Working with ranges: mismatched function return type inference

[TheFlyingFiddle via Digitalmars-d-learn]

On Tuesday, 11 October 2016 at 15:46:20 UTC, orip wrote:
> On Tuesday, 11 October 2016 at 13:06:37 UTC, pineapple wrote:
>> Rewrite `return chain(ints[0..5], ints[8..$]);` as `return 
>> ints[0..5] ~ ints[8..$];`
>>
>> The `chain` function doesn't return an array, it returns a 
>> lazily-evaluated sequence of an entirely different type from 
>> `int[]`.
>
> Of course it does! I would like the function to return an 
> "input range of int", no matter which one specifically. Is this 
> possible?

It is, but you will have to use an interface / class to achieve 
this behavior (or use some sort of polymorphic struct). Something 
like this will do the trick:

import std.range;
import std.stdio;

interface IInputRange(T)
{
     bool empty();
     T front();
     void popFront();
}

final class InputRange(Range) if(isInputRange!Range)
	: IInputRange!(ElementType!Range)
{
     Range r;
     this(Range r)
     {
         this.r = r;
     }

     bool empty() { return r.empty; }
     ElementType!Range front() { return r.front; }
     void popFront() { r.popFront; }
}

auto inputRange(Range)(Range r)
{
     return new InputRange!Range(r);
}

IInputRange!int foo(int[] ints)
{
     import std.range;
     if(ints.length > 10) {
         return inputRange(chain(ints[0 .. 5], ints[8 .. $]));
     } else {
         return inputRange(ints);
     }
}

void main()
{
     auto ir  = foo([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
     auto ir2 = foo([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
     writeln(ir);
     writeln(ir2);
}