Package options depending on compiler

Question

I'm experimenting with custom packages, and I try to create the "ultimate" package for keeping notes. Now I know it isn't great but it works. Now -because I'm indecisive- I want to be able to declare two options: one to use the corresponding commands when compiling with pdfLaTex, and one to use the corresponding when compiling with XeLaTeX.

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{notetaking}[2020/07/19 Necessary Packages for Note Keeping in LaTeX]

\newif\if@xetex
\DeclareOption{xetex}{\@xetextrue\@pdftexfalse}
\ProcessOptions\relax

\if@xetex
    \RequirePackage{fontspec}
    \RequirePackage{polyglossia}
    
    %%Language related settings.
    \setmainlanguage{greek}
    \setmainfont[
        Mapping = tex-text,
        Script = Greek,
        Extension = .ttf,
        BoldFont = timesbd,
        ItalicFont = timesi,
        BoldItalicFont = timesbi
    ]{times}
\fi

\newif\if@pdftex
\DeclareOption{pdftex}{\@pdftextrue\@xetexfalse}
\ProcessOptions\relax

\if@pdftex
    \RequirePackage[utf8]{inputenc}
    \RequirePackage[english, greek]{babel}
    
    %%Language related settings.
    \newcommand{\en}[1]{\selectlanguage{english}#1\selectlanguage{greek}}
\fi

\RequirePackage{amsmath}
\RequirePackage{amsfonts}
\RequirePackage{amssymb}
\RequirePackage{amsthm}
\RequirePackage{makeidx}
\RequirePackage{graphicx}
\RequirePackage{fancyhdr}
\RequirePackage{import}
\RequirePackage{lipsum}
\RequirePackage[left=1in,right=1in,top=1in,bottom=1in]{geometry}
\RequirePackage{titling}
\RequirePackage{etoolbox}
\RequirePackage{enumitem}

%%Title elated settings.
\newcommand{\subtitle}[1]{
    \posttitle{
        \par\end{center}
        \begin{center}\large#1\end{center}
        \vskip0.5em}
}

%%Page related settings
\appto\frontmatter{\pagestyle{empty}}
\appto\mainmatter{\pagestyle{fancy}}

\fancyhf{}
\fancyhead[LE,RO]{\leftmark}
\fancyhead[RE,LO]{\rightmark}
\fancyfoot[CE,CO]{Σελίδα \thepage}

%%Math related macros.
\newcommand{\R}{\mathbb{R}}
\newcommand{\br}[1]{\left\{ #1 \right\}}
\newcommand{\pr}[1]{\left( #1 \right)}
\newcommand{\nm}[1]{\left\lVert #1 \right\rVert}
\renewcommand{\vec}{\mathbf}

%% Title related macros.
\newcommand{\nd}[1]{\def\@notedt{#1}}

%%Theorem related settings.
\theoremstyle{plain}
    \newtheorem*{theorem}{Θεώρημα}
    \newtheorem*{lemma}{Λήμμα}
    \newtheorem*{corollary}{Πόρισμα}
    \newtheorem*{conjecture}{Υπόθεση}
    \newtheorem*{proposition}{Πρόταση}
    \newtheorem*{exercise}{Άσκηση}

\theoremstyle{definition}
    \newtheorem*{definition}{Ορισμός}
    \newtheorem*{problem}{Πρόβλημα}
    \newtheorem*{example}{Παράδειγμα}
    \newtheorem*{axiom}{Αξίωμα}

\theoremstyle{remark}
    \newtheorem*{remark}{Παρατήρηση}
    \newtheorem*{note}{Σημείωση}

\renewcommand\qedsymbol{$\blacksquare$}

When I try to compile my main.tex with the option pdftex I get this error:

! LaTeX Error: Unknown option `pdftex' for package `notetaking'.See the LaTeX manual or LaTeX Companion for explanation.Type H <return> for immediate help.... \ProcessOptions\relax

So what am I doing wrong? Is it even possible to do what I want? Thank you very much in advance.

Okay I got it, what @moewe suggested worked like a charm.

Answer 1

Your error is calling \ProcessOptions twice but you do not need an option here:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{notetaking}[2020/07/19 Necessary Packages for Note Keeping in LaTeX]

\RequirePackage{iftex}


\iftutex % xetex or luatex
    \RequirePackage{fontspec}
    \RequirePackage{polyglossia}
    
    %%Language related settings.
    \setmainlanguage{greek}
    \setmainfont[
        Mapping = tex-text,
        Script = Greek,
        Extension = .ttf,
        BoldFont = timesbd,
        ItalicFont = timesi,
        BoldItalicFont = timesbi
    ]{times}
\else
% classic 8bit tex (latex or pdftex)
    \RequirePackage[utf8]{inputenc}
    \RequirePackage[english, greek]{babel}
    
    %%Language related settings.
    \newcommand{\en}[1]{\selectlanguage{english}#1\selectlanguage{greek}}
\fi