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digitalmars.D - Bugs in template constraints

reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
There seem to be some bugs with template constraints. Here's a reduce example
(from TDPL) which will not compile:

import std.stdio;
import std.range;

 property bool empty(T)(T[] a) { return a.length == 0; }
 property ref T front(T)(T[] a) { return a[0]; }
void popFront(T)(ref T[] a) { a = a[1 .. $]; }


V reduce(alias fun, V, R)(V x, R range)
    if (is(typeof(x = fun(x, range.front)))
        && is(typeof(range.empty) == bool)
        && is(typeof(range.popFront())))
{
    //~ writeln(is(typeof(x = fun(x, range.front))));
    //~ writeln(is(typeof(range.empty) == bool));
    //~ writeln(is(typeof(range.popFront())));
    
    for ( ; !range.empty; range.popFront()) {
        x = fun(x, range.front);
    }
    return x;
}

unittest {
    int[] r = [10, 14, 3, 5, 23];
    
    // compute sum
    int sum = reduce!((a, b) { return a + b; })(0, r);
    assert(sum == 55);
    
    // compute minimum
    int min = reduce!((a, b) { return a < b ? a : b; })(r[0], r);
    assert(min == 3);
}

void main()
{
}

Errors:
reduce_original.d(28): Error: template reduce_original.reduce(alias fun,V,R) if
(is(typeof(x = fun(x,range.front))) && is(typeof(range.empty) == bool) &&
is(typeof(range.popFront()))) does not match any function template declaration
reduce_original.d(28): Error: template reduce_original.reduce(alias fun,V,R) if
(is(typeof(x = fun(x,range.front))) && is(typeof(range.empty) == bool) &&
is(typeof(range.popFront()))) cannot deduce template function from argument
types !(__dgliteral1)(int,int[])
reduce_original.d(28): Error: template instance errors instantiating template
reduce_original.d(32): Error: template reduce_original.reduce(alias fun,V,R) if
(is(typeof(x = fun(x,range.front))) && is(typeof(range.empty) == bool) &&
is(typeof(range.popFront()))) does not match any function template declaration
reduce_original.d(32): Error: template reduce_original.reduce(alias fun,V,R) if
(is(typeof(x = fun(x,range.front))) && is(typeof(range.empty) == bool) &&
is(typeof(range.popFront()))) cannot deduce template function from argument
types !(__dgliteral4)(int,int[])
reduce_original.d(32): Error: template instance errors instantiating template


If I comment out the constraints, and uncomment those writeln's - which are the
same as the constraints - like so:

V reduce(alias fun, V, R)(V x, R range)
    //~ if (is(typeof(x = fun(x, range.front)))
        //~ && is(typeof(range.empty) == bool)
        //~ && is(typeof(range.popFront())))
{
    writeln(is(typeof(x = fun(x, range.front))));
    writeln(is(typeof(range.empty) == bool));
    writeln(is(typeof(range.popFront())));
    
    for ( ; !range.empty; range.popFront()) {
        x = fun(x, range.front);
    }
    return x;
}

I will get all true results back:

true
true
true
true
true
true

I'm filing a bug unless something else is to blame here.
Aug 03 2010
next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
--00032555834a1645bf048cf0688d
Content-Type: text/plain; charset=ISO-8859-1

On Tue, Aug 3, 2010 at 20:59, Andrej Mitrovic <andrej.mitrovich gmail.com>wrote:


 There seem to be some bugs with template constraints. Here's a reduce
 example (from TDPL) which will not compile:


 V reduce(alias fun, V, R)(V x, R range)
    if (is(typeof(x = fun(x, range.front)))
        && is(typeof(range.empty) == bool)
        && is(typeof(range.popFront())))





 I'm filing a bug unless something else is to blame here.



I think that's because you cannot directly take the type of a statement. The
assignment in the first typeof() is to blame. To make a statement into an
expression, transform it into an anonymous void delegate(): put it in braces
(with a semicolon at the end) and call it like a function, like this:

V reduce(alias fun, V, R)(V x, R range)
    if (is({ typeof(x = fun(x, range.front);}()))
        && is(typeof(range.empty) == bool)
        && is(typeof(range.popFront())))
{...}

Or, more readable, wrap all the code you want to test into curly brackets
and evaluates its global return type:


V reduce(alias fun, V, R)(V x, R range)
   if (is(typeof({                              // I want to be able to do
that with an R and a V:
                 x = fun(x, range.front);
                 if (range.empty) {};
                 range.popFront();
   }())))

{...}

It's a D idiom you'll see in many places in the standard library. I
personally find it a _bit_ heavy on parenthesis, even though I like Lisp.


