Detecting the trivial can be non-trivial (was Why not allow empty code blocks?)

[Rustom Mody]

On Sunday, July 24, 2016 at 3:45:40 PM UTC+5:30, Ben Bacarisse wrote:
> Rustom Mody  writes:
> <snip>
> > For a long time the re → dfa transformation went and was taught the laborious
> > route:
> > re → nfa-with-ε-transitions → nfa-without-ε-transitions → dfa
> >
> > https://en.wikipedia.org/wiki/Thompson's_construction
> > https://en.wikipedia.org/wiki/Powerset_construction
> >
> > Now there is a direct, straightforward method which only becomes available
> >  (thinkable) when we have a null regular expression:
> > https://drona.csa.iisc.ernet.in/~deepakd/fmcs-06/seminars/presentation.pdf
> 
> "Now" seems an odd thing to say since the technique is quite old.  It
> would be better to say that it has been re-discovered.

The “Now” was not meant time-ly!
> 
> But thanks for the link -- I was unaware of the idea.  Unfortunately the
> material is not well presented there (lower-case phi for the empty set?)
> but in trying to understand what it was saying I found:
> 
> https://www.cl.cam.ac.uk/~so294/documents/jfp09.pdf
> 
> which, in my opinion, does it very much better.


Yeah
I think the one used when I was studying was this one
http://www2.in.tum.de/hp/file?fid=571

Anyway my main point was that getting trivial cases right is damn hard
And eliding the trivial case from a notation — because its too trivial to be useful
is damn stupid