Re: 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; }));
}