Philippe

--00032555834a1645bf048cf0688d
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

<br><br><div class=3D"gmail_quote">On Tue, Aug 3, 2010 at 20:59, Andrej Mit=
rovic <span dir=3D"ltr">&lt;<a href=3D"mailto:andrej.mitrovich gmail.com">a=
ndrej.mitrovich gmail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gma=
il_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-lef=
t:1ex;">
There seem to be some bugs with template constraints. Here&#39;s a reduce e=
xample (from TDPL) which will not compile:<br><br>
<br>
V reduce(alias fun, V, R)(V x, R range)<br>
 =A0 =A0if (is(typeof(x =3D fun(x, range.front)))<br>
 =A0 =A0 =A0 =A0&amp;&amp; is(typeof(range.empty) =3D=3D bool)<br>
 =A0 =A0 =A0 =A0&amp;&amp; is(typeof(range.popFront())))<br></blockquote><d=
iv>=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;bo=
rder-left:1px #ccc solid;padding-left:1ex;">I&#39;m filing a bug unless som=
ething else is to blame here.<br>

</blockquote></div><br><div>I think that&#39;s because you cannot directly =
take the type of a statement. The assignment in the first typeof() is to bl=
ame. To make a statement into an expression, transform it into an anonymous=
 void delegate(): put it in braces (with a semicolon at the end) and call i=
t like a function, like this:</div>
<div><br></div><div>V reduce(alias fun, V, R)(V x, R range)</div><div>=A0=
=A0 =A0if (is({ typeof(x =3D fun(x, range.front);}()))</div><div>=A0=A0 =A0=
 =A0 =A0&amp;&amp; is(typeof(range.empty) =3D=3D bool)</div><div>=A0=A0 =A0=
 =A0 =A0&amp;&amp; is(typeof(range.popFront())))</div>
<div>{...}</div><div><br></div><div>Or, more readable, wrap all the code yo=
u want to test into curly brackets and evaluates its global return type:</d=
iv><div><br></div><div><div><br></div><div>V reduce(alias fun, V, R)(V x, R=
 range)</div>
<div>=A0=A0 if (is(typeof({ =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
 =A0 =A0 =A0// I want to be able to do that with an R and a V:</div><div>=
=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 x =3D fun(x, range.front);</div><div>=A0=
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (range.empty) {};</div><div>=A0=A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 range.popFront();</div>
<div>=A0=A0 }())))</div><div>=A0</div></div><div>{...}</div><div><br></div>=
<div>It&#39;s a D idiom you&#39;ll see in many places in the standard libra=
ry. I personally find it a _bit_ heavy on parenthesis, even though I like L=
isp.</div>
<div><br></div><div><br></div><div>Philippe</div>

--00032555834a1645bf048cf0688d--
Aug 03 2010
prev sibling next sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
--0016364ef6faf62caa048cf0c7df
Content-Type: text/plain; charset=ISO-8859-1

On Tue, Aug 3, 2010 at 9:34 PM, Philippe Sigaud
<philippe.sigaud gmail.com>wrote:


 On Tue, Aug 3, 2010 at 20:59, Andrej Mitrovic
<andrej.mitrovich gmail.com>wrote:


 There seem to be some bugs with template constraints. Here's a reduce
 example (from TDPL) which will not compile:


 V reduce(alias fun, V, R)(V x, R range)
    if (is(typeof(x = fun(x, range.front)))
        && is(typeof(range.empty) == bool)
        && is(typeof(range.popFront())))





 I'm filing a bug unless something else is to blame here.



 I think that's because you cannot directly take the type of a statement.
 The assignment in the first typeof() is to blame. To make a statement into
 an expression, transform it into an anonymous void delegate(): put it in
 braces (with a semicolon at the end) and call it like a function, like this:

 V reduce(alias fun, V, R)(V x, R range)
     if (is({ typeof(x = fun(x, range.front);}()))
         && is(typeof(range.empty) == bool)
         && is(typeof(range.popFront())))
 {...}






 Or, more readable, wrap all the code you want to test into curly brackets
 and evaluates its global return type:


 V reduce(alias fun, V, R)(V x, R range)
    if (is(typeof({                              // I want to be able to do
 that with an R and a V:
                  x = fun(x, range.front);
                  if (range.empty) {};
                  range.popFront();
    }())))

 {...}

 It's a D idiom you'll see in many places in the standard library. I
 personally find it a _bit_ heavy on parenthesis, even though I like Lisp.


