On Wednesday, 15 February 2017 at 19:39:52 UTC, Andrei
Alexandrescu wrote:
On 02/15/2017 06:20 AM, Daniel N wrote:
On Wednesday, 15 February 2017 at 09:22:14 UTC, Daniel N wrote:
template every(T...)
{
template satisfies(U...)
{
enum satisfies = true;
}
}
(lunch-break => time to hack D!)
template every(T...)
{
template satisfies(U...)
{
enum satisfies = {
foreach(t; T)
foreach(u; U)
if(!u!t)
return false;
return true;
}();
}
}
That looks pretty neat. Can you find 4-5 cases in which this
could be used gainfully in Phobos? Thanks! -- Andrei
Slightly OT, but it is related so bear with me:
Design by introspection is great and prevent having to make new
names for every possible combo of features. Unfortunately, it
doesn't work so well for introspecting multiple types at once:
normally you have to then make a new symbol, with a new name, to
pass to e.g. allSatisfy. That annoys me, so I've been wanting
this sort of thing for a while (which would also make `every` way
better to use, instead of having to make a name for every
predicate:
You know how we have named parameterised enums and aliases?
enum isInt(a) = is(a == int);
and
alias sizeof(a) = a.sizeof;
why not allow *anonymous* parameterised enums and aliases, like
this:
enum areAllInt(TL ...) = allSatisfy!(enum(a) => is(a == int), TL);
void foo(TL...)(TL args)
if (allSatisfy!(enum(T) => T.sizeof <= 4, TL))
{
// ...
}
an example from phobos:
bool ordered(alias less = "a < b", T...)(T values)
if (T.length == 2 && is(typeof(binaryFun!less(values[1],
values[0])) : bool) || T.length > 2 &&
is(typeof(ordered!less(values[0..1 + $ / 2]))) &&
is(typeof(ordered!less(values[$ / 2..$]))))
becomes this:
bool ordered(alias less = "a < b", T...)(T values)
if (T.length > 2 &&
allSatisfy!(enum(size_t i) =>
is(typeof(binaryFun!less(values[i],
values[i+1])) : bool),
Iota!(T.length - 1))
I admit it's not a huge win, but hiding the log_2(N) template
depth behind allSatisfy is great: you shouldn't have to (and
people wont) think about that when writing a template constraint
like this.
There is a part of me that thinks all this meta-template stuff is
madness though and that modifications to ctfe could make it
mostly obsolete, but that's a story for later...
