Re: [boost] [Spirit-devel] [fusion] Short-circuited Exception-based View Iteration

From: Dean Michael Berris (mikhailberis_at_[hidden])
Date: 2007-09-24 00:39:33

On 9/23/07, Joel de Guzman <joel_at_[hidden]> wrote:
> Dean Michael Berris wrote:
> >
> > Almost... But 'any' will just return a boolean, but not which element
> > in seq (or a view of which elements in seq) where f returns true.
>
> Do it in f where you know both.
>

This may make f a bit bloated for my taste, but this will work. Thanks. :)

> > Perhaps 'filter' is what I'm looking for, but I'm just interested in
> > the first one where a predicate holds true, so even a find_if(...)
> > might suit my needs.
>
> No. There's no way to do that. You can't have a runtime predicate
> determine a compile time return type.
>

There might be special cases where this might happen though... Let me
illustrate below what I mean...

> > I'll explore a bit more.
> >
> > BTW, i haven't found a `find_first_if(...)` implementation (like what
> > the STL has) which works with a homogeneous tuple -- somehow that
> > container might be a special case that deserves attention (?) -- which
> > returns a single value? Thoughts on this?
> >
> > Maybe this will solve at least my problem... :D
>
> The world fusion (and for that matter, mpl) lives in has some peculiar
> constraints. As I noted above, one is that you cannot determine a type
> from a runtime predicate. You can, however, do it inside-out. IOTW,
> let the predicate itself get the info that you need. If you really
> need to return a type, somehow, you can, for example, use a pointer
> to a base class, hold the value in a boost::any, etc. In other words
> use type erasure. If I were you, I'd use the visitor, f, itself to
> do the action as soon as the correct type is obtained. No type
> erasure magic.
>

This makes sense, though there might be a possibility where a
homogeneous tuple would be able to return just a single value:

struct equals_2 {
  template <typename T>
  T operator() (T element) const {
    if (element == 2) return element;
  };
};

typedef fusion::tuple<int, int, int> point_type;
point_type point(1, 2, 3);
assert(fusion::is_homogenous<point_type>::value == true);
assert(fusion::find_first_if<equals_2>(point) == 2); // only works if
'point' is a homogeneous container

> HTH.

It does indeed. Thanks for the tips!

(Goes back to hacking... :D)

-- 
Dean Michael C. Berris
Software Engineer, Friendster, Inc.
[http://cplusplus-soup.blogspot.com/]
[mikhailberis_at_[hidden]]
[[phone]]
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