 Philippe



Yeah, those paranthesis are getting a bit scary now. :) I guess this one
goes to the TDPL errata.

Thanks for your help Philippe.

--0016364ef6faf62caa048cf0c7df
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

<br><br><div class=3D"gmail_quote">On Tue, Aug 3, 2010 at 9:34 PM, Philippe=
 Sigaud <span dir=3D"ltr">&lt;<a href=3D"mailto:philippe.sigaud gmail.com">=
philippe.sigaud gmail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gma=
il_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(20=
4, 204, 204); padding-left: 1ex;">
<br><br><div class=3D"gmail_quote"><div class=3D"im">On Tue, Aug 3, 2010 at=
 20:59, Andrej Mitrovic <span dir=3D"ltr">&lt;<a href=3D"mailto:andrej.mitr=
ovich gmail.com" target=3D"_blank">andrej.mitrovich gmail.com</a>&gt;</span=

 wrote:<br>



 border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div class=
=3D"im">
There seem to be some bugs with template constraints. Here&#39;s a reduce e=
xample (from TDPL) which will not compile:<br><br>
<br></div><div class=3D"im">
V reduce(alias fun, V, R)(V x, R range)<br>
 =A0 =A0if (is(typeof(x =3D fun(x, range.front)))<br>
 =A0 =A0 =A0 =A0&amp;&amp; is(typeof(range.empty) =3D=3D bool)<br>
 =A0 =A0 =A0 =A0&amp;&amp; is(typeof(range.popFront())))<br></div></blockqu=
ote><div>=A0</div><blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0p=
t 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"=

I&#39;m filing a bug unless something else is to blame here.<br>




</blockquote></div><br><div>I think that&#39;s because you cannot directly =
take the type of a statement. The assignment in the first typeof() is to bl=
ame. To make a statement into an expression, transform it into an anonymous=
 void delegate(): put it in braces (with a semicolon at the end) and call i=
t like a function, like this:</div>
<div class=3D"im">
<div><br></div><div>V reduce(alias fun, V, R)(V x, R range)</div><div>=A0=
=A0 =A0if (is({ typeof(x =3D fun(x, range.front);}()))</div><div>=A0=A0 =A0=
 =A0 =A0&amp;&amp; is(typeof(range.empty) =3D=3D bool)</div><div>=A0=A0 =A0=
 =A0 =A0&amp;&amp; is(typeof(range.popFront())))</div>

</div><div>{...}</div><div><br></div></blockquote><div><br>The { comes afte=
r &quot;typeof(&quot; as in your second example, and then it compiles.<br>=
=A0</div><blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8=
ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div></div><div>Or, more readable, wrap all the code you want to test into =
curly brackets and evaluates its global return type:</div><div><br></div><d=
iv><div class=3D"im"><div><br></div><div>V reduce(alias fun, V, R)(V x, R r=
ange)</div>

</div><div>=A0=A0 if (is(typeof({ =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0// I want to be able to do that with an R and a V:</div>=
<div>=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 x =3D fun(x, range.front);</div><di=
v>=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (range.empty) {};</div><div>
=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 range.popFront();</div>
<div>=A0=A0 }())))</div><div>=A0</div></div><div>{...}</div><div><br></div>=
<div>It&#39;s a D idiom you&#39;ll see in many places in the standard libra=
ry. I personally find it a _bit_ heavy on parenthesis, even though I like L=
isp.</div>

<div><br></div><font color=3D"#888888"><div><br></div><div>Philippe</div>
</font></blockquote></div><br>Yeah, those paranthesis are getting a bit sca=
ry now. :) I guess this one goes to the TDPL errata.<br><br>Thanks for your=
 help Philippe.<br><br><br>

--0016364ef6faf62caa048cf0c7df--
Aug 03 2010
prev sibling next sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
--0016368330526f4264048cf0d2bc
Content-Type: text/plain; charset=ISO-8859-1

Oh and there's a shorter way to write this example, by using isInputRange
from std.range, like so:

if (isInputRange!R && is(typeof({x = fun(x, range.front);})))

This was in TDPL (except the {}'s which are missing).

