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digitalmars.D - Built-in range type

reply Lionello Lunesu <lio lunesu.remove.com> writes:
Slices are one of D's greatest assets, and now in D2 we have the new
foreach range statement: foreach(i; 0..22) { ... }

Wouldn't it be nice to introduce a built-in for these int..int ranges?

auto range = 0..22;
auto slice = array[range];
foreach(i; range) {...}

L.
Apr 28 2010
parent reply bearophile <bearophileHUGS lycos.com> writes:
Lionello Lunesu:


Wouldn't it be nice to introduce a built-in for these int..int ranges?<


It was discussed in the past, but there are some design problems.

In theory it can be used to define overload of slices operator as:
opSliceAssign(int v, Slice s)

A problem is that slices can have more than just size_t indices, this code
works and 'i' is of type long:

import std.stdio: writeln;
void main() {
    foreach (i; (long.max - 10) .. (long.max - 1))
        writeln(typeid(typeof(i)), " ", i);
}

So the (immutable) Slice struct has to be a template:
opSliceAssign(int v, Slice!size_t s)


struct Slice(TIndex) {
    immutable TIndex start, stop;
    immutable int stride; // simplified
    immutable SliceType type;
    this(...) {...}
    // few methods: in operator, length, subslicing, etc.
}

So it's usually 4 CPU words long on 32 bit CPUs (it can be just 3 words long on
64 bit CPUs if stride and type are presented in 32 bit each).

The stride is the third optional value, usually it's 1, its syntax can be:
start .. stop : stride

The stride is an int because it can be negative. If start and stop are long,
you can't have a stride larger than an int min-max values.

'type' is a bitfield that encodes the kind of Slice. I think the cases are:

1 .. 7
0 .. $
0 .. $-10
1 .. 7 : 2
0 .. $ : 2
0 .. $-10 : 2
  .. 7
  .. $
  .. $-10
  .. 7 : 2
  .. $ : 2
  .. $-10 : 2


(The cases with no start are meant to be useful for intervals infinite on the
left. I am not sure this is an important group of cases.)


The Chapel language generalizes this idea into the idea of "domain", a
first-class index set.
I have explained it here, and I think it's an useful idea:
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=87311

Bye,
bearophile
Apr 28 2010
parent Lionello Lunesu <lio lunesu.remove.com> writes:
I appreciate your comments.

The introduction of a range type (as they appear in slices and foreach)
should not have to introduce new features. A simple int..int (or
uint/long/ulong) 'tuple' would do the trick. Internally the compiler
already has SliceExp with 'lwr' and 'upr' bounds. Exposing this to the
programmer might be useful.

Anyway, probably not useful enough :)

L.

On 29-4-2010 1:04, bearophile wrote:

 Lionello Lunesu:
 

 Wouldn't it be nice to introduce a built-in for these int..int ranges?<


 
 It was discussed in the past, but there are some design problems.
 
 In theory it can be used to define overload of slices operator as:
 opSliceAssign(int v, Slice s)
 
 A problem is that slices can have more than just size_t indices, this code
works and 'i' is of type long:
 
 import std.stdio: writeln;
 void main() {
     foreach (i; (long.max - 10) .. (long.max - 1))
         writeln(typeid(typeof(i)), " ", i);
 }
 
 So the (immutable) Slice struct has to be a template:
 opSliceAssign(int v, Slice!size_t s)
 
 
 struct Slice(TIndex) {
     immutable TIndex start, stop;
     immutable int stride; // simplified
     immutable SliceType type;
     this(...) {...}
     // few methods: in operator, length, subslicing, etc.
 }
 
 So it's usually 4 CPU words long on 32 bit CPUs (it can be just 3 words long
on 64 bit CPUs if stride and type are presented in 32 bit each).
 
 The stride is the third optional value, usually it's 1, its syntax can be:
 start .. stop : stride
 
 The stride is an int because it can be negative. If start and stop are long,
you can't have a stride larger than an int min-max values.
 
 'type' is a bitfield that encodes the kind of Slice. I think the cases are:
 
 1 .. 7
 0 .. $
 0 .. $-10
 1 .. 7 : 2
 0 .. $ : 2
 0 .. $-10 : 2
   .. 7
   .. $
   .. $-10
   .. 7 : 2
   .. $ : 2
   .. $-10 : 2
 
 
 (The cases with no start are meant to be useful for intervals infinite on the
left. I am not sure this is an important group of cases.)
 
 
 The Chapel language generalizes this idea into the idea of "domain", a
first-class index set.
 I have explained it here, and I think it's an useful idea:
 http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=87311
 
 Bye,
 bearophile
Apr 29 2010