UK TeX FAQ -- question label patch

### Patching existing commands

In the general case (possibly sticking something in the middle of an existing command) this is difficult. However, the common requirement, to add some code at the beginning or the end of an existing command, is conceptually quite easy. Suppose we want to define a version of a command that does some small extension of its original definition: we would naturally write:

>     \renewcommand{\splat}{\mumble\splat}

However, this would not work: a call to `\``splat` would execute `\``mumble`, and the call the redefined `\``splat` again; this is an unterminated recursion, that will quickly exhaust TeX’s memory.

Fortunately, the TeX primitive `\``let` command comes to our rescue; it allows us to take a “snapshot” of the current state of a command, which we can then use in the redefinition of the command. So:

>     \renewcommand{\smooth}{\mumble\OldSmooth}

effects the required patch, safely. Adding things at the end of a command works similarly. If `\``smooth` takes arguments, one must pass them on:

>     \renewcommand{\smooth}[2]{\mumble\OldSmooth{#1}{#2}}

The general case may be achieved in two ways. First, one can use the LaTeX command `\``CheckCommand`; this compares an existing command with the definition you give it, and issues a warning if two don’t match. Use is therefore:

> `\CheckCommand{\complex}{`<original definition>`}`
> `\renewcommand{\complex}{`<new definition>`}`

This technique is obviously somewhat laborious, but if the original command comes from a source that’s liable to change under the control of someone else, it does at least warn you that your patch is in danger of going wrong.

Otherwise, LaTeX users may use one of the patchcmd, ted or etoolbox packages.

The patchcmd package addresses a slightly simpler task, by restricting the set of commands that you may patch; you mayn’t patch any command that has an optional argument, though it does deal with the case of commands defined with `\``DeclareRobustCommand`. The package defines a `\``patchcommand` command, that takes three arguments: the command to patch, stuff to stick at the front of its definition, and stuff to stick on the end of its definition. So, after

>     \def\b{b}
>     \patchcmd\b{a}{c}

we will have a new version of `\``b` defined as “`abc`”.

The ted package is a ‘token list editor’, and provides a command `\``substitute` which will patch the contents of a macro, putting the result in a token-list, or (in the form `\``Substitute*`) using the result to (re)define a macro. The following example defines a simple macro, and then changes its definition:

>     \newcommand{\myfont}%
>         {\Large\sffamily\selectfont}
>     \Substitute*[\renewcommand{\myfont}]{\myfont}%
>         {\Large\sffamily}{\huge\itshape}

The macro’s definition is now:

>     \huge\itshape\selectfont

The package also offers a command `\``ShowTokens`, which shows the content of its argument, one token to a line, with details of the token’s category code, etc. This is recommended as a debugging tool.

The etoolbox (which provides user access to e-TeX’s programming facilities) provides a command `\``patchcmd` which is very similar to `\``Substitute`, except that it only replaces a single instance of the token(s) in its search pattern. Since not all commands may be patched in this way, `\``patchcmd` takes extra arguments for success and failure cases. The package also provides commands that prepend (`\``pretocmd`) or append (`\``apptocmd`) to the definition of a command. Not all commands may be patched in this way; the package provides a command `\``ifpatchable` which checks the prerequisites, and checks that the target command’s body does indeed include the patch string you propose to use. (The command `\``ifpatchable*` omits the test on the patch string.)

Finally, we’ll briefly consider a package that is (just about) usable, but not really recommendable. Patch gives you an ingenious (and difficult to understand) mechanism, and comes as an old-style LaTeX documented macro file, which can no longer be processed to produce formatted documentation, etc.. Fortunately, the file (patch.doc) may be input directly, and “documentation” may found by reading the source of the package. Roughly speaking, one gives the command a set of instructions analogous to sed substitutions, and it regenerates the command thus amended. Unless you can’t do your job any other way, patch is best avoided.

`etoolbox.sty`
  macros/latex/contrib/etoolbox (or browse the directory)
`patch.doc`
  macros/generic/misc/patch.doc
`patchcommand.sty`
  macros/latex/contrib/patchcmd (or browse the directory)
`ted.sty`
  macros/latex/contrib/ted (or browse the directory)

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=patch