Re: [q-lang-users] Special values or failing functions

[Albert G. <Dr....@t-...>]

Alexander Nickolsky wrote:
> len []    = 0;
> len [[H|Hs]|T] = _FAIL_;
> len [H|T]      = 1+len T;
> 
> so semantics of _FAIL_ would be : rewrite the current match with
> itself and return.
> So, 'fail' applies to the left side, but _FAIL_ applies to the right side.

I don't grok this last sentence. Should 'len [[1,2],3]' return 'len
[[1,2],3]' itself, or _FAIL_, or what? And what should 'len [1,[2,3]]'
return?

> no matter how you call it - reject, normal, agree, refuse - which
> strangely means the same.

reject sounds like a worthy candidate. Or we could just use _FAIL_, it's
a valid function identifier.

> X+Y = Y+X; // commutative rule
> will not work.

No, it just loops. In a CAS, you'd have to add a qualifying condition
there which makes sure that terms are ordered in some way.

Albert

-- 
Dr. Albert Gr"af
Dept. of Music-Informatics, University of Mainz, Germany
Email:  Dr....@t-..., ag...@mu...
WWW:    http://www.musikinformatik.uni-mainz.de/ag