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digitalmars.D - Re: problem with the variadic template

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.

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.


 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