Passing function as values and returning functions

[Xan]

On Wednesday, 11 April 2012 at 11:59:14 UTC, Jacob Carlborg wrote:
> On 2012-04-11 13:10, Xan wrote:
>> Hi,
>>
>> Following the thread of Higher-order functions, how can I do 
>> to pass a
>> function as a parameter and return a function. That is a 
>> something like:
>>
>> import std.functional, std.stdio;
>>
>> int f (int a) {
>> return 2*a;
>> }
>>
>> int delegate (int) g(int function(int a) p) {
>> return p;
>> }
>>
>>
>> void main() {
>> writeln(g(f)(1));
>> }
>>
>>
>> but it gives me:
>>
>> $ gdmd-4.6 functions.d
>> functions.d:8: Error: cannot implicitly convert expression (p) 
>> of type
>> int function(int a) to int delegate(int)
>> functions.d:13: Error: function functions.f (int a) is not 
>> callable
>> using argument types ()
>> functions.d:13: Error: expected 1 function arguments, not 0
>> functions.d:13: Error: function functions.g (int function(int 
>> a) p) is
>> not callable using argument types (int)
>> functions.d:13: Error: cannot implicitly convert expression 
>> (f()) of
>> type int to int function(int a)
>
> Use "delegate" or "function" both for the argument type and 
> return type.


How to do that?