Loading a package option conditionally - TeX

Phelype Oleinik replying to निरंजन

You're welcome :)

निरंजन

Okay. Thanks

Phelype Oleinik replying to निरंजन

Yes, in normal circumstances the end-line char is inserted after _every_ line, so that is `\newcommand{\foo}{ \textbf{bar} }` with two spaces.  If you are unsure that a macro has spurious spaces you can just do `a\foo b`: there should be no spaces between  `a`, the output of `\foo` and `b`.

निरंजन replying to Phelype Oleinik

Is it the same for the control sequences with arguments? eg.  
  
```  
\newcommand{\foo}{  
\textbf{bar}  
}  
```  
  
Here do we have an end-line-character after \textbf{bar}?

निरंजन replying to Phelype Oleinik

Oh!! Okay :)

Phelype Oleinik replying to निरंजन

If you think about it, it's the same case as before.  If you have no `%`, TeX inserts spaces at the end of the line, so you have:  
```  
\ifnum 1=0␣
  1␣
\else %  no space here (control sequence)
  0␣
\fi % no space here as well
```  
or, in one line:  

```  
\ifnum 1=0 1 \else0\fi
```  
which compares `\ifnum 1=0` and (obviously) returns false.  With the `%`, however:
```  
\ifnum 1=0% no space here because of the %
  1␣
\else %  no space here (control sequence)
  0␣
\fi % no space here as well
```    
or  
```  
\ifnum 1=01 \else0\fi
```    
and `\ifnum 1=01` is true.

Phelype Oleinik replying to निरंजन

No, it's hard to do so.  TeX can't know that you forgot a `\fi` before it sees the `\end` primitive that will stop TeX.  At that time, it has already finished TeX's main control routine, so it would be hard to issue an “usual” TeX error message and give control to the user: it would be a hard to control environment.

निरंजन replying to Phelype Oleinik

Okay I can see the difference, but can't guess the reason. Can you explain please?

निरंजन replying to Phelype Oleinik

Okay, got it :)

निरंजन replying to Phelype Oleinik

Is it absolutely not possible to turn them into errors?

निरंजन replying to Phelype Oleinik

Ah! A clever [jugaad](https://www.lexico.com/definition/jugaad) :P

Phelype Oleinik replying to निरंजन

You don't need them _always_ though.  After control sequences you don't need them:  
```  
\def\foo{%  
bar%
\baz
}  
```  
Also numerical conditionals should _not_ have a `%`, or you might get them wrong:  
```  
\ifnum 1=0% <- add/remove this % to see the difference  
  1  
\else  
  0  
\fi  
```

Phelype Oleinik replying to निरंजन

```  
because when you are writing a  
document you might want to insert  
line breaks for clarity without  
having to add \verb|\space| or  
\verb*|\ | to force a space at the  
end of the line, so \TeX\ inserts  
that endlinechar for you.  Remember:  
\TeX\ is meant for writing actual  
text, not clever code ;-)  
```

Phelype Oleinik replying to निरंजन

You do, but that incomplete conditional cannot be reliably detected until the end of the document.  When TeX stops you see:  
```  
(\end occurred when \ifx on line 8 was incomplete)
(\end occurred when \ifx on line 13 was incomplete)  
```  
but they (unfortunately) don't raise actual errors; only sneaky messages in the terminal.

निरंजन

I'll now onwards add `%` at the line-end.

निरंजन replying to Phelype Oleinik

Why is that character inserted when not asked for? What is its use?

Phelype Oleinik replying to निरंजन

Almost, yes, except that `\@` does not produce the an @ character when typeset (you can just write `@` for that).  That depends on what the catcode of `@` is at the time of the definition.  Compare:  
```  
\def\testa{a\@date}
\makeatletter
\def\testb{a\@date}
\makeatletter

\testa

\testb  
```

निरंजन replying to Phelype Oleinik

Okay. Now I understood it. Thanks

निरंजन

In [your answer](#a1728), you have said `\if<name>` is the conditional itself. I was wondering if this is the case I should have received an error saying incomplete `\if` because of `\DeclareOption{digital}{\ifdigital}`, right? I don't think I got it.  
  
```  
\begin{filecontents}[overwrite]{test.cls}
\newif\ifdigital
\DeclareOption{digital}{\ifdigital}
\ProcessOptions
\ifdigital
\LoadClass[oneside]{book}
\else
\LoadClass{book}
\fi
\end{filecontents}

\documentclass[digital]{test}
\title{abcd}

\begin{document}
\maketitle
\end{document}  
```

Phelype Oleinik replying to निरंजन

For context, `expl3` code does `\endlinechar=-1` so that TeX doesn't insert that end line character.

