biblatex: Is it possible to patch macros created with \newbibmacro?

Question

This question led to a new feature in a package:
xpatch

The etoolbox package provides various tools that "are useful to hook into or modify existing code" (etoolbox manual, section 3.4). The most important one is the \patchcmd command with the syntax

\patchcmd[<prefix>]{<command>}{<search>}{<replace>}{<success>}{<failure>}

which "extracts the replacement text of a <command>, replaces [the first occurence of] <search> with <replace>, and reassembles the <command>". Other useful macros are, e.g., \apptocmd and \pretocmd which will append resp. prepend code to the replacement text of a command.

The biblatex package features the commands \newbibmacro, \renewbibmacro, and \providebibmacro (plus their starred counterparts). These commands resemble \newcommand/\renewcommand/\providecommand "except that [the command name] may contain characters such as numbers and punctuation marks and does not start with a backslash" (biblatex manual, section 4.6.4). The "bibmacros" defined this way may be executed via \usebibmacro{<command name>}.

Is it possible to define a \patchbibmacro command that extracts and modifies the replacement text of macros created with \newbibmacro the same way \patchcmd does for macros created with \newcommand?

Answer 1

\makeatletter
\def\act@on@bibmacro#1#2{%
  \expandafter#1\csname abx@macro@\detokenize{#2}\endcsname}
\def\patchbibmacro{\act@on@bibmacro\patchcmd}
\def\pretobibmacro{\act@on@bibmacro\pretocmd}
\def\apptobibmacro{\act@on@bibmacro\apptocmd}
\def\showbibmacro{\act@on@bibmacro\show}
\makeatother

Now you have available

\patchbibmacro \pretobibmacro \apptobibmacro \showbibmacro

Explanation:

\newbibmacro{foo}[1]{-#1-} actually executes

\expandafter\newcommand\csname abx@macro@\detokenize{foo}\endcsname[1]{-#1-}

(I omit some gory details, but this is almost the truth).

This, of course, won't work for macros with optional arguments, but here's how:

\makeatletter
\def\act@on@bibmacroopt#1#2{%
  \expandafter#1\csname\expandafter\string\csname abx@macro@\detokenize{#2}\endcsname\endcsname}
\def\patchbibmacroopt{\act@on@bibmacroopt\patchcmd}
\def\pretobibmacroopt{\act@on@bibmacroopt\pretocmd}
\def\apptobibmacroopt{\act@on@bibmacroopt\apptocmd}
\def\showbibmacroopt{\act@on@bibmacroopt\show}
\makeatother

With \patchbibmacroopt you can define biblatex macros defined for instance by

\newbibmacro{baz}[2][x]{-#1-#2-}

One has to remember that with \newcommand{\baz}[2][x]{-#1-#2-} the macro doing the real work is

\\baz

that, of course, has to be accessed with

\csname\string\baz\endcsname

Easier solution

The question led to adding some features to the xpatch package, which now has

\xpatchbibmacro
\xpretobibmacro
\xapptobibmacro

which work out of the box even with bibmacros defined having an optional argument; the syntax is the same as the one for \patchcmd, \pretocmd and \apptocmd:

\xpatchbibmacro{<name>}{<search>}{<replace>}{<success>}{<failure>}
\xpretobibmacro{<name>}{<code to prepend>}{<success>}{<failure>}
\xapptobibmacro{<name>}{<code to append>}{<success>}{<failure>}

So, if foo has been defined with \newbibmacro{foo}[1]{-#1-} one can say, for instance,

\xpatchbibmacro{foo}{-}{+}{}{}

to change the first - into +.