On 08/21/2013 07:45 PM, Andrei Alexandrescu wrote:
On 8/21/13 4:52 AM, John Colvin wrote:
On Wednesday, 21 August 2013 at 02:46:06 UTC, Dylan Knutson wrote:
Hello,
I'd like to open up discussion regarding allowing foreach loops which
iterate over a tuple of types to exist outside of function bodies.
I would *LOVE* to have this.
however, once we go down that route, it would be very tempting to allow
a whole lot of other compile-time imperative style programming. Which
would be awesome.
Not necessarily. We could allow "static foreach" as a functional
construct that binds in turn a symbol to each element in a collection.
...
Yup.
A static foreach in conjunction with string mixins would be great for
simplifying code generation, e.g. "for each method in that class
generate a method here".
AFAIK one only reason why it hasn't made it in yet were implementation
issues related to DMD internals? The design was discussed in 2007's
dconf IIRC.
Andrei
We really need to define a consistent semantics for compile time symbol
manipulation though.
Eg:
class C{
int a;
static foreach(x;__traits(allMembers,C)){
mixin("int "~__traits(identifier,x)~"b;");
}
}
What is this supposed to do? "class C{ int a,ab; }"? Non-terminating
compilation? Error?
My best guess is that the above code should be illegal, but there is a
lot of similar code already out in the wild that works by chance. Eg. I
guess some of Manu's bindings only work by luck. Thrift for D also
contains one or two questionable constructs, as David has shown me
recently. (It is possible that this is related to the recent breakage,
DMD does not necessarily behave consistently w.r.t. to this kind of code
across versions.)
Most of the times this has been brought up it was simply ignored, but it
is a glaring hole in D's design.
