On Thursday, 29 October 2015 at 02:43:59 UTC, Shammah Chancellor
wrote:
Currently D has some very very powerful compile time features,
but they can get unwieldily quickly when doing compile time
reflection with mildly complicated logic. It's pretty
disappointing. I'd like to start a discussion around some of
the problems I've ran into using these features. If there
aren't bugzilla reports, I can open them but I'm not entirely
sure how to file these appropriately.
1) There seems to be many cases where __traits(allMembers, ...)
over a package produces symbol strings which cannot be
converted to a symbol using __traits(getMember). Also __traits
is not useable in reasonable contexts( alias foo =
__traits(..)). E.g.
This will fail due to weird symbols being returned from
allMembers.
```
import std.stdio;
enum hack(A...) = A;
int main() {
foreach(member; __traits(allMembers, std.stdio)) {
pragma(msg, member);
alias sym = hack!(__traits(getMember, std.stdio, member))[0];
}
return 0;
}
```
2) There are oddities around getting the types of functions
which are properties:
e.g. (Note the __compiles, and addressing the member):
```
//Gets the member type, even for properties.
private template MemberType(C, string memberName)
{
alias member = TypeTuple!(__traits (getMember, C,
memberName))[0];
static if (__traits (compiles, typeof(&member)))
{
static if (isSomeFunction!(typeof(&member)))
{
alias MemberType = typeof(&member);
}
else
{
alias MemberType = typeof(member);
}
} else {
alias MemberType = typeof(member);
}
}
```
3) Additionally, any kind of iteration becomes very unwieldy.
This kind of code becomes necessary in common cases. (E.g.
pass in a list of packages and reduce to a AliasSeq of type
symbols)
```
import std.stdio : writeln;
import std.typetuple;
// This won't work:
/+
private template stuff(Args...) {
/+ static +/ foreach(Arg; Args) {
...
}
}
+/
// Must do this instead:
private template expand(Args...)
{
static if (Args.length > 0)
{
static if (Args.length > 1)
{
enum stuff = AliasSeq!(repeat!(Args[0], Args.length),
stuff!(Args[1..$]));
}
else static if (Args.length == 1)
{
enum stuff = AliasSeq!(repeat!(Args[0], Args.length));
}
}
}
private template repeat(alias T, int times) {
static if ( times > 1 ) {
enum repeat = AliasSeq!(T, repeat!(T, times-1));
} else static if (times == 1) {
enum repeat = AliasSeq!(T);
}
}
enum things = expand!("A","B","C");
void main() {
pragma(msg, things);
writeln("Hello World");
}
```
I agree with you on an aspect: writing code with __traits() often
leads to a cascade of "unfriendly", "cryptic", "undigest",
static if and loops.
However your examples are discutables:
1/ The symbol is not always a type, so to test it there is
`if(is()){}`:
---
import std.stdio;
enum hack(A...) = A;
int main() {
foreach(member; __traits(allMembers, std.stdio)) {
pragma(msg, member);
static if (is(member))
alias sym = hack!(__traits(getMember, std.stdio,
member))[0];
}
}
---
2/ with only one way to define the alias it works, whatever is
the function type (property or not)
---
import std.traits, std.stdio, std.typetuple;
private template MemberType(C, string memberName)
{
alias member = TypeTuple!(__traits (getMember, C,
memberName))[0];
static if (isSomeFunction!(typeof(&member)))
alias MemberType = typeof(&member);
else static assert(0, "not handled here !");
}
struct Hop
{
void foo(uint a){}
@property uint bar(){return 0;}
uint baz(){return 0;}
@property void bla(uint h){}
}
void main()
{
MemberType!(Hop, "foo").stringof.writeln;
MemberType!(Hop, "bar").stringof.writeln;
MemberType!(Hop, "baz").stringof.writeln;
MemberType!(Hop, "bla").stringof.writeln;
}
---
3/ Your example doesn't compile
