Problem with ExplSyntax

Question

I'm trying to define a flexible "delimiter styling" command. I tried it in a package, and \str_case:nnF returned Undefined Control Sequence, whereas using \str_case:nnn went further and caused a different problem, which seemed to be related to the command itself. So I moved it into the document, getting the following code:

\documentclass[a4paper]{report}
\usepackage{amsthm,xparse}
\ExplSyntaxOn
\newcommand{\delim}[3][lr]{
 \str_case:nnF { #1 }
    {
    { b }{\def\lsize{\big}\def\rsize{\big}}
    { B }{\def\lsize{\Big}\def\rsize{\Big}}
    { x }{\def\lsize{\bigg}\def\rsize{\bigg}}
    { X }{\def\lsize{\Bigg}\def\rsize{\Bigg}}
    { lr }{\def\lsize{\left}\def\rsize{\right}}
    { bB }{\def\lsize{\big}\def\rsize{\Big}}
    { Bb }{\def\lsize{\Big}\def\rsize{\big}}
    { xb }{\def\lsize{\bigg}\def\rsize{\big}}
    { xB }{\def\lsize{\bigg}\def\rsize{\Big}}
    { bx }{\def\lsize{\big}\def\rsize{\bigg}}
    { Bx }{\def\lsize{\Big}\def\rsize{\bigg}}
    { bX }{\def\lsize{\big}\def\rsize{\Bigg}}
    { BX }{\def\lsize{\Big}\def\rsize{\Bigg}}
    { xX }{\def\lsize{\bigg}\def\rsize{\Bigg}}
    { Xb }{\def\lsize{\Bigg}\def\rsize{\big}}
    { XB }{\def\lsize{\Bigg}\def\rsize{\Big}}
    { Xx }{\def\lsize{\Bigg}\def\rsize{\bigg}}
}
 \str_case:nnF { #2 }
    {
    { s }{\def\ldel{[}\def\rdel{]}}
    { r }{\def\ldel{(}\def\rdel{)}}
    { b }{\def\ldel{\lbrace}\def\rdel{\rbrace}}
    { v }{\def\ldel{|}\def\rdel{|}}
    { a }{\def\ldel{\langle}\def\rdel{\rangle}}
    { dv }{\def\ldel{\|}\def\rdel{\|}}
    { rs }{\def\ldel{(}\def\rdel{]}}
    { sr }{\def\ldel{[}\def\rdel{)}}
    { rb }{\def\ldel{(}\def\rdel{\rbrace}}
    { sb }{\def\ldel{[}\def\rdel{\rbrace}}
    { br }{\def\ldel{\lbrace}\def\rdel{)}}
    { bs }{\def\ldel{\lbrace}\def\rdel{]}}
    { ra }{\def\ldel{(}\def\rdel{\rangle}}
    { sa }{\def\ldel{[}\def\rdel{\rangle}}
    { ba }{\def\ldel{\lbrace}\def\rdel{\rangle}}
    { ar }{\def\ldel{\langle}\def\rdel{)}}
    { as }{\def\ldel{\langle}\def\rdel{]}}
    { ab }{\def\ldel{\langle}\def\rdel{\rbrace}}
    { rv }{\def\ldel{(}\def\rdel{|}}
    { sv }{\def\ldel{[}\def\rdel{|}}
    { bv }{\def\ldel{\lbrace}\def\rdel{|}}
    { av }{\def\ldel{\langle}\def\rdel{|}}
    { vr }{\def\ldel{|}\def\rdel{)}}
    { vs }{\def\ldel{|}\def\rdel{]}}
    { vb }{\def\ldel{|}\def\rdel{\rbrace}}
    { va }{\def\ldel{|}\def\rdel{\rangle}}
    { rdv }{\def\ldel{(}\def\rdel{\|}}
    { sdv }{\def\ldel{[}\def\rdel{\|}}
    { bdv }{\def\ldel{\lbrace}\def\rdel{\|}}
    { adv }{\def\ldel{\langle}\def\rdel{\|}}
    { vdv }{\def\ldel{|}\def\rdel{\|}}
    { dvr }{\def\ldel{\|}\def\rdel{)}}
    { dvs }{\def\ldel{\|}\def\rdel{]}}
    { dvb }{\def\ldel{\|}\def\rdel{\rbrace}}
    { dva }{\def\ldel{\|}\def\rdel{\rangle}}
    { dvv }{\def\ldel{\|}\def\rdel{|}}
    { lr }{\def\ldel{(}\def\rdel{\@empty}}
    { rr }{\def\ldel{)}\def\rdel{\@empty}}
    { ls }{\def\ldel{[}\def\rdel{\@empty}}
    { rs }{\def\ldel{]}\def\rdel{\@empty}}
    { lb }{\def\ldel{\lbrace}\def\rdel{\@empty}}
    { rb }{\def\ldel{\rbrace}\def\rdel{\@empty}}
    { la }{\def\ldel{\langle}\def\rdel{\@empty}}
    { ra }{\def\ldel{\rangle}\def\rdel{\@empty}}
    { ov }{\def\ldel{|}\def\rdel{\@empty}}
    { odv }{\def\ldel{\|}\def\rdel{\@empy}}
    { ssi }{\def\ldel{[}\def\rdel{\mathclose{[}}}
    { sis }{\def\ldel{\mathopen{]}}\def\rdel{]}}
    { sisi }{\def\ldel{\mathopen{]}}\def\rdel{\mathclose{[}}}
}
\ifx\rdel\@empty
    \lsize\ldel
\else
    \lsize\ldel #3 \rsize\rdel
\fi
}
\ExplSyntaxOff
%\ignopt{greek}

