Please write more to this story

Not "this" story. Everything above has happened on several projects. The cat thing comes up because it is tricky to expand two macros next to each other without whitespace. So if you do:

  define(`foo',`hello')
  define(`bar',`world')
  foo bar
  foobar

You will get:

  hello world
  foobar

Working around this gets tricky, so someone inevitably ends up writing a cat-like macro such that you can do

  cat(foo,bar)

To get

  helloworld.

A side effect of this is that now "cat" is really "cat()" which expands to "". You can work around this by doing `cat'. However, if `cat' is used as an argument to another macro (such as a for loop), the quotation only prevents escaping the first time. When the for macro is expanded, the quotation marks are stripped, giving you just "cat", which gets expanded again. A correctly written for macro would add new quotes as needed, but I have never seen someone correctly write such a macro without just copying it.

Not sure if I have seen this interaction specifically with for and cat, but I have seen an interaction like it on almost every project that used m4.

You can place an empty "expansion" in that line to get the behavior you want without an additional cat-like function

    foo`'bar

I only know of this feature because recently read the manual page for m4, and it's mentioned rather early in there, but might have been not as well emphasized in past iterations of the manual.

For completions sake, though, the easy way to do it is:

    foo`'bar

The empty quotes make foo and bar separate words.

I've only ever used m4 via autoconf and sendmail configuration files, so I don't know if it's m4 that has the bizarre syntax or whether it's autoconf's and sendmail's use of it. I'm not sure I've ever tried to use m4 directly for anything.

That's why I wrote a "pinch" in the OC. m4 is arguably better than cat as a barebones template engine. The moment you start doing anything beyond simple includes and variable extrapolations, it is time to switch something more modern and robust.

I guess autoconf/sendmail still use m4 because there wasn't anything better at the time that doesn't come with a kitchensink attached.

I know that story too well. Finally, I thought that if I have to code, I should just use a programming language.

Now, I use nodeJS to replace every m4 file with mustache.js and some JS logic and I don't feel limited anymore. The complexity doesn't increase much.