• Andrej Mitrovic (2/2) Apr 09 2011 If they're evaluated at the instantiation scope, and not in the point of...
• Jonathan M Davis (22/26) Apr 09 2011 Probably because statements are only valid in functions, and a template ...
• Cliff Hudson (5/38) Apr 09 2011 Couldn't you also just have your mixin define a function which - as a ne...

Andrej Mitrovic <none none.none> writes:

If they're evaluated at the instantiation scope, and not in the point of
definition, then why are statements disallowed?

Where does the limitation come from?

Jonathan M Davis <jmdavisProg gmx.com> writes:

 If they're evaluated at the instantiation scope, and not in the point of
 definition, then why are statements disallowed?
 
 Where does the limitation come from?

Probably because statements are only valid in functions, and a template is a 
set of declarations. It certainly wouldn't make any sense to have a normal 
template include statements, and the whole template mixin thing grew out of 
normal templates, so even if it makes perfect sense for template mixins to 
include statements, since they came out of normal templates in the first place 
(which can't have statements, since it would make no sense), they wouldn't 
wouldn't be able to have statements unless Walter thought to add that 
capabality specifically with mixing in in mind.

But while templates that you mixin are generally designed for that and 
therefore designed differently than normal templates, I don't believe that 
there's any really difference in them except in how you instantiate them, so 
the compiler is going to end up treating them the same as far as the lexer and 
parser go. And since it makes no sense for normal templates to have 
statements, templates that you mixin don't either. But think of the mess that 
you'd have if you tried to instantiate a template as a normal template when it 
held statements? You'd probably get some pretty weird errors if that were 
allowed.

So, I think that the limitation comes primarily from the fact that prior to 
mixins it made no sense for templates to contain naked statements, and 
templates intended for being used in mixins aren't really any different from 
normal templates from the compiler's perspective.

- Jonathan M Davis

Cliff Hudson <cliff.s.hudson gmail.com> writes:

Couldn't you also just have your mixin define a function which - as a nested
function - would have access to the stack of the enclosing block?  Slightly
less pretty, and slightly different (since variables declared in the mixin
wouldn't exist in the enclosing scope)... ok, yeah, rather different.

On Sat, Apr 9, 2011 at 3:07 PM, Jonathan M Davis <jmdavisProg gmx.com>wrote:

 If they're evaluated at the instantiation scope, and not in the point of
 definition, then why are statements disallowed?

 Where does the limitation come from?

 Probably because statements are only valid in functions, and a template is
 a
 set of declarations. It certainly wouldn't make any sense to have a normal
 template include statements, and the whole template mixin thing grew out of
 normal templates, so even if it makes perfect sense for template mixins to
 include statements, since they came out of normal templates in the first
 place
 (which can't have statements, since it would make no sense), they wouldn't
 wouldn't be able to have statements unless Walter thought to add that
 capabality specifically with mixing in in mind.

 But while templates that you mixin are generally designed for that and
 therefore designed differently than normal templates, I don't believe that
 there's any really difference in them except in how you instantiate them,
 so
 the compiler is going to end up treating them the same as far as the lexer
 and
 parser go. And since it makes no sense for normal templates to have
 statements, templates that you mixin don't either. But think of the mess
 that
 you'd have if you tried to instantiate a template as a normal template when
 it
 held statements? You'd probably get some pretty weird errors if that were
 allowed.

 So, I think that the limitation comes primarily from the fact that prior to
 mixins it made no sense for templates to contain naked statements, and
 templates intended for being used in mixins aren't really any different
 from
 normal templates from the compiler's perspective.

 - Jonathan M Davis