digitalmars.D.learn - Cannot alias expression
- John Colvin (12/12) Oct 21 2013 I suspect I'm being very dumb here, but I can't get my head
- Dicebot (6/19) Oct 21 2013 I think this is a good match for a gold collection of "awkward
- simendsjo (3/23) Oct 21 2013 I didn't know the B template would work... Is this correct
- Dicebot (3/5) Oct 21 2013 http://dlang.org/template.html#TemplateAliasParameter
- simendsjo (5/29) Oct 21 2013 I've used quite some T(A...), so I was used to literals not being
- simendsjo (5/29) Oct 21 2013 I've used quite some T(A...), so I was used to literals not being
- Dicebot (6/11) Oct 21 2013 Literals are never symbols. It is a special case for template
- Jesse Phillips (4/17) Oct 21 2013 I say file it as a bug. That will make for an official answer on
- John Colvin (16/29) Oct 21 2013 Also:
- simendsjo (4/34) Oct 21 2013 Alias doesn't take primitive types. I usually add a regular B(A)
- John Colvin (2/34) Oct 21 2013 Why on earth not?
- Dicebot (5/6) Oct 21 2013 By spec / definition. Alias is intended to only work on symbols
- Dicebot (2/2) Oct 21 2013 Please check the specification I have linked ;) It is a weird
- Timon Gehr (12/23) Oct 22 2013 It is awkward design. I think it should just work. In case you needed to...
I suspect I'm being very dumb here, but I can't get my head
around this:
template B(alias A)
{
alias B = A;
}
template C(A ...)
{
alias C = A[0];
}
static assert(B!1 == 1); //fine
static assert(C!1 == 1); //Error: cannot alias an expression 1
Oct 21 2013
On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:
I suspect I'm being very dumb here, but I can't get my head
around this:
template B(alias A)
{
alias B = A;
}
template C(A ...)
{
alias C = A[0];
}
static assert(B!1 == 1); //fine
static assert(C!1 == 1); //Error: cannot alias an
expression 1
I think this is a good match for a gold collection of "awkward
mismatch of template alias vs normal alias". I am quite surprised
former template work actually, aliases are not supposed to handle
expressions at all. But magic of template alias parameter turns
expression into symbol and here we go.
Oct 21 2013
On Monday, 21 October 2013 at 15:00:26 UTC, Dicebot wrote:On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:I didn't know the B template would work... Is this correct semantics according to the spec?I suspect I'm being very dumb here, but I can't get my head around this: template B(alias A) { alias B = A; } template C(A ...) { alias C = A[0]; } static assert(B!1 == 1); //fine static assert(C!1 == 1); //Error: cannot alias an expression 1I think this is a good match for a gold collection of "awkward mismatch of template alias vs normal alias". I am quite surprised former template work actually, aliases are not supposed to handle expressions at all. But magic of template alias parameter turns expression into symbol and here we go.
Oct 21 2013
"Dicebot" <public dicebot.lv> On Monday, 21 October 2013 at 16:51:02 UTC, simendsjo wrote:I didn't know the B template would work... Is this correct semantics according to the spec?http://dlang.org/template.html#TemplateAliasParameter "Literals can also be used as arguments to alias parameters."
Oct 21 2013
On Monday, 21 October 2013 at 16:51:02 UTC, simendsjo wrote:On Monday, 21 October 2013 at 15:00:26 UTC, Dicebot wrote:I've used quite some T(A...), so I was used to literals not being aliasable. A bit strange that literals "is symbols" (or how I should phrase it) when using T(alias A), but not T(A...).On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:I didn't know the B template would work... Is this correct semantics according to the spec?I suspect I'm being very dumb here, but I can't get my head around this: template B(alias A) { alias B = A; } template C(A ...) { alias C = A[0]; } static assert(B!1 == 1); //fine static assert(C!1 == 1); //Error: cannot alias an expression 1I think this is a good match for a gold collection of "awkward mismatch of template alias vs normal alias". I am quite surprised former template work actually, aliases are not supposed to handle expressions at all. But magic of template alias parameter turns expression into symbol and here we go.
Oct 21 2013
On Monday, 21 October 2013 at 16:51:02 UTC, simendsjo wrote:On Monday, 21 October 2013 at 15:00:26 UTC, Dicebot wrote:I've used quite some T(A...), so I was used to literals not being aliasable. A bit strange that literals "is symbols" (or how I should phrase it) when using T(alias A), but not T(A...).On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:I didn't know the B template would work... Is this correct semantics according to the spec?I suspect I'm being very dumb here, but I can't get my head around this: template B(alias A) { alias B = A; } template C(A ...) { alias C = A[0]; } static assert(B!1 == 1); //fine static assert(C!1 == 1); //Error: cannot alias an expression 1I think this is a good match for a gold collection of "awkward mismatch of template alias vs normal alias". I am quite surprised former template work actually, aliases are not supposed to handle expressions at all. But magic of template alias parameter turns expression into symbol and here we go.
