• Vlad Levenfeld (17/17) May 21 2015 How possible is something like this:
• extrawurst (2/19) May 21 2015 that would be great!
• Dennis Ritchie (12/12) May 21 2015 Is it possible to write something like this in the compile time?
• ZombineDev (18/30) May 21 2015 Although the compiler already has enough information, this is not
• Dennis Ritchie (48/81) May 21 2015 Yes, but I can do that I need check use strings ( source is taken
• Dennis Ritchie (3/10) May 21 2015 Link wrong:

"Vlad Levenfeld" <vlevenfeld gmail.com> writes:

How possible is something like this:

   auto foo (T)(T t)
   if (is(Eq!T) && is(Num!T) && T.sizeof == 4)
   {...}

   foo ("hello");

then you get some output from the compiler, something like

   Error: template foo cannot deduce function from argument types 
!()(string),
   candidates are:

     foo(T)(T x)
     pass is(Eq!T)
     fail is(Num!T)
     fail T.sizeof == 4

Since template code (and by extension its constraint) is known to 
the compiler, would it break anything to have it parse the 
logical connectives and state specifically what fails for each 
attempted match?

"extrawurst" <stephan extrawurst.org> writes:

On Thursday, 21 May 2015 at 08:17:47 UTC, Vlad Levenfeld wrote:
 How possible is something like this:

   auto foo (T)(T t)
   if (is(Eq!T) && is(Num!T) && T.sizeof == 4)
   {...}

   foo ("hello");

 then you get some output from the compiler, something like

   Error: template foo cannot deduce function from argument 
 types !()(string),
   candidates are:

     foo(T)(T x)
     pass is(Eq!T)
     fail is(Num!T)
     fail T.sizeof == 4

 Since template code (and by extension its constraint) is known 
 to the compiler, would it break anything to have it parse the 
 logical connectives and state specifically what fails for each 
 attempted match?

that would be great!

"Dennis Ritchie" <dennis.ritchie mail.ru> writes:

Is it possible to write something like this in the compile time?

auto foo(T)(T t)
     if (is(T == immutable(int)) && t == 5)
     // Error: variable t cannot be read at compile time
{
     // ...
}

void main()
{
     immutable(int) n = 5;
     foo(n);
}

"ZombineDev" <valid_email he.re> writes:

On Thursday, 21 May 2015 at 11:05:10 UTC, Dennis Ritchie wrote:
 Is it possible to write something like this in the compile time?

 auto foo(T)(T t)
     if (is(T == immutable(int)) && t == 5)
     // Error: variable t cannot be read at compile time
 {
     // ...
 }

 void main()
 {
     immutable(int) n = 5;
     foo(n);
 }

Although the compiler already has enough information, this is not 
possible because the template constraints are not allowed to use 
the runtime parameters (they are compile-time only constraints). 
It would be more doable like this:

auto foo(T)(T t)
     if (is(T == immutable(int)))
{
     assert(t == 5);
}

void main()
{
     immutable int n = 6;
     foo(n); // Possible in the future:
     // dmd test.d --deep-asserts-verification
     // Error: function foo has an assert which can't satisfied:
     // assert(5 == n) <- n is 6
}

"Dennis Ritchie" <dennis.ritchie mail.ru> writes:

On Thursday, 21 May 2015 at 11:16:47 UTC, ZombineDev wrote:
 On Thursday, 21 May 2015 at 11:05:10 UTC, Dennis Ritchie wrote:
 Is it possible to write something like this in the compile 
 time?

 auto foo(T)(T t)
    if (is(T == immutable(int)) && t == 5)
    // Error: variable t cannot be read at compile time
 {
    // ...
 }

 void main()
 {
    immutable(int) n = 5;
    foo(n);
 }

 Although the compiler already has enough information, this is 
 not possible because the template constraints are not allowed 
 to use the runtime parameters (they are compile-time only 
 constraints). It would be more doable like this:

 auto foo(T)(T t)
     if (is(T == immutable(int)))
 {
     assert(t == 5);
 }

 void main()
 {
     immutable int n = 6;
     foo(n); // Possible in the future:
     // dmd test.d --deep-asserts-verification
     // Error: function foo has an assert which can't satisfied:
     // assert(5 == n) <- n is 6
 }

Yes, but I can do that I need check use strings ( source is taken 
away http://ddili.org/ders/d.en/mixin.html ):

import std.conv : to;
import std.string : split;
import std.stdio : writeln;

int[] filter(string predicate)(in int[] numbers)
     if (predicate.split[0] == "number" &&
         predicate.split[1] == ">" &&
         predicate.split[2].to!int <= int.max)
{
     int[] result;

     foreach (number; numbers) {
         if (mixin (predicate)) {
             result ~= number;
         }
     }

     return result;
}

void main()
{
     auto a = [1, 2, 3, 4, 5];

     writeln(a.filter!"number > 3"); // [4, 5]
}
-----
http://rextester.com/LNTB90172

Code Ali is not safe, because I can refer to the inner filter 
function directly in a function call :)

import std.conv : to;
import std.string : split;
import std.stdio : writeln;

int[] filter(string predicate)(in int[] numbers)
{
     int[] result;

     foreach (number; numbers) {
         if (mixin (predicate)) {
             result ~= number;
         }
     }

     return result;
}

void main()
{
     auto a = [1, 2, 3, 4, 5];

     writeln(a.filter!"result != [1, 2]"); // [1, 2]
}
-----
http://rextester.com/LNTB90172

"Dennis Ritchie" <dennis.ritchie mail.ru> writes:

On Thursday, 21 May 2015 at 13:25:56 UTC, Dennis Ritchie wrote:
 void main()
 {
     auto a = [1, 2, 3, 4, 5];

     writeln(a.filter!"result != [1, 2]"); // [1, 2]
 }
 -----
 http://rextester.com/LNTB90172

Link wrong:
http://rextester.com/UPUB47730