On Monday, 25 July 2016 at 12:37:18 UTC, Bahman Movaqar wrote:
Suppose I have the following function:
public auto max(alias comp, Range)(Range r)
in {
assert(r !is null && !r.empty);
}
body {
// ...
}
When the function after a series of chained `map` operations, I
get the following error:
Error: incompatible types for ((r) !is (null)):
'MapResult!(__lambda2, SInvoiceLine[])' and 'typeof(null)'
Of course if I remove `r !is null` from the `in` block,
everything will work. But I'm curious; how can I check for a
`null` in this case?
In the general case, the vast majority of ranges you work with
will be value types. In the rare case where it's a reference
type, you can use static if to specialize the assert. Two
possibilities have already been suggested in this thread, but
given that ranges are nearly always going to be a struct or a
class, then you might do this inside the template:
static if(is(Range == class)) { // check for null }
You may also want to add a constraint:
import std.range : isInputRange;
public auto max(alias comp, Range)(Range r)
if(isInputRange!Range) {
static if(is(Range == class)) assert(r !is null && !r.empty);
else assert(!r.empty);
}
