,0) -- (#1,1);
}
{
    \end{tikzpicture}
}

\newenvironment{envirB}[1]
{
    \newcommand{\mycommandB}[1]
    { 
        \draw (0,1) -- (##1,#1);
    }       
    \begin{tikzpicture}
    \draw (0,0) -- (#1,1);
}
{
    \end{tikzpicture}
}
\endinput 

Answer 1

I guess that the definition of \mycommandA in myenvirA (I won't use numbers, which are illegal in command names) depends on the argument passed to the environment, otherwise the problem is easy to solve, by just defining \mycommandA directly.

\newcommand{\temporarycommandname}{} % initialize
\newenvironment{myenvirA}[1]
 {%
  \renewcommand{\temporarycommandname}[1]{something with #1 and ##1}%
  \global\let\mycommandA\temporarycommandname
  % whatever else
 }
 {%
  % the end part
 }

Now \mycommandA is usable everywhere, after \begin{myenvirA} has been executed. Further calls of myenvirA will change the meaning of \mycommandA, of course.

If you want that \mycommandA is defined only once, at the first call of myenvirA, and not modified by subsequent calls of the environment, do

\newcommand{\temporarycommandname}{} % initialize
\newenvironment{myenvirA}[1]
 {%
  \ifdefined\mycommandA\else
    \renewcommand{\temporarycommandname}[1]{something with #1 and ##1}%
    \global\let\mycommandA\temporarycommandname
  \fi
  % whatever else
 }
 {%
  % the end part
 }

Answer 2

You can't grab a macro definition from another environment where it is defined, since the environment necessarily provides a limited scope for that macro (as it is grouped).

% \mycommand does not exist here
\begin{myenvironment}
  % _Local_ definition of \mycommand
  \newcommand{\mycommand}{<cmd definition>}
  % \mycommand exists here
  % ...
\end{myenvironment}
% \mycommand does not exist here

Perhaps there are ways to do that, but it's not worth the effort. You should instead define the macro to be globally available, or read it in from an external file via \input every time you need it (if properly installed within your installation folder). The latter option would slow down compilation.

The question you have to ask yourself is why you want to have the definition only be available locally to an environment. Do you want to save memory? That would have been a problem a couple of decades ago, but not anymore. Do you think it will provide for a cleaner package? If so, rather define a "namespace" by preceding each macro for your package mypackage by \@mypackage@. This way your macros will look like

\@mypackage@mycommandA
\@mypackage@mycommandB
...

The @ symbol in macros can be used without official declaration of a \makeatletter...\makeatother pair within a package, making it somewhat hidden to the end user. The idea of a "namespace" or @-macros is fairly common in packages.