On Tue, Aug 3, 2010 at 10:01 PM, Andrej Mitrovic <andrej.mitrovich gmail.com

 wrote:




 On Tue, Aug 3, 2010 at 9:34 PM, Philippe Sigaud <philippe.sigaud gmail.com

 wrote:




 On Tue, Aug 3, 2010 at 20:59, Andrej Mitrovic <andrej.mitrovich gmail.com

 wrote:




 There seem to be some bugs with template constraints. Here's a reduce
 example (from TDPL) which will not compile:


 V reduce(alias fun, V, R)(V x, R range)
    if (is(typeof(x = fun(x, range.front)))
        && is(typeof(range.empty) == bool)
        && is(typeof(range.popFront())))





 I'm filing a bug unless something else is to blame here.



 I think that's because you cannot directly take the type of a statement.
 The assignment in the first typeof() is to blame. To make a statement into
 an expression, transform it into an anonymous void delegate(): put it in
 braces (with a semicolon at the end) and call it like a function, like this:

 V reduce(alias fun, V, R)(V x, R range)
     if (is({ typeof(x = fun(x, range.front);}()))
         && is(typeof(range.empty) == bool)
         && is(typeof(range.popFront())))
 {...}



 compiles.



 Or, more readable, wrap all the code you want to test into curly brackets
 and evaluates its global return type:


 V reduce(alias fun, V, R)(V x, R range)
    if (is(typeof({                              // I want to be able to do
 that with an R and a V:
                  x = fun(x, range.front);
                  if (range.empty) {};
                  range.popFront();
    }())))

 {...}

 It's a D idiom you'll see in many places in the standard library. I
 personally find it a _bit_ heavy on parenthesis, even though I like Lisp.


 Philippe



 Yeah, those paranthesis are getting a bit scary now. :) I guess this one
 goes to the TDPL errata.

 Thanks for your help Philippe.



--0016368330526f4264048cf0d2bc
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

Oh and there&#39;s a shorter way to write this example, by using isInputRan=
ge from std.range, like so:<br><br>if (isInputRange!R &amp;&amp; is(typeof(=
{x =3D fun(x, range.front);})))<br><br>This was in TDPL (except the {}&#39;=
s which are missing).<br>
<br><div class=3D"gmail_quote">On Tue, Aug 3, 2010 at 10:01 PM, Andrej Mitr=
ovic <span dir=3D"ltr">&lt;<a href=3D"mailto:andrej.mitrovich gmail.com">an=
drej.mitrovich gmail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gmai=
l_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204=
, 204, 204); padding-left: 1ex;">
<br><br><div class=3D"gmail_quote"><div class=3D"im">On Tue, Aug 3, 2010 at=
 9:34 PM, Philippe Sigaud <span dir=3D"ltr">&lt;<a href=3D"mailto:philippe.=
sigaud gmail.com" target=3D"_blank">philippe.sigaud gmail.com</a>&gt;</span=

 wrote:<br>



r-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<br><br><div class=3D"gmail_quote"><div>On Tue, Aug 3, 2010 at 20:59, Andre=
j Mitrovic <span dir=3D"ltr">&lt;<a href=3D"mailto:andrej.mitrovich gmail.c=
om" target=3D"_blank">andrej.mitrovich gmail.com</a>&gt;</span> wrote:<br>
</div><blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex;=
 border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div>
There seem to be some bugs with template constraints. Here&#39;s a reduce e=
xample (from TDPL) which will not compile:<br><br>
<br></div><div>
V reduce(alias fun, V, R)(V x, R range)<br>
 =A0 =A0if (is(typeof(x =3D fun(x, range.front)))<br>
 =A0 =A0 =A0 =A0&amp;&amp; is(typeof(range.empty) =3D=3D bool)<br>
 =A0 =A0 =A0 =A0&amp;&amp; is(typeof(range.popFront())))<br></div></blockqu=
ote><div>=A0</div><blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0p=
t 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"=

I&#39;m filing a bug unless something else is to blame here.<br>





</blockquote></div><br><div>I think that&#39;s because you cannot directly =
take the type of a statement. The assignment in the first typeof() is to bl=
ame. To make a statement into an expression, transform it into an anonymous=
 void delegate(): put it in braces (with a semicolon at the end) and call i=
t like a function, like this:</div>