Phelype Oleinik replying to निरंजन

At the end of every line of input (usually) TeX inserts a character called “end-line char” (the default is a space).  For example:
```  
\def\foo{
bar
}
```
contains three lines, so three end-line characters (here spaces):
```  
\def\foo{␣
bar␣
}␣
```  
or, in one line, `\def\foo{ bar }`, which has two extra spaces that `\def\foo{bar}` doesn't.  Adding the `%` at the end of the line essentially comments out the end-line character

Phelype Oleinik replying to निरंजन

That is for things that store user input. `\title` and `\author` and `\date` are like that.  For example: `\title` is `\def\title#1{\gdef\@title{#1}}`.  When the user calls `\title{foo}` then it expands and becomes `\gdef\@title{foo}`, which will define `\@title`.  `\@title` is then used for typesetting.  The `\let\@title\@empty` is there to give it an initial value, so that you don't get an `Undefined control sequence` error when using `\@title` without the user giving a `\title`.  LaTeX defines the initial value for `\@title` as `\def\@title{\@latex@error{No \noexpand\title given}\@ehc}` to give an error instead

निरंजन replying to Phelype Oleinik

Oh okay. So when `@` is defined as a letter `\@title` is a macro and when it is `other` than a letter `\@` will be understood as @ and `title` will be followed by it, so I will get something like `@ title` in the output, right?

निरंजन

Also why is `%` required at the end of the line? I have seen it at many places. I assume it is for readability and not a strict requirement of the code itself.

निरंजन replying to Phelype Oleinik

I was actually following [this](https://tex.stackexchange.com/a/525452/174620) answer. I thought it is necessary to `\let` a command be empty first and then define it conditionally if we want it to be null at times, but I understand how to achieve this because of your code now! Thanks

निरंजन replying to Phelype Oleinik

Thanks for the explanation. It worked well now :)

Phelype Oleinik replying to निरंजन

Also in `\renewcommand{\maketitle}{\makeatletter\@title\makeatother}`, the `\makeat...` are useless: TeX sets catcodes when it _sees_ a character for the first time: in your case it saw the characters when _defining_ `\maketitle`, so `\@title` is already tokenised as `\@title` rather than `\@ title`.

Phelype Oleinik replying to निरंजन

I didn't understand why you had both `\printlof` and `\@printlof`, so I left the internal name only.  Also you had a couple missing `%` at the end of lines:  I marked them in the code above.

Phelype Oleinik

```  
\begin{filecontents}[overwrite]{test.cls}
\LoadClass{book}
\newif\iflof % declare \iflof (initially false)
\DeclareOption{lof}{\loftrue} % declare the 'lof' option
\ProcessOptions % process the options
\iflof % if 'lof' is given, \iflof is now true
  \def\@printlof{% <- missing %
    \listoffigures
  }%
\else
  \let\@printlof\@empty
\fi
\renewcommand{\maketitle}{% <- missing %
  \@title
  \@printlof
}
\end{filecontents}

\documentclass[lof]{test}
\title{abcd}

\begin{document}
\maketitle
\end{document}  
```

Phelype Oleinik replying to निरंजन

Your timing is wrong.  Line 2 declares `\iflof` (which starts as false), then line 3 _declares_ the `lof` option, but does not evaluate it.  The options given are only evaluated at `\ProcessOptions`, so on the `\iflof` line (4) of the class is always false.  You need to move `\ProcessOptions` earlier:

निरंजन

@Phelype I need another conditional package option which (if used) must affect the `maketitle` command.  See this not working minimal code -  

```   
\LoadClass{book}
\newif\iflof  
\DeclareOption{lof}{\loftrue}
\iflof
\let\@printlof\@empty
\def\printlof{
  \def\@printlof{
    \listoffigures
  }
}
\else
\fi
\ProcessOptions  
\renewcommand{\maketitle}{  
   \makeatletter  
   \@title  
   \makeatother  
   \printlof
}
```  
  
Let's assume this code is a class file `test` and our document is something like - 
  
```  
\documentclass[lof]{test}
\title{abcd}
  
\begin{document}  
\maketitle  
\end{document}
```  
  
the output should produce a document in which the title is followed by the list of figures.

निरंजन

@Phelype, re: [your answer](#a1728), Thank you so much! I will go with your `PassOptionsToClass`  suggestion. It seems to be much cleaner than the nested conditional approach.

Phelype Oleinik

I don't see why that shouldn't work, except that the line `\DeclareOption{digital}{\ifdigital}` should probably be `\DeclareOption{digital}{\digitaltrue}`

1 Answer