Map and arrays

[Jonathan M Davis]

On Sunday 28 November 2010 11:37:26 Tom wrote:
> El 28/11/2010 04:11, Jonathan M Davis escribió:
> > On Saturday 27 November 2010 22:48:28 Tom wrote:
> >> Hi,
> >> 
> >> I wonder how to solve this kind of stuff...
> >> 
> >> void foo(string[] sarray) {
> >> 
> >>       // do something with sarray
> >> 
> >> }
> >> 
> >> void bar(int[] iarray) {
> >> 
> >>       auto sarray = map!(to!string)(iarray);
> >>       foo(sarray);
> >> 
> >> }
> >> 
> >> And get...
> >> 
> >> foo (string[] sarray) is not callable using argument types
> >> (Map!(to,int[]))
> >> 
> >> What should I do? Is there some way to get the original array type?
> > 
> > Many functions in std.algorithm return range types specific to that
> > function. A number of these do lazy evaluation, so the actual work of
> > the function is done when iterating over the resulting range. map
> > happens to be one of those.
> > 
> > For all such functions, if you want an array out of the deal, what you do
> > is pass the result to std.array.array(). That will take an arbitrary
> > range and return its contents as an array - which also forces the
> > processing of the range in the case of lazy evaluation like you get with
> > map. So, if you want to delay the evaluation at all, or if it's really
> > only go to make sense that some of the range be processed, you may not
> > want to actually put it in an array. The combination of auto and
> > templates can avoid making you actually turn it into an array in many
> > cases. But if you do want an array, std.array.array() is what you're
> > looking for.
> > 
> > - Jonathan M Davis
> 
> Thank you very much.
> 
> Is there some documentation (besides digitalmars ddoc's, which I find
> very chaotic for a starting point) that explains these structures, when
> to use them (instead of arrays), and how they interact with arrays? It's
> very difficult to find that kind of documentation, if there's such.

Typically the point is _not_ to care about them. You use auto, so you don't care 
what the type actually is. If you want to pass them to a function, you have it 
be a templated function. Perhaps it has a template constraint to verify that the 
range is a particular type of range (such as std.range.isForwardRange!()) rather 
than a totally arbitrary type, but you're not supposed to have to care that map 
returns a Map object. Heck, exactly what type of range it is depends on what 
type of range the one you gave to map was (e.g. if you gave map a random access 
range - such as an array - than map returns a version of Map which is random 
access range, whereas if you fed it a range which isn't a random access range, 
then it isn't a random access range). You're not supposed to have to care. So, 
to some extent at least, the documentation tries to bury it (which is definitely 
better than telling you all of its gory details which is more along the lines of 
what it did in the past).

If you want to know what the return type of a function is, use typeof. And if 
you _really_ want to know what that return type actually looks like, then you 
need to look at the source code (which would be in 
dmd2/src/phobos/std/algorithm.d in the after you unzip the dmd zip file). But 
you're just not supposed to have to care. I'm sure that the documentation could 
be improved, but it is better than it used to be, and auto removes most of the 
need to worry about what the exact type a return value is.

> Also I wish to point out that the layout and presentation of some of
> phobos ddoc pages are, IMHO and as I said before, a little bit of a
> mess. Wish they could look more like javadoc, or at least more organized.

It's better than it used to be. I'm sure that it will improve more in the 
future. But there are higher priorities (like bug fixing), so it's not something 
that's likely to happen right away - certainly not massive change. I don't 
expect that it will ever look quite like javadoc does, but it should be getting 
better eventually. In theory, ddoc output can be made to look like pretty much 
anything (javadoc included) - and what's up on the site is definitely better than 
what you get by default - but that means that someone has to do the work to make 
it look like whatever looks really good (which is one of those things which is 
highly subjective). It's improving, but slowly.

- Jonathan M Davis