Re: Any basic cpp capability m4 cannot do?

[Daniel Goldman]

Hi Eric,

Upon further reflection, I would have to agree you right, that m4 seems 
to be Turing Complete. Yes, m4 cannot be used by itself to write 
Dungeons and Dragons or the UNIX OS. But all that matters is that it can 
emulate the Turing Machine.

As you mention, being Turing Complete does not indicate usefulness. Of 
course the reason for my post was of a more practical nature. So I might 
reword my question to: Can anyone suggest some non-trivial 
pre-processing capability cpp can readily do that m4 cannot readily do? 
The word "readily" indicates the focus on practicality.

Unless anyone has more suggestions, it seems that m4 can readily do any 
pre-processing task cpp can readily do, with the possible exception of 
"\"strings\" within strings" variations, which cpp does automatically. 
On the reverse, there are many basic things m4 can readily do that cpp 
cannot readily do (that's why I started the conversion!).

Thanks,
Daniel

On 5/10/2014 12:52 PM, Eric Blake wrote:
> On 05/10/2014 01:39 PM, Daniel Goldman wrote:
>
> > Not to go too much on a tangent, I feel uncomfortable labeling m4
> > "Turing Complete". I am no expert, but based on my best understanding,
> > "Turing Complete" refers to programming languages, and does not apply to
> > pre-processors.
>
> M4 is a pre-processor that also happens to be a programming language.
> It may not be the most efficient language, and you are correct that no
> one would use it to write an OS, but that does not diminish the fact
> that it is still a programming language.
>
> > But I think maybe it's going to far to say m4 is "Turing Complete" and
> > "can do anything". Maybe it's enough that it's very useful, and can do
> > needed pre-processing tasks.
>
> Just because something is Turing Complete doesn't mean it is useful in
> an efficient time-frame for all tasks, merely that all tasks that can be
> done in finite resources can be done in that language.  You are correct
> that different languages have different strengths, and that m4's
> strength is in pre-processing tasks, while other tasks easy in many
> languages are rather convoluted in m4.