Template elegance?

[anonymous via Digitalmars-d-learn]

On Wednesday, 3 June 2015 at 09:10:22 UTC, David Monagle wrote:
> What I was looking for is a more elegant way of defining those 
> secondary functions. Originally I was hoping I could do 
> something like:
>
> enum shouldEqual(E, V) = shouldValue((e, v) => e == v, "equal", 
> E, V);

Here you go:
----
template shouldValue(alias operation, string description)
{
     bool shouldValue(E, V)(lazy E expression, V value, string 
name="Value",
         string file = __FILE__, typeof(__LINE__) line = __LINE__)
     {
         if (operation(expression, value)) return true;
         throw new Exception(format("%s should %s %s, but was 
actually %s",
             name, description, value, expression), file, line);
     }
}

alias shouldEqual = shouldValue!((e, v) => e == v, "equal");
alias shouldBeGreaterThan = shouldValue!((e, v) => e > v, "be 
greater than");
----

The trick is to split `shouldValue` into two nested templates.

The outer template has those parameters that cannot be deduced: 
`operation` and `description`.

The inner template has the parameters that should be deduced: `E` 
and `V`.