\begin{document}
$$\delim[lr]{r}{\fr{a}{b}}$$
\end{document}

And then the strangest thing happened:

./delim_attempt.tex:3: Undefined control sequence.
<recently read> \ExplSyntaxOn 

l.3 \ExplSyntaxOn

And I'm like, "WHAT????" How is that possible?

Update: Having taken the comments below into account, I corrected [r] to {r}, loaded xparse, and updated Expl, meaning \str_case:nnF now works. So the code has been changed accordingly. Btw suspecting the spaces might be troublesome I tried removing them (e.g. { lr }->{lr}), and the wrong typesetting of the code above is still there. That is, as it is, the code gives:

./delim_attempt.tex:87: Undefined control sequence.
\\delim ...\@empty\lsize \ldel \else \lsize \ldel 
                                                  #3\rsize \rdel \fi 
l.87 $$\delim[lr]{r}{\frac{a}{b}}
                                 $$

and by commenting the \lsize\ldel #3 \rsize\rdel in the \else branch, I get to typeset:

rsrbvadvrssrrbsbbrbsrasabaarasabrvsvbvavvrvsvbvardvsdvbdvadvvdvdvrdvsdvbdvadvvlrrrlsrslbrblaraovod

which of course simpy isn't what I want.

Answer 1

You're misusing expl3; here's a conforming version:

\documentclass[a4paper]{report}
\usepackage{amsthm,xparse}
\ExplSyntaxOn
\NewDocumentCommand{\delim}{ O{lr} m m }
 {
  \mickg_delim:nnn { #1 } { #2 } { #3 }
 }

\tl_new:N \l_mickg_lsize_tl
\tl_new:N \l_mickg_rsize_tl
\tl_new:N \l_mickg_ldel_tl
\tl_new:N \l_mickg_rdel_tl

