Immutable separator to join() doesn't work

[Timon Gehr]

Jonathan M Davis wrote:
> On 2011-07-11 09:06, Timon Gehr wrote:
> > Jonathan M Davis wrote:
> > > On Sunday 10 July 2011 21:09:27 Mehrdad wrote:
> > >> I noticed that the code below doesn't work, and I was wondering if it's
> > >>
> > >> by design (hopefully not):
> > >> immutable SEP = ", ";
> > >> ["a", "b"].join(SEP);
> > >>
> > >> The fact that SEP is immutable(char[]) instead of immutable(char)[]
> > >> shouldn't break the function.
> > >
> > > It most definitely breaks the function, and it's a limitation of
> > > templates. Templates are instantiated with the exact type that they're
> > > given, so the compiler tries to instantiate join with immutable(char[]),
> > > but join _can't_ work with immutable(char[]), because it needs a mutable
> > > range. immutable ranges are worthless. If the compiler were smart enough
> > > to realize that it could instantiate join with immutable(char)[] and it
> > > would work, then you could use immutable(char[]), but since it isn't
> > > that smart, it doesn't work. The same problem happens with static
> > > arrays. They can't be used as ranges, so even though they'd work if the
> > > compiler picked a dynamic range as the type for the function, they don't
> > > work, because the compiler isn't that smart.
> > >
> > > The problem may be fixed at some point, but as it stands, it just doesn't
> > > work to use immutable arrays with range-based functions.
> > >
> > > - Jonathan M Davis
> >
> > There is no such thing as an immutable range because the range abstraction
> > is based on mutation.
> > This does not make sense, because intuitively, immutable(char[]) is a range
> > type just as immutable(char)[] or char[] is a range type.
> > It is quite odd that a data structure can only be a D range if it may be
> > modified...
> >
> > It is not really a compiler issue, but one of library design. Maybe adding
> > a wrapper range to Phobos to iterate over immutable 'ranges' would be an
> > improvement to the current situation.
>
> ??? A range is only of any value if you can process it. That generally
> requires calling popFront on it. That mutates it. So, sure, you can an
> immutable range, but it's useless, because you can't process it.

import std.range;
void main(){
    int[] arr;
    auto a=cast(immutable)map!"a"(arr);
    assert(!isInputRange!(typeof(a))); // this passes

}

So no, you cannot have an immutable range because it cannot fulfill the range
interface.

> And why should that be surprising?

This is surprising:
immutable SEP = ", ";

["a","b"].join(SEP);
// -- nope! Guess what? That is because your array is not a range! Why? Easy! It
cannot be changed! See?
// -- I don't care, but this should work... I'll implement the function myself!

immutable SEP = ", ";
string sep=SEP;
["a","b"].join(sep); //sure thing

> There are plenty of things that are essentially
> useless if they're immutable. Take a stream for instance. Like a range, it's
> altered as you use. So, you can't have an immutable stream - or if you did, it
> would be useless.

You can. You can even have immutable IO if you choose the right abstractions ;).

> Immutability can be very useful, but not being able to
> mutate something can really get in the way of doing anything with it. The same
> goes for const.

You can always use it's value to create others.

>
> There has been some discussion in the past of trying to find a way to convert
> an immutable or const range to a tail-immutable or tail-const range (similar
> to how you can pass immutable(char[]) to a function which takes
> immutable(char)[] and have it work),
> but the language doesn't currently
> provide any way to do that, and it wouldn't necessarily make sense for all
> range types anyway. It's highly dependent on how their implemented and what
> they're actually iterating over.

I completely agree. It would be unnecessarily complicated (impossible?) to make
all ranges work as immutable that would meaningfully be able to. Also it would
give almost no benefit.

But immutable(T[]) should clearly be accepted where immutable(T)[] would be. I've
run across that problem myself
It is just annoying, even more so if you do not actually need the generosity
provided by those functions at all.

>
> It's definitely true that the template situation could use some improvement to
> better deal with immutable arrays, and an improvement to the type system to
> better handle tail-const for ranges where applicable would be desirable, but
> the fact remains that not everything can be fully const or immutable and be
> useful. It's just a fact of how const and immutable work. There's stuff that
> needs to be mutable, and when you make it so that you can't mutate them,
> they're useless.
>
> - Jonathan M Davis

Nothing needs to be mutable. It is possible to work with immutable values only,
but it is no always efficient.
Having an immutable range interface would be quite easy. popFront would just have
to return the next range instead of changing the current one.


Cheers,
-Timon