I'm trying to store a value in a macro that is constructed from the arguments of another macro, but I must be missing something because the macro is not created.

MWE:

documentclassarticle

RequirePackagepgfkeys

newcommandzkeys[1]pgfkeys/prefix/.cd,#1

newcommandzsetup[2]
 zkeys
 #1/.store in=z#2,
 

zsetupkeyastoragea
zsetupkeybstorageb

zkeys
 keya=test,


begindocument

zstoragea % Undefined control sequence.

enddocument

New solution

Another, more flexible approach is to extend the handlers pgfkeys accepts by a new one .store in cs which does basically the same as .store in, but doesn't take a complete control sequence name as value but a list of characters from that the final control sequence is built. So the following calls would be equal:

foo/.store in=mymacro
foo/.store in cs=mymacro

The full example then looks like

documentclassarticle
RequirePackagepgfkeys

newcommandzkeys[1]pgfkeys/prefix/.cd,#1

pgfkeys/handlers/.store in cs/.code=pgfkeysalso%
 pgfkeyscurrentpath/.code=expandafterdefcsname#1endcsname##1%


newcommandzsetup[2]%
 zkeys
 #1/.store in cs=z#2,
 %

zsetupkeyastoragea
zsetupkeybstorageb

zkeys
 keya=test,


begindocument

zstoragea % Undefined control sequence.

enddocument

Old solution

When you type z#2, TeX parses this as the command name z followed by the tokens inserted from the second argument. If you want to build a new control sequence from a series of characters/tokens, you have to use the sequence csname ...endcsname, where ... would be z#2 in this case.

However, in this specific situation store in=csname z#2endcsname wouldn't work, because the csname call must be expanded exactly once to build the actual new control sequence from the characters, but not more than once, otherwise the built macro would be tried to expanded itself.

A possible solution is to wrap the whole key definitions into an edef, prefix all commands in it by noexpand, and use unexpandedexpandafter... in each place, we want exactly one expansion step:

newcommandzsetup[2]
 edeftemp%
 noexpandzkeys
 #1/.store in=unexpandedexpandaftercsname z#2endcsname,
 %
 temp

zstoragea will then expand to test.

Here are two solutions. They both take care not to expand the first argument of zsetup before passing it to zkeys, nor to define or overwrite any macro in the current group as a side effect.

First solution

documentclassarticle
usepackagepgfkeys

newcommandzkeys[1]pgfkeys/prefix/.cd,#1

newcommand*zsetup[2]%
 begingroup
 edefargunexpanded#1/.store in=%
 expandafternoexpandcsname z#2endcsname%
 expandafter
 endgroup
 expandafterzkeysexpandafterarg%


zsetupkeyastoragea
zsetupkeybstorageb

zkeys
 keya=test,


begindocument

zstoragea % Print 'test'

enddocument

Second solution

Same code, except for the definition of zsetup:

newcommand*zsetup[2]%
 begingroup
 deftmp##1zkeys#1/.store in=##1%
 expandafterexpandafterexpandafter
 endgroup
 expandaftertmpcsname z#2endcsname