\cs_new_protected:Npn \mickg_delim:nnn #1 #2 #3
 {
  \str_case:nnF { #1 }
   {
    { b }{\tl_set:Nn \l_mickg_lsize_tl {\big} \tl_set:Nn \l_mickg_rsize_tl {\big}}
    { B }{\tl_set:Nn \l_mickg_lsize_tl {\Big} \tl_set:Nn \l_mickg_rsize_tl {\Big}}
    { x }{\tl_set:Nn \l_mickg_lsize_tl {\bigg} \tl_set:Nn \l_mickg_rsize_tl {\bigg}}
    { X }{\tl_set:Nn \l_mickg_lsize_tl {\Bigg} \tl_set:Nn \l_mickg_rsize_tl {\Bigg}}
    { lr }{\tl_set:Nn \l_mickg_lsize_tl {\left} \tl_set:Nn \l_mickg_rsize_tl {\right}}
    { bB }{\tl_set:Nn \l_mickg_lsize_tl {\big} \tl_set:Nn \l_mickg_rsize_tl {\Big}}
    { Bb }{\tl_set:Nn \l_mickg_lsize_tl {\Big} \tl_set:Nn \l_mickg_rsize_tl {\big}}
    { xb }{\tl_set:Nn \l_mickg_lsize_tl {\bigg} \tl_set:Nn \l_mickg_rsize_tl {\big}}
    { xB }{\tl_set:Nn \l_mickg_lsize_tl {\bigg} \tl_set:Nn \l_mickg_rsize_tl {\Big}}
    { bx }{\tl_set:Nn \l_mickg_lsize_tl {\big} \tl_set:Nn \l_mickg_rsize_tl {\bigg}}
    { Bx }{\tl_set:Nn \l_mickg_lsize_tl {\Big} \tl_set:Nn \l_mickg_rsize_tl {\bigg}}
    { bX }{\tl_set:Nn \l_mickg_lsize_tl {\big} \tl_set:Nn \l_mickg_rsize_tl {\Bigg}}
    { BX }{\tl_set:Nn \l_mickg_lsize_tl {\Big} \tl_set:Nn \l_mickg_rsize_tl {\Bigg}}
    { xX }{\tl_set:Nn \l_mickg_lsize_tl {\bigg} \tl_set:Nn \l_mickg_rsize_tl {\Bigg}}
    { Xb }{\tl_set:Nn \l_mickg_lsize_tl {\Bigg} \tl_set:Nn \l_mickg_rsize_tl {\big}}
    { XB }{\tl_set:Nn \l_mickg_lsize_tl {\Bigg} \tl_set:Nn \l_mickg_rsize_tl {\Big}}
    { Xx }{\tl_set:Nn \l_mickg_lsize_tl {\Bigg} \tl_set:Nn \l_mickg_rsize_tl {\bigg}}
   }
   { \tl_clear:N \l_mickg_lsize_tl \tl_clear:N \l_mickg_rsize_tl }
  \str_case:nnF { #2 }
   {
    { s }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_set:Nn \l_mickg_rdel_tl {]}}
    { r }{\tl_set:Nn \l_mickg_ldel_tl {(} \tl_set:Nn \l_mickg_rdel_tl {)}}
    { b }{\tl_set:Nn \l_mickg_ldel_tl {\lbrace} \tl_set:Nn \l_mickg_rdel_tl {\rbrace}}
    { v }{\tl_set:Nn \l_mickg_ldel_tl {|} \tl_set:Nn \l_mickg_rdel_tl {|}}
    { a }{\tl_set:Nn \l_mickg_ldel_tl {\langle} \tl_set:Nn \l_mickg_rdel_tl {\rangle}}
    { dv }{\tl_set:Nn \l_mickg_ldel_tl {\|} \tl_set:Nn \l_mickg_rdel_tl {\|}}
    { rs }{\tl_set:Nn \l_mickg_ldel_tl {(} \tl_set:Nn \l_mickg_rdel_tl {]}}
    { sr }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_set:Nn \l_mickg_rdel_tl {)}}
    { rb }{\tl_set:Nn \l_mickg_ldel_tl {(} \tl_set:Nn \l_mickg_rdel_tl {\rbrace}}
    { sb }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_set:Nn \l_mickg_rdel_tl {\rbrace}}
    { br }{\tl_set:Nn \l_mickg_ldel_tl {\lbrace} \tl_set:Nn \l_mickg_rdel_tl {)}}
    { bs }{\tl_set:Nn \l_mickg_ldel_tl {\lbrace} \tl_set:Nn \l_mickg_rdel_tl {]}}
    { ra }{\tl_set:Nn \l_mickg_ldel_tl {(} \tl_set:Nn \l_mickg_rdel_tl {\rangle}}
    { sa }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_set:Nn \l_mickg_rdel_tl {\rangle}}
    { ba }{\tl_set:Nn \l_mickg_ldel_tl {\lbrace} \tl_set:Nn \l_mickg_rdel_tl {\rangle}}
    { ar }{\tl_set:Nn \l_mickg_ldel_tl {\langle} \tl_set:Nn \l_mickg_rdel_tl {)}}