<div>
<div><br></div><div>V reduce(alias fun, V, R)(V x, R range)</div><div>=A0=
=A0 =A0if (is({ typeof(x =3D fun(x, range.front);}()))</div><div>=A0=A0 =A0=
 =A0 =A0&amp;&amp; is(typeof(range.empty) =3D=3D bool)</div><div>=A0=A0 =A0=
 =A0 =A0&amp;&amp; is(typeof(range.popFront())))</div>


</div><div>{...}</div><div><br></div></blockquote></div><div><br>The { come=
s after &quot;typeof(&quot; as in your second example, and then it compiles=
.<br>=A0</div><div class=3D"im"><blockquote class=3D"gmail_quote" style=3D"=
margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); paddi=
ng-left: 1ex;">

<div></div><div>Or, more readable, wrap all the code you want to test into =
curly brackets and evaluates its global return type:</div><div><br></div><d=
iv><div><div><br></div><div>V reduce(alias fun, V, R)(V x, R range)</div>


</div><div>=A0=A0 if (is(typeof({ =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0// I want to be able to do that with an R and a V:</div>=
<div>=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 x =3D fun(x, range.front);</div><di=
v>=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (range.empty) {};</div><div>

=A0=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 range.popFront();</div>
<div>=A0=A0 }())))</div><div>=A0</div></div><div>{...}</div><div><br></div>=
<div>It&#39;s a D idiom you&#39;ll see in many places in the standard libra=
ry. I personally find it a _bit_ heavy on parenthesis, even though I like L=
isp.</div>


<div><br></div><font color=3D"#888888"><div><br></div><div>Philippe</div>
</font></blockquote></div></div><br>Yeah, those paranthesis are getting a b=
it scary now. :) I guess this one goes to the TDPL errata.<br><br>Thanks fo=
r your help Philippe.<br><br><br>
</blockquote></div><br>

--0016368330526f4264048cf0d2bc--
Aug 03 2010
prev sibling next sibling parent Philippe Sigaud <philippe.sigaud gmail.com> writes:
--00151747c070b37826048cf11611
Content-Type: text/plain; charset=ISO-8859-1

On Tue, Aug 3, 2010 at 22:04, Andrej Mitrovic <andrej.mitrovich gmail.com>wrote:


 Oh and there's a shorter way to write this example, by using isInputRange
 from std.range, like so:

 if (isInputRange!R && is(typeof({x = fun(x, range.front);})))



Does this work, without the () after the } ?



 This was in TDPL (except the {}'s which are missing).




As I said, abstracting away common constraints is a common trick. Have a
look at std.range, you'll see a bunch of these.


Philippe

--00151747c070b37826048cf11611
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

<br><br><div class=3D"gmail_quote">On Tue, Aug 3, 2010 at 22:04, Andrej Mit=
rovic <span dir=3D"ltr">&lt;<a href=3D"mailto:andrej.mitrovich gmail.com">a=
ndrej.mitrovich gmail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gma=
il_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-lef=
t:1ex;">
Oh and there&#39;s a shorter way to write this example, by using isInputRan=
ge from std.range, like so:<br><br>if (isInputRange!R &amp;&amp; is(typeof(=
{x =3D fun(x, range.front);})))<br></blockquote><div><br></div><div>Does th=
is work, without the () after the } ?</div>
<div>=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;=
border-left:1px #ccc solid;padding-left:1ex;">This was in TDPL (except the =
{}&#39;s which are missing).</blockquote><div><br></div><div>As I said, abs=
tracting away common constraints is a common trick. Have a look at std.rang=
e, you&#39;ll see a bunch of these.</div>
<div><br></div><div><br></div><div>Philippe=A0</div></div>

--00151747c070b37826048cf11611--
Aug 03 2010
prev sibling next sibling parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
--0016e649d5dc0222a4048cf1b4d0
Content-Type: text/plain; charset=ISO-8859-1

On Tue, Aug 3, 2010 at 10:23 PM, Philippe Sigaud
<philippe.sigaud gmail.com>wrote:


 On Tue, Aug 3, 2010 at 22:04, Andrej Mitrovic
<andrej.mitrovich gmail.com>wrote:


 Oh and there's a shorter way to write this example, by using isInputRange
 from std.range, like so:

 if (isInputRange!R && is(typeof({x = fun(x, range.front);})))



 Does this work, without the () after the } ?



