std.algorithm.reduce on an array of structs

[via Digitalmars-d-learn]

On Thursday, 11 September 2014 at 13:06:05 UTC, Colin wrote:
> I have this test code:
>
> struct Thing {
>     uint x;
> }
>
> void main(){
>     uint[] ar1 = [1, 2, 3, 4, 5];
>     auto min1 = ar1.reduce!((a,b) => a < b);
>     writefln("%s", min1);  // prints 1 as expected
>
>     Thing[] ar2 = [Thing(1), Thing(2), Thing(4)];
>     auto min2 = ar2.reduce!((a,b) => a.x < b.x);  //  <- Wont 
> Compile
>     writefln("%s", min2);
> }
>
> The line with "Wont Compile" on it has this error message:
> /usr/include/dmd/phobos/std/algorithm.d(770): Error: cannot 
> implicitly convert expression (__lambda2(result, 
> front(_param_1))) of type bool to Thing
> /usr/include/dmd/phobos/std/algorithm.d(791): Error: template 
> instance t.main.reduce!((a, b) => a.x < b.x).reduce!(Thing, 
> Thing[]) error instantiating
> t.d(16):        instantiated from here: reduce!(Thing[])
>
>
> Any idea what I'm doing wrong here?
> To me, the operation on ar2 should be pretty much identical to 
> ar1, except for the use of the struct.

I think you want to use `filter()` (for both Thing and uint), not 
`reduce()`. The former produces a range with only the elements 
that match the predicate, while the latter produces _one_ element 
according to the given rules, e.g.

     my_int_array.reduce!((result,a) => result+a);

produces the sum of all elements. In your example, the first use 
only compiles because `bool` happens to be implicitly convertible 
to `uint`.