    { as }{\tl_set:Nn \l_mickg_ldel_tl {\langle} \tl_set:Nn \l_mickg_rdel_tl {]}}
    { ab }{\tl_set:Nn \l_mickg_ldel_tl {\langle} \tl_set:Nn \l_mickg_rdel_tl {\rbrace}}
    { rv }{\tl_set:Nn \l_mickg_ldel_tl {(} \tl_set:Nn \l_mickg_rdel_tl {|}}
    { sv }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_set:Nn \l_mickg_rdel_tl {|}}
    { bv }{\tl_set:Nn \l_mickg_ldel_tl {\lbrace} \tl_set:Nn \l_mickg_rdel_tl {|}}
    { av }{\tl_set:Nn \l_mickg_ldel_tl {\langle} \tl_set:Nn \l_mickg_rdel_tl {|}}
    { vr }{\tl_set:Nn \l_mickg_ldel_tl {|} \tl_set:Nn \l_mickg_rdel_tl {)}}
    { vs }{\tl_set:Nn \l_mickg_ldel_tl {|} \tl_set:Nn \l_mickg_rdel_tl {]}}
    { vb }{\tl_set:Nn \l_mickg_ldel_tl {|} \tl_set:Nn \l_mickg_rdel_tl {\rbrace}}
    { va }{\tl_set:Nn \l_mickg_ldel_tl {|} \tl_set:Nn \l_mickg_rdel_tl {\rangle}}
    { rdv }{\tl_set:Nn \l_mickg_ldel_tl {(} \tl_set:Nn \l_mickg_rdel_tl {\|}}
    { sdv }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_set:Nn \l_mickg_rdel_tl {\|}}
    { bdv }{\tl_set:Nn \l_mickg_ldel_tl {\lbrace} \tl_set:Nn \l_mickg_rdel_tl {\|}}
    { adv }{\tl_set:Nn \l_mickg_ldel_tl {\langle} \tl_set:Nn \l_mickg_rdel_tl {\|}}
    { vdv }{\tl_set:Nn \l_mickg_ldel_tl {|} \tl_set:Nn \l_mickg_rdel_tl {\|}}
    { dvr }{\tl_set:Nn \l_mickg_ldel_tl {\|} \tl_set:Nn \l_mickg_rdel_tl {)}}
    { dvs }{\tl_set:Nn \l_mickg_ldel_tl {\|} \tl_set:Nn \l_mickg_rdel_tl {]}}
    { dvb }{\tl_set:Nn \l_mickg_ldel_tl {\|} \tl_set:Nn \l_mickg_rdel_tl {\rbrace}}
    { dva }{\tl_set:Nn \l_mickg_ldel_tl {\|} \tl_set:Nn \l_mickg_rdel_tl {\rangle}}
    { dvv }{\tl_set:Nn \l_mickg_ldel_tl {\|} \tl_set:Nn \l_mickg_rdel_tl {|}}
    { lr }{\tl_set:Nn \l_mickg_ldel_tl {(} \tl_clear:N \l_mickg_rdel_tl}
    { rr }{\tl_set:Nn \l_mickg_ldel_tl {)} \tl_clear:N \l_mickg_rdel_tl}
    { ls }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_clear:N \l_mickg_rdel_tl}
    { rs }{\tl_set:Nn \l_mickg_ldel_tl {]} \tl_clear:N \l_mickg_rdel_tl}
    { lb }{\tl_set:Nn \l_mickg_ldel_tl {\lbrace} \tl_clear:N \l_mickg_rdel_tl}
    { rb }{\tl_set:Nn \l_mickg_ldel_tl {\rbrace} \tl_clear:N \l_mickg_rdel_tl}
    { la }{\tl_set:Nn \l_mickg_ldel_tl {\langle} \tl_clear:N \l_mickg_rdel_tl}
    { ra }{\tl_set:Nn \l_mickg_ldel_tl {\rangle} \tl_clear:N \l_mickg_rdel_tl}
    { ov }{\tl_set:Nn \l_mickg_ldel_tl {|} \tl_clear:N \l_mickg_rdel_tl}
    { odv }{\tl_set:Nn \l_mickg_ldel_tl {\|} \tl_clear:N \l_mickg_rdel_tl}
    { ssi }{\tl_set:Nn \l_mickg_ldel_tl {[} \tl_set:Nn \l_mickg_rdel_tl {\mathclose{[}}}
    { sis }{\tl_set:Nn \l_mickg_ldel_tl {\mathopen{]}} \tl_set:Nn \l_mickg_rdel_tl {]}}
    { sisi }{\tl_set:Nn \l_mickg_ldel_tl {\mathopen{]}} \tl_set:Nn \l_mickg_rdel_tl {\mathclose{[}}}
   }
   { \tl_set:Nn \l_mickg_ldel_tl {(} \tl_set:Nn \l_mickg_rdel_tl {)} } % some default
  \tl_if_empty:NTF \l_mickg_rdel_tl
   {
    \l_mickg_lsize_tl \l_mickg_ldel_tl #3
   }
   {
    \l_mickg_lsize_tl \l_mickg_ldel_tl #3 \l_mickg_rsize_tl \l_mickg_rdel_tl
   }
 }
\ExplSyntaxOff