I haven't even noticed those.

In the following, If I add the pair of ()'s I get void as a return type. If
I remove them, I get void delegate():

writeln(typeid(typeof( delegate void () {int x = 1;}())));    // writes void
writeln(typeid(typeof( delegate void () {int x = 1;})));      // writes void
delegate()

So I definitely need to add them. When not added the expression evaluates to
void delegate(), which is a valid type and the constraint then passes.

If I understood everything, this code in the constraint:

is(typeof({x = fun(x, range.front);}() )))

creates an anonymous function, the compiler sees it's trying to access x so
it makes it a delegate, and it infers that the function takes no arguments
and the return type is void. Did I get this right?


This was in TDPL (except the {}'s which are missing).

 As I said, abstracting away common constraints is a common trick. Have a
 look at std.range, you'll see a bunch of these.


 Philippe



--0016e649d5dc0222a4048cf1b4d0
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Content-Transfer-Encoding: quoted-printable

<br><br><div class=3D"gmail_quote">On Tue, Aug 3, 2010 at 10:23 PM, Philipp=
e Sigaud <span dir=3D"ltr">&lt;<a href=3D"mailto:philippe.sigaud gmail.com"=

philippe.sigaud gmail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gm=



04, 204, 204); padding-left: 1ex;">
<br><br><div class=3D"gmail_quote"><div class=3D"im">On Tue, Aug 3, 2010 at=
 22:04, Andrej Mitrovic <span dir=3D"ltr">&lt;<a href=3D"mailto:andrej.mitr=
ovich gmail.com" target=3D"_blank">andrej.mitrovich gmail.com</a>&gt;</span=

 wrote:<br>



r-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Oh and there&#39;s a shorter way to write this example, by using isInputRan=
ge from std.range, like so:<br><br>if (isInputRange!R &amp;&amp; is(typeof(=
{x =3D fun(x, range.front);})))<br></blockquote><div><br></div></div><div>
Does this work, without the () after the } ?</div><div class=3D"im">
<div>=A0</div></div></div></blockquote><div><br>I haven&#39;t even noticed =
those. <br><br>In the following, If I add the pair of ()&#39;s I get void a=
s a return type. If I remove them, I get void delegate():<br><br>writeln(ty=
peid(typeof( delegate void () {int x =3D 1;}())));=A0=A0=A0 // writes void<=
br>
writeln(typeid(typeof( delegate void () {int x =3D 1;})));=A0=A0=A0=A0=A0 /=
/ writes void delegate()<br>=A0<br>So I definitely need to add them. When n=
ot added the expression evaluates to void delegate(), which is a valid type=
 and the constraint then passes.<br>
<br>If I understood everything, this code in the constraint:<br><br>is(type=
of({x =3D fun(x, range.front);}() )))<br><br>creates an anonymous function,=
 the compiler sees it&#39;s trying to access x so it makes it a delegate, a=
nd it infers that the function takes no arguments and the return type is vo=
id. Did I get this right?<br>
<br><br></div><blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0p=
t 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><di=
v class=3D"gmail_quote"><div class=3D"im"><blockquote class=3D"gmail_quote"=
 style=3D"margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 2=
04); padding-left: 1ex;">
This was in TDPL (except the {}&#39;s which are missing).</blockquote><div>=
<br></div></div><div>As I said, abstracting away common constraints is a co=
mmon trick. Have a look at std.range, you&#39;ll see a bunch of these.</div=



<div><br></div><font color=3D"#888888"><div><br></div><div>Philippe=A0</div=

</font></div>




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 Aug 03 2010



  parent  Pelle <pelle.mansson gmail.com>  writes:


On 08/03/2010 11:07 PM, Andrej Mitrovic wrote:

 On Tue, Aug 3, 2010 at 10:23 PM, Philippe Sigaud
 <philippe.sigaud gmail.com <mailto:philippe.sigaud gmail.com>> wrote:



     On Tue, Aug 3, 2010 at 22:04, Andrej Mitrovic
     <andrej.mitrovich gmail.com <mailto:andrej.mitrovich gmail.com>> wrote:

         Oh and there's a shorter way to write this example, by using
         isInputRange from std.range, like so:

         if (isInputRange!R && is(typeof({x = fun(x, range.front);})))


     Does this work, without the () after the } ?


 I haven't even noticed those.

