mjhjmtu

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

I tried this:

 newcommandfork[6]
 begincenter
 begintikzcd
 #1 arrow[r, "#2"] & #3 arrow[r,shift left, "#4"] 
 arrow[r,shift right, "#5"'] & #6
 endtikzcd
 endcenter 

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 &:

newcommandfork[6]%
 begintikzcd[ampersand replacement=&]
 #1 arrow[r, "#2"] & #3 arrow[r,shift left, "#4"] 
 arrow[r,shift right, "#5"'] & #6
 endtikzcd%

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

[
forkKkAf0B
]

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.

documentclassarticle
usepackageamsmath
usepackagetikz-cd
usepackagexparse

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,
 

NewDocumentCommandforkm
 
 keys_set:nn soap/forks #1 
 begintikzcd[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
 
 endtikzcd
 

ExplSyntaxOff

begindocument

begingather
forks=A,t=B,e=K,u=f,d=0,c=k
\
fork*,s=A,t=B,e=K,u=f,d=0,c=k
endgather

enddocument