Is there some facility in the LaTeX3 kernel to modify existing commands like etoolbox's \preto \appto and \patchcmd do?
No. The expl3 language is useful for both programming with existing LaTeX2e code and creating new code, and so there are some facilities which are '2e-facing'. However, patching existing commands is not an area we wish to encourage: it is much better if interfaces are provided for altering code flows, etc. Patching for LaTeX2e can as you note be achieved using etoolbox, whilst it should not be used for expl3 code at all (rather discussions should be undertaken to provide appropriate interfaces).
Note that \preto and appto are more-or-less the same at \tl_put_left:Nn and \tl_put_right:Nn, but there are no equivalents of \pretocmd/\apptocmd.
answered Sep 10 '15 at 12:33
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I tried to
\tl_put_left:Nn \existingmacro { \new \directives }without success. Am I right to assume they only work on initialized token lists and not for arbitrary macros? – XZS Sep 10 '15 at 12:46 -
@XZS As the name says, it's adding to the left of an existing token list (macro): the 'all vars must be declared' position is pretty clear in
expl3. – Joseph Wright♦ Sep 10 '15 at 12:58 -
1@JosephWright (off-topic question): it seems
LaTeX3parlance often refers totoken listswhich are not the same as thetoken list registers or variablesfromTeX. Am I wrong, and if not isn't it a bit confusing ? – user4686 Sep 10 '15 at 13:10 -
2@jfbu I guess I should have put
tl varin the above comment: we usetl varto refer to a TeX macro used as a storage bin. We don't usetoks (token list registers)at all inexpl3(well, there are a few internal places but we don't expose them). The point thus is from anexpl3point of view there is no need to discusstoksat all. – Joseph Wright♦ Sep 10 '15 at 13:24 -
1@jfbu The intention has always been that
expl3will be 'stand-alone', i.e. documented fully on its own terms rather than implicitly relying on (bits of) The TeXbook as a reference. We are not there yet but that is our direction. – Joseph Wright♦ Sep 10 '15 at 13:59