 In the following, If I add the pair of ()'s I get void as a return type.
 If I remove them, I get void delegate():

 writeln(typeid(typeof( delegate void () {int x = 1;}())));    // writes void
 writeln(typeid(typeof( delegate void () {int x = 1;})));      // writes
 void delegate()

 So I definitely need to add them. When not added the expression
 evaluates to void delegate(), which is a valid type and the constraint
 then passes.

 If I understood everything, this code in the constraint:

 is(typeof({x = fun(x, range.front);}() )))

 creates an anonymous function, the compiler sees it's trying to access x
 so it makes it a delegate, and it infers that the function takes no
 arguments and the return type is void. Did I get this right?



You only need to call it if you want to check the return type. You 
cannot create a function with content that can't compile, so in this 
case, the () isn't needed.

Correct me if I'm wrong, of course. :)



 Aug 03 2010



prev sibling next sibling parent  Philippe Sigaud <philippe.sigaud gmail.com>  writes:


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On Tue, Aug 3, 2010 at 23:07, Andrej Mitrovic <andrej.mitrovich gmail.com>wrote:


 If I understood everything, this code in the constraint:

 is(typeof({x = fun(x, range.front);}() )))

 creates an anonymous function, the compiler sees it's trying to access x so
 it makes it a delegate, and it infers that the function takes no arguments
 and the return type is void. Did I get this right?



Yes, that's it. If the enclosed statements are OK, then the whole delegate
compiles, gets a type (void delegate() ), and so on, though is(typeof()).

So, using it as a way to get template constraints to work on many statements
was not planned that way, I think. But it works :)

Basically any D block statement: { statements;} can be seen as a void
delegate().

see http://www.digitalmars.com/d/2.0/lazy-evaluation.html
and http://www.digitalmars.com/d/2.0/statement.html#ScopeStatement for an
example of this.

Philippe

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<div class=3D"gmail_quote">On Tue, Aug 3, 2010 at 23:07, Andrej Mitrovic <s=
pan dir=3D"ltr">&lt;<a href=3D"mailto:andrej.mitrovich gmail.com">andrej.mi=
trovich gmail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote=
" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div class=3D"gmail_quote"><div><br>If I understood everything, this code i=
n the constraint:<br><br>is(typeof({x =3D fun(x, range.front);}() )))<br><b=
r>creates an anonymous function, the compiler sees it&#39;s trying to acces=
s x so it makes it a delegate, and it infers that the function takes no arg=
uments and the return type is void. Did I get this right?<br>
</div></div></blockquote><div><br></div><div>Yes, that&#39;s it. If the enc=
losed statements are OK, then the whole delegate compiles, gets a type (voi=
d delegate() ), and so on, though is(typeof()).</div><div><br></div><div>
So, using it as a way to get template constraints to work on many statement=
s was not planned that way, I think. But it works :)</div><div><br></div><d=
iv>Basically any D block statement: { statements;} can be seen as a void de=
legate().=A0</div>
<div><br></div><div>see=A0<a href=3D"http://www.digitalmars.com/d/2.0/lazy-=
evaluation.html">http://www.digitalmars.com/d/2.0/lazy-evaluation.html</a><=
/div><div>and=A0<a href=3D"http://www.digitalmars.com/d/2.0/statement.html#=
ScopeStatement">http://www.digitalmars.com/d/2.0/statement.html#ScopeStatem=
ent</a>=A0for an example of this.</div>
<div><br></div><div>Philippe</div><div><br></div></div>

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 Aug 03 2010



prev sibling  parent  Andrej Mitrovic <andrej.mitrovich gmail.com>  writes:


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I guess that would make sense. With {}() I could add a comparison for the
return type if I ever needed that.

I did take a look in std.range, and pretty much all the templates there use
the {}() syntax.

On Tue, Aug 3, 2010 at 11:53 PM, Pelle <pelle.mansson gmail.com> wrote:


 On 08/03/2010 11:07 PM, Andrej Mitrovic wrote:


 On Tue, Aug 3, 2010 at 10:23 PM, Philippe Sigaud
 <philippe.sigaud gmail.com <mailto:philippe.sigaud gmail.com>> wrote:



    On Tue, Aug 3, 2010 at 22:04, Andrej Mitrovic
    <andrej.mitrovich gmail.com <mailto:andrej.mitrovich gmail.com>>
 wrote:

        Oh and there's a shorter way to write this example, by using
        isInputRange from std.range, like so:

        if (isInputRange!R && is(typeof({x = fun(x, range.front);})))


    Does this work, without the () after the } ?


