Create a command using a `tasks` environment

Question

I would like to use a command bitcol (see below) with a tasks environment, but the following code give an error. How can I fix it ?

\documentclass[a4paper]{book}
\usepackage{tasks}
\usepackage{fontawesome}
\NewTasksEnvironment[style=itemize,label=\faCaretRight]{itcol}[\item]
%
\newcommand{\bitcol}[1][2]{\begin{itcol}(#1)}
\newcommand{\eitcol}{\end{itcol}}
%
\begin{document}
% OK
\begin{itcol}(3)
\item 1
\item 2
\item 3
\item 4 
\end{itcol}
% KO
\bitcol[3]
\item 1
\item 2
\item 3
\item 4 
\eitcol
\end{document}

Answer 1

It cannot work this way. In order to do its job, the defined itcol environment has to explicitly see \end{itcol}.

I see no real reason to type

\bitcol[3]
...
\eitcol

instead of

\begin{itcol}(3)
...
\end{itcol}

If your aim is to use a standard optional argument, you can do as follows. But don't use the not recommended way.

\documentclass[a4paper]{book}
\usepackage{tasks}
\usepackage{fontawesome}
\NewTasksEnvironment[style=itemize,label=\faCaretRight]{itcolinner}[\item]
\NewDocumentEnvironment{itcol}{O{2}+b}
 {\begin{itcolinner}(#1)#2\end{itcolinner}}
 {}
%%% this part is for the not recommended way
\NewDocumentCommand{\bitcol}{O{2}}{\dobitcol{#1}}
\newcommand{\dobitcol}{}% for safety
\def\dobitcol#1#2\eitcol{\begin{itcolinner}(#1)#2\end{itcolinner}}
%%% end of code for the not recommended way
\begin{document}
% recommended way
\begin{itcol}[3]
\item 1
\item 2
\item 3
\item 4 
\end{itcol}
% NOT RECOMMENDED WAY
\bitcol[3]
\item 1
\item 2
\item 3
\item 4 
\eitcol
\end{document}