A user of one of my classes exceptionally wants to modify something that is hard-coded in a \cs_new_protected:Nn function. I can give her the means to redefine this function using \cs_set_protected:Nn (between \ExplSyntaxOn and \ExplSyntaxOff); but it would be more practical to rely on a hook. Is this possible?

  • You can but if you are defining the command better to add a specific hook rather than rely on the generic hook mechanism to patch it. Commented Jun 13 at 8:15
  • My command is wonderful: apart from this exceptional request, there's no need to modify it ;) So what's the trick to patching it using the generic hook mechanism? Commented Jun 13 at 8:26
  • It should "just work" shouldn't? as long as you do it inside ExplSyntaxOn but haven't tried, perhaps I misunderstand the question. Commented Jun 13 at 8:30
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    well it depends on what must be changed inside the function. No generic hook can replace e.g. a string in the middle of some complicated code. Commented Jun 13 at 8:37
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    As you are using \cs_new_protected:Nn, this is presumably a code-level function: that really means you need a documented API to extend it
    – Joseph Wright
    Commented Jun 13 at 8:40

1 Answer 1


It should just work with no special things needed:

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But you shouldn't be patching L3 level commands in this way (and certainly not internal __ commands) If you are defining the command and want to allow an extension point you can add that as part of the definition, either a hook or a socket or simply a command initially empty that the user can redefine.

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