\begin{document}
\[
\delim[lr]{r}{\frac{a}{b}}
\]
\end{document}

Check texdoc interface3 for more information. The \str_case:nnF function requires three arguments: the first is the string to check, the second is a set of pairs {<string>}{<code>} for deciding what to do; the final argument is for code to be executed in the “none of the above” case.

Please, don't use $$ in LaTeX. It's wrong.

Answer 2

Here is an alternative to egreg's answer, which mainly aims to tidy up your proposed syntax a bit and adds some warnings and error messages.

Notice also that there is still a problem in egreg's answer with cases like \delim{ssi}{\frac{a}{b}} that carries over from your code to his (the \mathopen and \mathclose are the culprits).

In this version, both the size and the delimiter options take two letters. If only one letter is given, it is used for both delimiters (the only exceptions are "l" and "r" for size, either of which will give a \left and \right pairing).

For delimiter types, lowercase letters give the standard orientation and uppercase letters give the inverted orientation (e.g. "rr" will give "(" and ")", while "RR" will give ")" and "(" ). The exception are "v" and "V", which give a single bar and a double bar, respectively. When "-" is given, the corresponding delimiter is omitted.

Both the specification arguments are optional and default to "lr" and "rr", respectively.

\documentclass[a4paper]{report}
\usepackage{xparse,pgffor}

\ExplSyntaxOn

\tl_new:N \l_mickg_left_delim_size_tl
\tl_new:N \l_mickg_right_delim_size_tl
\tl_new:N \l_mickg_left_delim_tl
\tl_new:N \l_mickg_right_delim_tl

