complement to $

[Nick Sabalausky]

"Steven Schveighoffer" <schveiguy at yahoo.com> wrote in message 
news:op.vco5zwhreav7ka at localhost.localdomain...
> Currently, D supports the special symbol $ to mean the end of a 
> container/range.
>
> However, there is no analogous symbol to mean "beginning of a 
> container/range".  For arrays, there is none necessary, 0 is always the 
> first element.  But not all containers are arrays.
>
> I'm running into a dilemma for dcollections, I have found a way to make 
> all containers support fast slicing (basically by imposing some 
> limitations), and I would like to support *both* beginning and end 
> symbols.
>
> Currently, you can slice something in dcollections via:
>
> coll[coll.begin..coll.end];
>
> I could replace that end with $, but what can I replace coll.begin with? 
> 0 doesn't make sense for things like linked lists, maps, sets, basically 
> anything that's not an array.
>
> One thing that's nice about opDollar is I can make it return coll.end, so 
> I control the type.  With 0, I have no choice, I must take a uint, which 
> means I have to check to make sure it's always zero, and throw an 
> exception otherwise.
>
> Would it make sense to have an equivalent symbol for the beginning of a 
> container/range?
>
> In regex, ^ matches beginning of the line, $ matches end of the line --  
> would there be any parsing ambiguity there?  I know ^ is a binary op, and 
> $ means nothing anywhere else, so the two are not exactly equivalent.  I'm 
> not very experienced on parsing ambiguities, but things like ~ can be 
> unambiguous as binary and unary ops, so maybe it is possible.
>
> So how does this look:  coll[^..$];
>
> Thoughts? other ideas?
>

I think 0 makes perfect sense for any ordered container, or, really, 
anything for which $ makes sense (plus some things for which $ doesn't make 
sense, like an right-infinite range). However, the rest of your argument 
convinced me.