Command not 'legale' in .sty file

Question

Consider the following style file goat.sty:

\NeedsTeXFormat{LaTeX2e}[2014/05/01]
\ProvidesPackage{goat}[2014/10/16 goat]

\DeclareOption{mammalbirdfish}{
\makeatletter
 \begingroup
  \catcode`\_=\active
  \protected\gdef_{\@ifnextchar|\subtextup\sb}
 \endgroup
\def\subtextup|#1|{\sb{\textup{#1}}}
\AtBeginDocument{\catcode`\_=12 \mathcode`\_=32768}
\makeatother}

\ExecuteOptions{mammalbirdfish}
\ProcessOptions

When I try to load the package in my LaTeX document be executing \usepackage{goat}, I get the following error:

! Missing control sequence inserted.
<inserted text> 
                \inaccessible 
l.13 \ExecuteOptions{mammalbirdfish}

How do I get the above chunk of code into my style file?

P.S. I've never written a style file before so please forgive me if my question has a trivial answer.

Update

I also can't put

\ExplSyntaxOn
  \cs_new_eq:NN \calc \fp_eval:n
\ExplSyntaxOff

into my style file

\NeedsTeXFormat{LaTeX2e}[2014/05/01]
\ProvidesPackage{goat}[2014/10/16 goat]

\RequirePackage{expl3}

\DeclareOption{macros}{
\ExplSyntaxOn
  \cs_new_eq:NN \calc \fp_eval:n
\ExplSyntaxOff
}

\ExecuteOptions{macros}
\ProcessOptions

When I compile the doucment

\documentclass{article}

\usepackage{goat}

\begin{document}

test

\end{document}

it gives the error

! Undefined control sequence.
\ds@makroer ...scriptfalse \par \ExplSyntaxOn \cs 
                                                  _new_eq:NN \calc \fp _eval...

How do I fix this?

(Let my know if I should ask the update question as a new one; it was put here because I think it might be a similar problem.)

Answer 1

Let's see what happens with

\DeclareOption{mammalbirdfish}{
\makeatletter
 \begingroup
  \catcode`\_=\active
  \protected\gdef_{\@ifnextchar|\subtextup\sb}
 \endgroup
\def\subtextup|#1|{\sb{\textup{#1}}}
\AtBeginDocument{\catcode`\_=12 \mathcode`\_=32768}
\makeatother}

\ExecuteOptions{mammalbirdfish}
\ProcessOptions

The code from \makeatletter to \makeatother is given as argument to \DeclareOption, so it has already been tokenized and, in particular, the _ after \gdef has category code 8, so it's illegal after \gdef when the code is executed (storing it is not a problem, as TeX doesn't interpret commands at that moment).

It's (almost) never needed to issue \makeatletter and \makeatother in a class or package file, because they are read with implicit \makeatletter and \makeatother at the end. To be more precise, these commands in a class or package file usually are wrong, unless they appear as part of a macro definition. In this case they are wrong, because when \ProcessOptions expands the code for the mammalbirdfish option, \makeatother will ruin all subsequent reading of the code in the class or package.

Note that setting that code as an option which is executed no matter what is nonsense: such an option should never be passed to \ExecuteOptions.

However, the most important problem is the category code of _. I'll show a different strategy than setting a conditional and doing the code after \ProcessOptions if the conditional is true.

\begingroup\lccode`\~=`\_
\lowercase{\endgroup
  \DeclareOption{mammalbirdfish}{%
    \protected\gdef~{\@ifnextchar|\subtextup\sb}%
    \def\subtextup|#1|{\sb{\textup{#1}}}%
    \AtBeginDocument{\catcode`\_=12 \mathcode`\_=\string"8000 }%
  }%
}

which works because there's no uppercase character token in the code and so only ~ (which is already active) will be converted to (active) _.

Similar problems are with

\DeclareOption{macros}{
\ExplSyntaxOn
  \cs_new_eq:NN \calc \fp_eval:n
\ExplSyntaxOff
}

which should be

\ExplSyntaxOn
\DeclareOption{macros}{
  \cs_new_eq:NN \calc \fp_eval:n
}
\ExplSyntaxOff

otherwise the characters : and _ would have be tokenized as “other characters”.

Answer 2

I'm not sure my comment isn't less clear than mud, but this is too much code for a comment - sorry. Happy to delete it later if best.

\NeedsTeXFormat{LaTeX2e}[2014/05/01]
\ProvidesPackage{goat}[2014/10/16 goat]

\newif\ifmammalbirdfish
\mammalbirdfishfalse
\DeclareOption{mammalbirdfish}{%
  \mammalbirdfishtrue}

\ExecuteOptions{mammalbirdfish}
\ProcessOptions
\ifmammalbirdfish
  \begingroup
    \catcode`\_=\active
    \protected\gdef_{\@ifnextchar|\subtextup\sb}%
  \endgroup
  \def\subtextup|#1|{\sb{\textup{#1}}}%
  \AtBeginDocument{%
    \catcode`\_=12 \mathcode`\_=32768}%
\fi