\msg_new:nnnn { MickG } { Left ~ delimiter ~ size }
  { Incorrectly ~ specified ~ left ~ delimiter ~ size.  } {}
\msg_new:nnnn { MickG } { Right ~ delimiter ~ size }
  { Incorrectly ~ specified ~ right ~ delimiter ~ size. } {}
\msg_new:nnnn { MickG } { Delimiter ~ size }
  { Incorrect ~ delimiter ~ size ~ specification. } {}
\msg_new:nnnn { MickG } { Left ~ delimiter }
  { Incorrectly ~ specified ~ left ~ delimiter.  } {}
\msg_new:nnnn { MickG } { Right ~ delimiter }
  { Incorrectly ~ specified ~ right ~ delimiter. } {}
\msg_new:nnnn { MickG } { Delimiters }
  { Incorrect ~ delimiter ~ specification. } {}

\cs_new_protected:Npn \mickg_delims_size_set:n #1
  {
    % Test if one or two letters provided.
    \int_case:nnF { \tl_count:n {#1} }
      {
        { \c_two } { \tl_set:Nn \l_tmpa_tl {#1}   } % If two, use both.
        { \c_one } { \tl_set:Nn \l_tmpa_tl {#1#1} } % If one, use it twice.
      }
      % If not one or two letters, set to 'lr' and issue error.
      {
        \tl_set:Nn \l_tmpa_tl { lr }
        \msg_error:nn { MickG } { Delimiter ~ size }
      }
    % Ensure \left is always paired with \right
    \str_case_x:nnTF { \l_tmpa_tl }
      {
        { lr } { \prg_do_nothing: }
        { ll } { \msg_warning:nn { MickG } { Delimiter ~ size } } % Warn if 'l' was copied.
        { rr } { \msg_warning:nn { MickG } { Delimiter ~ size } } % Warn if 'r' was copied.
      }
      {
        \tl_set_eq:NN \l_mickg_left_delim_size_tl  \left
        \tl_set_eq:NN \l_mickg_right_delim_size_tl \right
      }
      % If not using \left and \rigth, set sizes independently.
      {
        \str_case_x:nnF { \tl_head:N \l_tmpa_tl }
          {
            { b } { \tl_set_eq:NN \l_mickg_left_delim_size_tl \bigl  }
            { B } { \tl_set_eq:NN \l_mickg_left_delim_size_tl \Bigl  }
            { x } { \tl_set_eq:NN \l_mickg_left_delim_size_tl \biggl }
            { X } { \tl_set_eq:NN \l_mickg_left_delim_size_tl \Biggl }
          }
          % Error if no match; default to empty token list.
          { 
            \tl_clear:N \l_mickg_left_delim_size_tl
            \msg_error:nn { MickG } { Left ~ delimiter ~ size } 
          } 
        \str_case_x:nnF { \tl_tail:N \l_tmpa_tl }
          {
            { b } { \tl_set_eq:NN \l_mickg_right_delim_size_tl \bigr   }
            { B } { \tl_set_eq:NN \l_mickg_right_delim_size_tl \Bigr   }
            { x } { \tl_set_eq:NN \l_mickg_right_delim_size_tl \biggr  }
            { X } { \tl_set_eq:NN \l_mickg_right_delim_size_tl \Biggr  }
          }
          % Error if no match; default to empty token list. 
          { 
            \tl_clear:N \l_mickg_right_delim_size_tl
            \msg_error:nn { MickG } { Right ~ delimiter ~ size } 
          }
      }
  }