Oct 21 2013
On Monday, 21 October 2013 at 19:33:07 UTC, simendsjo wrote:I've used quite some T(A...), so I was used to literals not being aliasable. A bit strange that literals "is symbols" (or how I should phrase it) when using T(alias A), but not T(A...).Literals are never symbols. It is a special case for template alias parameter. But alias parameter itself it a symbol and thus can be used with normal alias. Variadic template argument list infers literal as a value, not as a template alias parameter. Values can't be used with normal aliases and thus it fails.
Oct 21 2013
On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:
I suspect I'm being very dumb here, but I can't get my head
around this:
template B(alias A)
{
alias B = A;
}
template C(A ...)
{
alias C = A[0];
}
static assert(B!1 == 1); //fine
static assert(C!1 == 1); //Error: cannot alias an
expression 1
I say file it as a bug. That will make for an official answer on
what is correct. Could be that template B shouldn't be valid
either.
Oct 21 2013
On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:
I suspect I'm being very dumb here, but I can't get my head
around this:
template B(alias A)
{
alias B = A;
}
template C(A ...)
{
alias C = A[0];
}
static assert(B!1 == 1); //fine
static assert(C!1 == 1); //Error: cannot alias an
expression 1
Also:
struct S{}
template B(alias A)
{
alias B = A;
}
template C(A ...)
{
alias C = A[0];
}
pragma(msg, B!int); //Error: template instance B!(int)
//does not match template declaration B(alias A)
pragma(msg, B!S); // S
pragma(msg, C!int); // int
pragma(msg, C!S); // S
Oct 21 2013
On Monday, 21 October 2013 at 18:42:27 UTC, John Colvin wrote:On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:Alias doesn't take primitive types. I usually add a regular B(A) template overload too. Note that when you have a alias A overload, structs uses that rather than the basic A template.I suspect I'm being very dumb here, but I can't get my head around this: template B(alias A) { alias B = A; } template C(A ...) { alias C = A[0]; } static assert(B!1 == 1); //fine static assert(C!1 == 1); //Error: cannot alias an expression 1Also: struct S{} template B(alias A) { alias B = A; } template C(A ...) { alias C = A[0]; } pragma(msg, B!int); //Error: template instance B!(int) //does not match template declaration B(alias A)pragma(msg, B!S); // S pragma(msg, C!int); // int pragma(msg, C!S); // S
Oct 21 2013
On Monday, 21 October 2013 at 19:14:25 UTC, simendsjo wrote:On Monday, 21 October 2013 at 18:42:27 UTC, John Colvin wrote:Why on earth not?On Monday, 21 October 2013 at 12:58:55 UTC, John Colvin wrote:Alias doesn't take primitive types.I suspect I'm being very dumb here, but I can't get my head around this: template B(alias A) { alias B = A; } template C(A ...) { alias C = A[0]; } static assert(B!1 == 1); //fine static assert(C!1 == 1); //Error: cannot alias an expression 1Also: struct S{} template B(alias A) { alias B = A; } template C(A ...) { alias C = A[0]; } pragma(msg, B!int); //Error: template instance B!(int) //does not match template declaration B(alias A)
Oct 21 2013
On Monday, 21 October 2013 at 20:20:45 UTC, John Colvin wrote:Why on earth not?By spec / definition. Alias is intended to only work on symbols (that is the very definition of alias). Unfortunately, template alias parameter has also extra special cases for now good reason which adds confusion.
Oct 21 2013
Please check the specification I have linked ;) It is a weird design but implementation currently conforms to it.
Oct 21 2013
On 10/21/2013 02:58 PM, John Colvin wrote:
I suspect I'm being very dumb here, but I can't get my head around this:
template B(alias A)
{
alias B = A;
}
template C(A ...)
{
alias C = A[0];
}
static assert(B!1 == 1); //fine
static assert(C!1 == 1); //Error: cannot alias an expression 1
It is awkward design. I think it should just work. In case you needed to
this for anything, the following works:
template ID(alias a){ alias ID = a; } // "identity function on symbols"
template B(alias A){
alias B = A;
}
template C(A ...){
alias C = ID!(A[0]);
}
static assert(B!1 == 1); // fine
static assert(C!1 == 1); // fine
Oct 22 2013