Krdytkyu

I am trying to create a macro to easily draw forks as this one:

I tried this:

 newcommand{fork}[6]{

 begin{center}

 begin{tikzcd}

 #1 arrow[r, "#2"] & #3 arrow[r,shift left, "#4"] 

 arrow[r,shift right, "#5"'] & #6

 end{tikzcd}

 end{center} 

}

But I get an error "Single ampersand used with wrong catcode."

You don't really want to use the center environment, but the main problem is the &:

newcommand{fork}[6]{%

  begin{tikzcd}[ampersand replacement=&]

  #1 arrow[r, "#2"] & #3 arrow[r,shift left, "#4"] 

  arrow[r,shift right, "#5"'] & #6

  end{tikzcd}%

}

The key ampersand replacement is necessary each time a TikZ matrix (such as a tikzcd diagram) is used in the argument to another command.

Use it in a display environment, say

[

fork{K}{k}{A}{f}{0}{B}

]

Here's another suggestion for coping with both equalizer and coequalizer diagrams. Each item in the diagram is defined with a key-value approach (use different keys, if you like); a * tells to draw a coequalizer diagram. Like in most key-value approaches, the order of the keys is irrelevant.

documentclass{article}

usepackage{amsmath}

usepackage{tikz-cd}

usepackage{xparse}



ExplSyntaxOn



keys_define:nn { soap/forks }

 {

  s .tl_set:N = l__soap_fork_source_tl,

  t .tl_set:N = l__soap_fork_target_tl,

  e .tl_set:N = l__soap_fork_end_tl,

  u .tl_set:N = l__soap_fork_up_tl,

  d .tl_set:N = l__soap_fork_down_tl,

  c .tl_set:N = l__soap_fork_center_tl,

  * .bool_set:N = l__soap_fork_co_bool,

  * .default:n = true,

 }



NewDocumentCommand{fork}{m}

 {

  keys_set:nn { soap/forks } { #1 }

  begin{tikzcd}[ampersand~replacement=&]

  bool_if:NF l__soap_fork_co_bool

   {% false, equalizer

    l__soap_fork_end_tl

    arrow[r,"l__soap_fork_center_tl"] &

   }

  l__soap_fork_source_tl

  arrow[r,shift~left,"l__soap_fork_up_tl"]

  arrow[r,shift~right,"l__soap_fork_down_tl",swap] &

  l__soap_fork_target_tl

  bool_if:NT l__soap_fork_co_bool

   {% true, coequalizer

    arrow[r,"l__soap_fork_center_tl"] &

    l__soap_fork_end_tl

   }

  end{tikzcd}

 }



ExplSyntaxOff



begin{document}



begin{gather}

fork{s=A,t=B,e=K,u=f,d=0,c=k}

\

fork{*,s=A,t=B,e=K,u=f,d=0,c=k}

end{gather}



end{document}