On 02/03/2011 02:25 PM, Lars T. Kyllingstad wrote:
On Thu, 03 Feb 2011 13:53:44 +0100, spir wrote:
On 02/03/2011 01:17 PM, Lars T. Kyllingstad wrote:
Why the reluctance to use template constraints? They're so flexible!
:)
I cannot stand the "is()" idiom/syntax ;-) Dunno why. Would happily get
rid of it in favor of type-classes (built eg as an extension to current
interfaces). For instance, instead of:
void func (T) (T t)
if (is(someConstraint1)&& is(someConstraint2))
{
...
}
use:
void func (SomeTypeClass T) (T t)
{
...
}
For instance (untested):
void func (T) (T t)
if (isInputRange(T)&& is(ElementType!T == E))
-->
void func (InputRange!E T) (T t)
where InputRange is a (templated) interface / type-class.
Type-class checks on /type/ /template/ parameters (as opposed to type
checks on regular value parameters) would be performed structurally (as
opposed to nominally). D knows how to do this, since that's what it
needs to perform when checking is() constraints.
I agree that is() is rather ugly. Same with __traits. If you haven't
already done so, I suggest you vote up this issue:
http://d.puremagic.com/issues/show_bug.cgi?id=3702
Done!
(I did not get all the details 'cause no time for a deep look, but anything
impulsed by the motivation of getting rid of is() and __traits can hardly be a
Bad Thing ;-)
What do you think of type classes, as an alternative to Don's proposal in issue
#3702.
See also "Type Classes as Objects and Implicits":
http://ropas.snu.ac.kr/~bruno/papers/TypeClasses.pdf
Anyway, you can hide is()'s ugliness in the most common cases, though, by
defining new templates. For instance, I wouldn't mind having the
following in std.range as an overload of isInputRange:
template isInputRange(R, T)
{
enum isInputRange = isInputRange!R&& is(ElementType!R == T);
}
Then, you'd simply write
void func(R)(R range) if (isInputRange!(R, E)) { ... }
-Lars
A great improvement, indeed.
While we're at defining a set of constraints in a template, let us make it an
interface / type-class that the E must (structurally) satisfy, and just write:
void func(InputRange!E R)(R range) { ... }
What do you think?
Note: a template is not always required, I guess:
void writeElements (Iterable Elements) (Elements elements) {
foreach (element, elements) {
write(element,' ');
}
}
(In this case, because write is itself generic.)
Denis
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