Ygthkb

The version A part in the following example is to define header/footer by macros from package fancyhdr.

Though this version is workable, it's a awkward achievement because the same part of code(ifnum...) is written twice.

Except the optional argument #2, all other code is exactly the same. To improve it, I tried the version B below, but it does not work at all.

It will be awfull especially when the same part of code is large.

So, to sumarize, is there a better way of handling optional arguments (to be there or not) to avoid heavy duplicate code.

documentclass{article}

usepackage{geometry,fancyhdr,xparse}

geometry{showframe}



begin{document}

first pageparvspace{80em} second page

pagestyle{fancy}

fancyhf{}



%Version A: workable %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

NewDocumentCommand{firstpage}{momm}{%

  IfNoValueTF{#2}

  {#1{%

      ifnumvalue{page}=1

      #3% content of header/footer only at the first page

      else

      #4% content of header/footer at the rest pages

      fi

    }

  }

  {#1[#2]% the only diferrence between the two branches of IfNoValueTF is, whether introducing the optional argument #2

    {%

      ifnumvalue{page}=1

      #3% content of header/footer only at the first page

      else

      #4% content of header/footer at the rest pages

      fi

    }

  }

}%firstpage



% Test

firstpage{fancyhead}[c]{only at the first page}{at rest pages}

firstpage{rhead}{r first}{r rest}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



% Version B: not workable %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

NewDocumentCommand{firstpageA}{momm}{%

   #1 IfNoValueTF{#2}{}{[#2]}

      ifnumvalue{page}=1

      #3% content of header/footer only at the first page

      else

      #4% content of header/footer at the rest pages

      fi

}%firstpageA



% Test

firstpageA{fancyhead}[c]{only at the first page}{at rest pages}

firstpageA{rhead}{r first}{r rest}  



end{document}

I believe it's not useful to try and fit an awkward syntax into a command like that.

It's more rewarding to spend a bit more time in devising a friendlier syntax. Here I propose

sethf{

  head = <contents of all headers, except possibly the first>,

  foot = <contents of all footers, except possibly the first>,

  pos = <l|c|r>,

  first = <possible exception for first header/footer>,

}

Only one among head or foot should be used in a call of sethf.

documentclass{article}

usepackage{fancyhdr,xparse}



usepackage[paper=a6paper]{geometry} % smaller pictures

usepackage{lipsum} % mock text



pagestyle{fancy}

fancyhf{}



ExplSyntaxOn

NewDocumentCommand{sethf}{m}

 {

  tl_clear:N l__lyl_hf_pos_tl

  tl_clear:N l__lyl_hf_first_tl

  tl_clear:N l__lyl_hf_headfoot_tl

  keys_set:nn { lyl/hf } { #1 }

  lyl_hf_set:

 }



keys_define:nn { lyl/hf }

 {

  head  .code:n   = __lyl_hf_aux:Nn fancyhead { #1 },

  foot  .code:n   = __lyl_hf_aux:Nn fancyfoot { #1 },

  pos   .tl_set:N = l__lyl_hf_pos_tl,

  first .tl_set:N = l__lyl_hf_first_tl,

 }



tl_new:N l__lyl_hf_headfoot_tl



cs_new_protected:Nn __lyl_hf_aux:Nn

 {

  cs_set_eq:NN __lyl_hf_temp:w #1

  tl_set:Nn l__lyl_hf_headfoot_tl { #2 }

 }



cs_new_protected:Nn lyl_hf_set:

 {

  tl_if_empty:NT l__lyl_hf_first_tl

   {

    tl_set_eq:NN l__lyl_hf_first_tl l__lyl_hf_headfoot_tl

   }

  __lyl_hf_set:NVVV 

   __lyl_hf_temp:w        % fancyhead or fancyfoot

   l__lyl_hf_pos_tl       % position

   l__lyl_hf_first_tl     % first page

   l__lyl_hf_headfoot_tl  % other pages

 }

cs_new_protected:Nn __lyl_hf_set:Nnnn

 {

  #1 [ #2 ] { int_compare:nTF { value{page} = 1 } { #3 } { #4 } }

 }

cs_generate_variant:Nn __lyl_hf_set:Nnnn { NVVV }

ExplSyntaxOff



sethf{

  head = other pages,

  first = first page,

  pos = c,

}



sethf{pos = r, foot=thepage}



begin{document}



lipsum[1-2]



end{document}

To answer your question: use a macro for the duplicate code.

documentclass{article}

usepackage{fancyhdr,xparse}



usepackage[paper=a6paper]{geometry} % smaller pictures

usepackage{lipsum} % mock text



pagestyle{fancy}

fancyhf{}



NewDocumentCommand{firstpage}{momm}{%

  IfNoValueTF{#2}

   {setfirstpage{#1}{#3}{#4}}

   {setfirstpage{#1[#2]}{#3}{#4}}%

}

NewDocumentCommand{setfirstpage}{mmm}

 {

  #1{ifnumvalue{page}=1 #2else #3fi}

 }



firstpage{fancyhead}[c]{first page}{other pages}

% can also be firstpage{chead}{first page}{other pages}



begin{document}



lipsum[1-2]



end{document}