 I haven't even noticed those.

 In the following, If I add the pair of ()'s I get void as a return type.
 If I remove them, I get void delegate():

 writeln(typeid(typeof( delegate void () {int x = 1;}())));    // writes
 void
 writeln(typeid(typeof( delegate void () {int x = 1;})));      // writes
 void delegate()

 So I definitely need to add them. When not added the expression
 evaluates to void delegate(), which is a valid type and the constraint
 then passes.

 If I understood everything, this code in the constraint:

 is(typeof({x = fun(x, range.front);}() )))

 creates an anonymous function, the compiler sees it's trying to access x
 so it makes it a delegate, and it infers that the function takes no
 arguments and the return type is void. Did I get this right?



 create a function with content that can't compile, so in this case, the ()
 isn't needed.

 Correct me if I'm wrong, of course. :)



--0016367fbad467f19a048cf28f18
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

I guess that would make sense. With {}() I could add a comparison for the r=
eturn type if I ever needed that.<br><br>I did take a look in std.range, an=
d pretty much all the templates there use the {}() syntax.<br><br><div clas=
s=3D"gmail_quote">
On Tue, Aug 3, 2010 at 11:53 PM, Pelle <span dir=3D"ltr">&lt;<a href=3D"mai=
lto:pelle.mansson gmail.com">pelle.mansson gmail.com</a>&gt;</span> wrote:<=
br><blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; bo=
rder-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
On 08/03/2010 11:07 PM, Andrej Mitrovic wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; borde=
r-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;"><div class=3D"im"=



<br>
On Tue, Aug 3, 2010 at 10:23 PM, Philippe Sigaud<br>
&lt;<a href=3D"mailto:philippe.sigaud gmail.com" target=3D"_blank">philippe=
.sigaud gmail.com</a> &lt;mailto:<a href=3D"mailto:philippe.sigaud gmail.co=
m" target=3D"_blank">philippe.sigaud gmail.com</a>&gt;&gt; wrote:<br>
<br>
<br>
<br>
 =A0 =A0On Tue, Aug 3, 2010 at 22:04, Andrej Mitrovic<br></div><div class=
=3D"im">
 =A0 =A0&lt;<a href=3D"mailto:andrej.mitrovich gmail.com" target=3D"_blank"=

andrej.mitrovich gmail.com</a> &lt;mailto:<a href=3D"mailto:andrej.mitrovi=



e:<br>
<br>
 =A0 =A0 =A0 =A0Oh and there&#39;s a shorter way to write this example, by =
using<br>
 =A0 =A0 =A0 =A0isInputRange from std.range, like so:<br>
<br>
 =A0 =A0 =A0 =A0if (isInputRange!R &amp;&amp; is(typeof({x =3D fun(x, range=
.front);})))<br>
<br>
<br>
 =A0 =A0Does this work, without the () after the } ?<br>
<br>
<br></div><div class=3D"im">
I haven&#39;t even noticed those.<br>
<br>
In the following, If I add the pair of ()&#39;s I get void as a return type=
.<br>
If I remove them, I get void delegate():<br>
<br>
writeln(typeid(typeof( delegate void () {int x =3D 1;}()))); =A0 =A0// writ=
es void<br>
writeln(typeid(typeof( delegate void () {int x =3D 1;}))); =A0 =A0 =A0// wr=
ites<br>
void delegate()<br>
<br>
So I definitely need to add them. When not added the expression<br>
evaluates to void delegate(), which is a valid type and the constraint<br>
then passes.<br>
<br></div><div class=3D"im">
If I understood everything, this code in the constraint:<br>
<br>
is(typeof({x =3D fun(x, range.front);}() )))<br>
<br>
creates an anonymous function, the compiler sees it&#39;s trying to access =
x<br>
so it makes it a delegate, and it infers that the function takes no<br>
arguments and the return type is void. Did I get this right?<br>
<br>
</div></blockquote>
<br>
You only need to call it if you want to check the return type. You cannot c=
reate a function with content that can&#39;t compile, so in this case, the =
() isn&#39;t needed.<br>
<br>
Correct me if I&#39;m wrong, of course. :)<br>
</blockquote></div><br>

--0016367fbad467f19a048cf28f18--



 Aug 03 2010