tdlp: higher-order functions

[Alex Rønne Petersen]

On 20-01-2012 15:32, Jerome BENOIT wrote:
> Hello List:
>
> In tDlp book in section 5.6 entitled `Higher-Order Functions. Function
> Literals,
> the first code example is:
>
> -----------------------------------------------------------------
> T[] find(alias pred, T)(T[] input)
> if (is(typeof(pred(input[0])) == bool)) {
> for(; input.length > 0; input = input[1 .. $]) {
> if (pred(input[0])) break;
> }
> return input;
> }
> -----------------------------------------------------------------
>
> I can play it. Nevertheless, it is not clear for me right now
> what is the role played by `T' in the generic argument list (alias pred,
> T).
> (Its meaning is bypassed in this section via a `deduction' way.)
>
> Any hint is welcome.
>
> Thanks in advance,
> Jerome
>

It's important to realize that D does not have generics; rather, it has 
templates. What (alias pred, T) means is that it takes (virtually) any 
argument as the first template parameter (function literals included) 
and a type for the second parameter.

You can call this function like so:

auto ints = find!((x) { return x % 2 != 0; })([1, 2, 3, 4, 5]);

Here, the type parameter T gets deduced from the argument, which is an 
array of ints.

-- 
- Alex