Re: tdlp: higher-order functions

[Jerome BENOIT]

Thanks.

Let go further.


On 20/01/12 15:58, Alex Rønne Petersen wrote:

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.

What would be the non-deducible version ?

Jerome