Thanks for this very good explanations and examples. This "Range" topic
is a constant confusion, see one of many examples:
http://forum.dlang.org/thread/[email protected]
I don't understand why D cannot be simple AND efficient.
I think every function, which returns a kind of a "Range", should also
always be able to return "char", "char[]", "char[][]", "string" and
"string[]" per default, depending on what is expected.
On 23.02.2013 11:14, Peter Alexander wrote:
On Saturday, 23 February 2013 at 08:48:25 UTC, Allen Nelson wrote:
Why are filter and map returning these weird one-off types in the
first place? Why would filter return anything but the same type of
list that it was passed as an argument? Why would map return anything
other than an array of whatever type matches the return value of the
operator? And if there's some good reason why these things are the
case, why then at the very least would you not be able to easily cast
the return type into the return type that you want?
If you are using an array, you can construct an array from the result
using std.array.array
int[] evens = [1, 2, 3, 4, 5].map!(x => 2*x)().array();
The reason map, filter, and most other range functions don't return the
same type of array is because they are lazy. For example:
int[] twoFourSix = [1, 2, 3, 4, 5].map!(x => 2*x)().take(3).array();
Here, the mapping function is only called on the first three elements of
the array. If map returned the array [2, 4, 6, 8, 10] then it would be
doing more work than necessary since the take only cares about the first
3 elements.
Also consider infinite ranges:
auto twoFourSixAdInfinitum = cycle([1, 2, 3]).map!(x => 2*x)();
What should map return here? You cannot have infinitely sized arrays!
There's no way to have map return the same range as the input, or even
just an array, while also being lazy. If you want an array, use
std.array.array().
Haskell manages to get around this by adding an extra layer of
indirection around every operation, which hurts performance. D's
standard library is designed to be high performance by default, which
unfortunately does hurt expressiveness a little bit.
