Append to 'map' result

[Ali Çehreli via Digitalmars-d-learn]

On 07/04/2017 04:57 PM, Jean-Louis Leroy wrote:
> On Tuesday, 4 July 2017 at 23:26:28 UTC, H. S. Teoh wrote:
>> On Tue, Jul 04, 2017 at 11:27:25PM +0000, Jean-Louis Leroy via
>> Digitalmars-d-learn wrote:
>>> I want to create a range that consists of the result of a map()
>>> followed by a value, e.g.:
>>>
>>>   int[] x = [ 1, 2, 3];
>>>   auto y = map!(x => x * x)(x);
>>>   auto z = y ~ 99; // how???
>>>
>>> I have tried several variations: convert 99 to a dynamic array, to a
>>> range, convert range to dynamic array (couldn't even figure that
>>> one); to no avail.
>> [...]
>>
>> Try:
>>
>>     auto z = chain(y, only(99));
>
> Thanks!
>
> However, when I try to use this construct in my real code, I still have
> a problem:
>
>     // k is a ClassInfo
>     chain(map!(i => i.classinfo)(k.interfaces), only(k.base))
>
> meth.d(311): Error: template std.range.chain cannot deduce function from
> argument types !()(MapResult!(__lambda3, Interface[]),
> OnlyResult!(TypeInfo_Class, 1LU)), candidates are:
> /usr/include/d/std/range/package.d(795):
> std.range.chain(Ranges...)(Ranges rs) if (Ranges.length > 0 &&
> allSatisfy!(isInputRange, staticMap!(Unqual, Ranges)) &&
> !is(CommonType!(staticMap!(ElementType, staticMap!(Unqual, Ranges))) ==
> void))
>
>
>
No time to dig deeper but this is because the two ranges that chain() 
receives do not have a common type. (Rather, that type is 'void'):

import std.algorithm;
import std.range;
import std.traits;

class C {
}

void main() {
     auto k = C.classinfo;
     auto m = map!(i => i.classinfo)(k.interfaces);
     auto b = only(k.base);

     // Types
     alias M = typeof(m);
     alias B = typeof(b);

     // Element types
     alias EM = ElementType!M;
     alias EB = ElementType!B;

     // Two of chain's constraints are satisfied:
     static assert(isInputRange!M);
     static assert(isInputRange!B);
     // Now it needs to see a CommonType of range elements

     pragma(msg, EM);    // const(void)[]()
     pragma(msg, EB);    // const(void)[]()

     pragma(msg, CommonType!(EM, EB)); // void

     // For that reason, the following does not work:
     // chain(m, b);
}

Ali