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digitalmars.D - problem with the variadic template

reply baleog <maccarka yahoo.com> writes:
Hello
I want to write "map" function with the Common Lisp "mapcar" behaviour. But i
don't understand how to write one template for the N-argument delegate. 
This is how it works now:

T[] map(T, T1)(T delegate(T1) fun, T1[] xs) {
  T[] result; result.length = xs.length;
  foreach (i,x; xs)
    {
      result[i] = fun(x);
    }
  return result;
}
T[] map(T,T1,T2)(T delegate(T1, T2) fun, T1[] xs, T2[] ys) {
  T[] result; result.length = reduce!(min)(xs.length,[ys.length]);
  for (int i = 0; i<result.length; ++i)
    {
      result[i] = fun(xs[i], ys[i]);
    }
  return result;
}

How can i improve the code?

Thank you
Jul 05 2008
next sibling parent "Koroskin Denis" <2korden gmail.com> writes:
On Sun, 06 Jul 2008 02:19:09 +0400, baleog <maccarka yahoo.com> wrote:


 Hello
 I want to write "map" function with the Common Lisp "mapcar" behaviour=


.  =


 But i don't understand how to write one template for the N-argument  =



 delegate.
 This is how it works now:

 T[] map(T, T1)(T delegate(T1) fun, T1[] xs) {
   T[] result; result.length =3D xs.length;
   foreach (i,x; xs)
     {
       result[i] =3D fun(x);
     }
   return result;
 }
 T[] map(T,T1,T2)(T delegate(T1, T2) fun, T1[] xs, T2[] ys) {
   T[] result; result.length =3D reduce!(min)(xs.length,[ys.length]);
   for (int i =3D 0; i<result.length; ++i)
     {
       result[i] =3D fun(xs[i], ys[i]);
     }
   return result;
 }

 How can i improve the code?

 Thank you


There already exists a map function in D2, see std.algorithm  =

(http://www.digitalmars.com/d/2.0/phobos/std_algorithm.html), take a loo=
p  =

at it.
Jul 05 2008
prev sibling parent reply baleog <maccarka yahoo.com> writes:
Koroskin Denis Wrote:


 There already exists a map function in D2, see std.algorithm  
 (http://www.digitalmars.com/d/2.0/phobos/std_algorithm.html), take a loop  
 at it.


Standard map works only with string mixin (not with the delegate)  and multiple
arrays it concatenates into 1. So std map mixin 1-argument function.
Jul 05 2008
parent "Koroskin Denis" <2korden gmail.com> writes:
On Sun, 06 Jul 2008 02:38:35 +0400, baleog <maccarka yahoo.com> wrote:


 Koroskin Denis Wrote:


 There already exists a map function in D2, see std.algorithm
 (http://www.digitalmars.com/d/2.0/phobos/std_algorithm.html), take a =




 =


 loop
 at it.


 Standard map works only with string mixin (not with the delegate)  and=


  =


 multiple arrays it concatenates into 1. So std map mixin 1-argument  =



 function.



Ok, let's generalize your solution slightly.

T[] map(T, T1)(T delegate(T1) fun, T1[] xs) {
   T[] result; result.length =3D xs.length;
   foreach (i,x; xs)
     {
       result[i] =3D fun(x);
     }
   return result;
}

T[] map(T,T1,T2)(T delegate(T1, T2) fun, T1[] xs, T2[] ys) {
   T[] result; result.length =3D reduce!(min)(xs.length,[ys.length]);
   for (int i =3D 0; i<result.length; ++i)
     {
       result[i] =3D fun(xs[i], ys[i]);
     }
   return result;
}

First, replace T1,T2,T3.... TN with a variadic template:

T[] map(T, TT...)(T delegate(TT) fun, ArgsType args) {
    // ...
}

Now what's the type of args? It's better seen by example:
TT              ArgsType
(int, int)      (int[], int[])
(char, long)    (char[], long[])

So, we need some template that would convert TT to ArgsType. And it's  =

simple!

// convert T to T[]
template TArr(T) {
     alias T[] TArr;
}

// create a tuple
template S(T...) {
     alias T S;
}

// create tuple of type[] from tuple of type
template SA(T...) {
     static if (T.length =3D=3D 1) {
         alias S!(TArr!(T[0])) SA;
     } else {
         alias S!(TArr!(T[0]), SA!(T[1..$])) SA;
     }
}

Now map's signature becomes like this:
T[] map(T, TT...)(T delegate(TT) fun, SA!(TT) args);

And the only thing left is an implementation:

T[] map(T, TT...)(T delegate(TT) fun, SA!(TT) args) {
     T[] result;
     int length =3D args[0].length; // you may also put an assert to ens=
ure  =

args lengths are same
     result.length =3D length;	 // preallocate memory

     for (int i =3D 0; i < length; ++i) {
         // now tricky part: we need extract data from args as a TT
         TT tt =3D void;
         // there should really be a more simple approach, but I found  =

none, so let's fill the tuple it by hand:
         foreach (j, arg; args) {
             tt[j] =3D arg[i];
         }

         // invoke the function and store result
         result[i] =3D fun(tt);
     }

     return result;
}

It works with both D1 and D2, but there is a problem - IFTI doesn't work=
  =

anymore :(
P.S. Public domain, of course :)
Jul 05 2008