\cs_new_protected:Npn \mickg_delims_type_set:n #1
  {
    % Test if one or two letters provided.
    \int_case:nnF { \tl_count:n {#1} }
      {
        { \c_two } { \tl_set:Nn \l_tmpa_tl {#1}   } % If two, use both.
        { \c_one } { \tl_set:Nn \l_tmpa_tl {#1#1} } % If one, use it twice.
      }
      % If not one or two letters, set to 'rr' and issue error.
      {
        \tl_set:Nn \l_tmpa_tl { rr }
        \msg_error:nn { MickG } { Delimiters }
      }
    % Test first letter.
    \str_case_x:nnF { \tl_head:N \l_tmpa_tl }
      {
        { r } { \tl_set_eq:NN \l_mickg_left_delim_tl (       }
        { R } { \tl_set_eq:NN \l_mickg_left_delim_tl )       }
        { s } { \tl_set_eq:NN \l_mickg_left_delim_tl [       }
        { S } { \tl_set_eq:NN \l_mickg_left_delim_tl ]       }
        { b } { \tl_set_eq:NN \l_mickg_left_delim_tl \lbrace }
        { B } { \tl_set_eq:NN \l_mickg_left_delim_tl \rbrace }
        { a } { \tl_set_eq:NN \l_mickg_left_delim_tl \langle }
        { A } { \tl_set_eq:NN \l_mickg_left_delim_tl \rangle }
        { v } { \tl_set_eq:NN \l_mickg_left_delim_tl |       }
        { V } { \tl_set_eq:NN \l_mickg_left_delim_tl \|      }
        { - } { \tl_set_eq:NN \l_mickg_left_delim_tl .       }
      }
      % Error if no match; default to "(" 
      { 
        \tl_set_eq:NN \l_mickg_left_delim_tl (
        \msg_error:nn { MickG } { Left ~ delimiter } 
      }
    % Test second letter.
    \str_case_x:nnF { \tl_tail:N \l_tmpa_tl }
      {
        { r } { \tl_set_eq:NN \l_mickg_right_delim_tl )       }
        { R } { \tl_set_eq:NN \l_mickg_right_delim_tl (       }
        { s } { \tl_set_eq:NN \l_mickg_right_delim_tl ]       }
        { S } { \tl_set_eq:NN \l_mickg_right_delim_tl [       }
        { b } { \tl_set_eq:NN \l_mickg_right_delim_tl \rbrace }
        { B } { \tl_set_eq:NN \l_mickg_right_delim_tl \lbrace }
        { a } { \tl_set_eq:NN \l_mickg_right_delim_tl \rangle }
        { A } { \tl_set_eq:NN \l_mickg_right_delim_tl \langle }
        { v } { \tl_set_eq:NN \l_mickg_right_delim_tl |       }
        { V } { \tl_set_eq:NN \l_mickg_right_delim_tl \|      }
        { - } { \tl_set_eq:NN \l_mickg_right_delim_tl .       }
      }
      % Error if no match; default to ")" 
      { 
        \tl_set_eq:NN \l_mickg_right_delim_tl )
        \msg_error:nn { MickG } { Right ~ delimiter } 
      }
  }

\cs_new_protected:Npn \mickg_delims_print:nnn #1 #2 #3
  {
    \mickg_delims_size_set:n {#1}
    \mickg_delims_type_set:n {#2}
    \l_mickg_left_delim_size_tl  \l_mickg_left_delim_tl
    #3
    \l_mickg_right_delim_size_tl \l_mickg_right_delim_tl
  }

\NewDocumentCommand \delim { O{ lr } O{ r } m }
  {
    \mickg_delims_print:nnn {#1} {#2} {#3}
  }

\ExplSyntaxOff

\begin{document}

% Test by iterating through the combinations.
\foreach \x in {b,B,x,X} {
  \foreach \y in {b,B,x,X} {
    \foreach \z in {r,R,s,S,b,B,a,A,v,V,-} {
      \foreach \w in {r,R,s,S,b,B,a,A,v,V,-} {
        % \delim does not expand the optional arguments but this could
        % easily be changed; I expand them here with an \edef
        \edef\foo{\noexpand\delim[\x\y][\z\w]}
        $\displaystyle\foo{\frac{a}{b}}$\quad
      }
    }
  }
}

\end{document}

Answer 3

1) xparse or a similar package is needed for Expl.

2) The second argument is mandatory, so \delim[lr]{r}{\fr{a}{b}} is right. By the way, \fr=\displaystyle\frac.

3) You need a "non of those cases" code, otherwise strange errors are produced with \str_case:nnF; alternately, you can use \str_case:nn.

4) Btw, without updating Expl I don't know if those two functions are both available. One thing is sure: \str_case:nnF is called \str_case:nnn, or such a name is borne by a similar function.

Now I'll just test the rest of this command's possibilities.