Related: http://d.puremagic.com/issues/show_bug.cgi?id=8676
This is something to think about for D3 (or maybe even D2 if it
doesn't break code): it should be possible to inject default
parameters in the function header rather than the body when using
variadic templates. It would make auto-documentation possible and
would make it easier to write template constraints. E.g. the current
function header for lockstep() in std.range is:
auto lockstep(Args...)(Args args)
if (allSatisfy!(isInputRange, staticMap!(Unqual, Args)) || (
allSatisfy!(isInputRange, staticMap!(Unqual, Args[0..$ - 1])) &&
is(Args[$ - 1] == StoppingPolicy))
and the body:
static if (is(Args[$ - 1] == StoppingPolicy))
{
alias args[0..$ - 1] ranges;
alias Args[0..$ - 1] Ranges;
alias args[$ - 1] stoppingPolicy;
}
else
{
alias Args Ranges;
alias args ranges;
auto stoppingPolicy = StoppingPolicy.shortest;
}
A lot of code could be stripped if there was a way to set an optional
last parameter for variadics. lockstep could then be written as:
auto newLockstep(Args...)(Args args, StoppingPolicy stoppingPolicy =
StoppingPolicy.shortest)
if (allSatisfy!(isInputRange, staticMap!(Unqual, Args)))
{
static if (Args.length > 1)
{
return Lockstep!(Args)(args, stoppingPolicy);
}
else
{
return args[0];
}
}
So why doesn't this currently work? Using this template does compile:
int[] arr1 = [1, 2, 3];
int[] arr2 = [1, 2, 3, 4];
foreach (x, y; newLockstep(arr1, arr2, StoppingPolicy.shortest))
{
writeln(x, " ", y);
}
but if you don't specify the last StoppingPolicy argument at the call
site it won't compile:
foreach (x, y; newLockstep(arr1, arr2)) { }
test.d(32): Error: template test.newLockstep does not match any
function template declaration
test.d(14): Error: template test.newLockstep(Args...) if
(allSatisfy!(isInputRange,staticMap!(Unqual,Args))) cannot deduce
template function from argument types !()(int[],int[])
