Higher-order functions?

[Timon Gehr]

On 04/11/2012 11:51 AM, Xan wrote:
> On Wednesday, 11 April 2012 at 09:43:27 UTC, Timon Gehr wrote:
>> On 04/11/2012 11:37 AM, Xan wrote:
>>> On Wednesday, 11 April 2012 at 09:17:12 UTC, Jacob Carlborg wrote:
>>>> On 2012-04-11 10:45, Xan wrote:
>>>>> Good answer.
>>>>>
>>>>> For the other hand, what is the simplest method for implementing this
>>>>> (in pseucode) in D:
>>>>>
>>>>> Sure:
>>>>>
>>>>> FUNC someprocedure(int a, int b, func<int, int: int> f) int
>>>>> RETURN f(a, b)
>>>>> }
>>>>>
>>>>> And call it with:
>>>>>
>>>>> IO.writeLine("add: " .. someprocedure(2, 3, { a, b => a + b }))
>>>>> IO.writeLine("multiply: " .. someprocedure(2, 3, { a, b => a * b }))
>>>>>
>>>>> (Read the => as "gives")
>>>>>
>>>>> Is it possible to have this?
>>>>>
>>>>
>>>> If I understand the above code correctly:
>>>>
>>>> import std.stdio;
>>>>
>>>> int someprocedure (int a, int b, int delegate (int, int) f)
>>>> {
>>>>    return f(a, b);
>>>> }
>>>>
>>>
>>>
>>> Yes, you undertood correcty.
>>>
>>> Your code gives me an error:
>>>
>>> $ gdmd-4.6 funcions.d
>>> funcions.d:10: expression expected, not '>'
>>> funcions.d:10: found 'a' when expecting ','
>>> funcions.d:11: expression expected, not '>'
>>> funcions.d:11: found 'a' when expecting ','
>>>
>>>> void main ()
>>>> {
>>>>    writeln("add: ", someprocedure(2, 3, (a, b) => a + b));
>>>>    writeln("multiply: ", someprocedure(2, 3, (a, b) => a * b));
>>>> }
>>>
>>>
>>
>> AFAIK GDC does not yet support the new lambda literal syntax.
>>
>> You can use
>>
>> void main ()
>> {
>>     writeln("add: ", someprocedure(2, 3, (a, b) { return a + b; }));
>>     writeln("multiply: ", someprocedure(2, 3, (a, b) { return a * b; }));
>> }
>
> Better but with error ;-)
>
> $ gdmd-4.6 func2.d
> func2.d:10: Error: undefined identifier a
> func2.d:10: Error: undefined identifier b
> func2.d:10: Error: function func2.someprocedure (int a, int b, int
> delegate(int, int) f) is not callable using argument types
> (int,int,_error_ delegate(_error_, _error_))
> func2.d:10: Error: cannot implicitly convert expression (__dgliteral1)
> of type _error_ delegate(_error_, _error_) to int delegate(int, int)
>
>
> With:
>
> import std.stdio;
>
> int someprocedure (int a, int b, int delegate (int, int) f)
> {
>      return f(a, b);
> }
>
> void main ()
> {
>      writeln("add: ", someprocedure(2, 3, (a, b) { return a + b; }  ));
>      writeln("multiply: ", someprocedure(2, 3, (a, b) { return a * b; } ));
> }
>
>
> What is the error?
>

Apparently your compiler does not support parameter type deduction yet.

void main ()
{
     writeln("add: ", someprocedure(2, 3, (int a, int b) { return a + b; 
}));
     writeln("multiply: ", someprocedure(2, 3, (int a, int b) { return a 
* b; }));
}