On Thursday, 5 September 2013 at 09:33:00 UTC, John Colvin wrote:
On Thursday, 5 September 2013 at 09:11:06 UTC, John Colvin
wrote:
On Thursday, 5 September 2013 at 09:00:27 UTC, John Colvin
wrote:
On Tuesday, 3 September 2013 at 11:51:37 UTC, Carl Sturtivant
wrote:
I'm confused as to what you're trying to do... your example
code is equivalent to
import std.stdio;
int x = 3;
int scale( int s) { return x * s; }
auto f = &scale;
writeln( f(7) );
No it isn't according to dmd.
My code is a minimal piece that produces the same error as
some real code. The higher order generic function muddle in
the real code is supposed to transform one delegate into
another, but I still get the template problem if muddle is
the identity function (given here).
My example code isn't equivalent to the above according to
the compiler. Why is that? And how can I make it work?
Ok, that makes more sense.
The reason why it doesn't work is that you're effectively
asking the compiler to work backwards from {the type of the
delegate passed to muddle} to {the type parameters that would
have to be used in Dptr to generate that delegate type}.
I'm no expert on type-deduction algorithms, but I seriously
doubt that's a solvable problem in the general case,
especially considering that the definition of Dptr could
contain string mixins etc. In the general case, all code is
forwards-only.
Anyhow, it's easy to work around:
import std.traits : ReturnType, ParameterTypeTuple;
template Dptr( T, U...) {
alias T delegate( U args) Dptr;
}
auto muddle( DT)( DT f) {
alias T = ReturnType!f;
alias U = ParameterTypeTuple!f;
//use T and U
return f; //or make another delegate in real code
}
unittest {
import std.stdio;
int x = 3;
int scale( int s) { return x * s; }
Dptr!(int,int) f = muddle( &scale);
writeln( f(7));
}
Having said that, in the case where you explicitly set the
template parameters to muddle I think it *should* work and is
probably a bug (or at least a simple enhancement) that it
doesn't.
I filed a bug report for it
http://d.puremagic.com/issues/show_bug.cgi?id=10969
and only 26.5 mins after posting the bug, there's a pull request
to fix it: https://github.com/D-Programming-Language/dmd/pull/2526
That's what I call service :p
