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?

Thanks,
-- 
Bahman

static if (is(typeof(r is null))) { ...you can do your assert here... }

On Monday, 25 July 2016 at 12:37:18 UTC, Bahman Movaqar wrote:
>     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?

You can't. null is not a valid value for the return type of map.

On Monday, 25 July 2016 at 12:37:18 UTC, Bahman Movaqar wrote:
But I'm curious; how can I check for a
> `null` in this case?

Well, if you're happy with assertion failure by access violation, you may not even need to check for null, because generally if you try to call .empty on a null pointer you'll get an access violation (killing two birds with one stone).

Otherwise you could try
(!__traits(compiles, r is null) || r !is null) && !r.empty

On Monday, 25 July 2016 at 12:47:25 UTC, Cauterite wrote:
> (!__traits(compiles, r is null) || r !is null) && !r.empty

Ah, whoops that's wrong, looks like ketmar had the right idea.

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);
}

On 07/25/2016 05:07 PM, 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?

Thank you people for the answers.
From what I could gather, it's not possible to check for `null` at
runtime for reference based types.  Am I right?

-- 
Bahman

On Monday, 25 July 2016 at 13:09:22 UTC, Bahman Movaqar wrote:
> From what I could gather, it's not possible to check for `null` at
> runtime for reference based types.  Am I right?

No, it is only possible to check for null for reference based types. But map's result is not a reference based type.

On 07/25/2016 05:47 PM, Adam D. Ruppe wrote:
> On Monday, 25 July 2016 at 13:09:22 UTC, Bahman Movaqar wrote:
>> From what I could gather, it's not possible to check for `null` at runtime for reference based types.  Am I right?
> 
> No, it is only possible to check for null for reference based types. But map's result is not a reference based type.

Oh, I see now.  Thanks.

-- 
Bahman