On 10/03/2015 12:45 PM, Abdulhaq wrote:
Perhaps the answer to this is obvious, but what's harder to write from
scratch - a C++ compiler or a D compiler? :-)

We know Walter wrote a C++ compiler single handedly, does anyone else
recall the C++ Grandmaster qualification, the free course where
participants get to write a complete C++ compiler from scratch? I think
it's dead now but I can't find any real info about that despite a
serious google. What's the chances of anyone single-handedly writing a D
compiler from scratch in the future? I know deadalnix is writing SDC in
D - sounds interesting.

I have also started a similar project four years ago (it's currently just an incomplete compiler front-end), but I have not been able to work much on it during the last year. (It only compiles with DMD 2.060 though, because of issues similar to what I discuss below.)

Is the D language well enough documented /
specified for a complete D implementation to be even possible (as things
stand now)?



Well, not really. The main impediment to a fully formal specification is the interplay of forward references and the meta-programming system. (DMD just does a best-effort kind of thing, where the common case scenarios for which there were bug reports work, but in general the semantics of the resulting code can depend on things like the order that modules are passed on the command line, or perfectly valid code is rejected with a "forward reference error".) The documentation just specifies that everything works, but there is no consistent way to interpret it.

E.g.:

static if(!is(typeof(x))) enum y=2;
static if(!is(typeof(y))) enum x=2;

Arbitrarily abstruse examples can be constructed, e.g. this is from my test suite:

struct TestInvalidInheritance{
    class A{ int string; } // error
    template Mixin(string s){
        mixin("alias "~s~" Mixin;");
    }
    class D: Mixin!({D d = new E; return d.foo();}()){
        int foo(int x){ return 2;}
        string foo(){ return "X"; }
    }
    class E: D{
        override int foo(int x){ return super.foo(x); }
        override string foo(){ return "A"; }
    }
}

(I currently accept this code if the declaration of int string in class A is removed. Otherwise the code is analyzed until the point when it is clear that A is in D's superclass chain, and it is also clear that the symbol 'string' which was necessary to resolve in order to discover this fact was resolved incorrectly. The compiler then gives up and prints an error message:

example.d:3:18: error: declaration of 'string' is invalid
    class A{ int string; } // error
                 ^─────
example.d:9:9: note: this lookup on subclass 'D' should have resolved to it
        string foo(){ return "X"; })
        ^─────

I and SDC have different ways to deal with those kinds of examples, and I think the SDC way does not work (unless things have changed since I have looked at it). It assumes that declarations can be ordered and that it is fine to depend on the order of declarations.

My implementation is designed to be independent of declaration order and to reject the cases where there is no single obvious and consistent interpretation of the program. The drawbacks currently are:

- It is overly conservative in some cases, especially when string mixins are involved. E.g., the following code has only one consistent interpretation, but it is rejected (as is any reordering of those declarations):

enum x = "enum xx = q{int y = 0;};";

struct SS{
    mixin(xx);
    mixin(x);
}

- The current implementation is somewhat slow. IIRC, N-fold recursive template instantiation currently runs in Ω(N²). It's clear that this needs to be improved. If this is to be adopted as the official solution, it should not make the compiler any slower, at least in the common case.


There are also some other, more minor issues. For example, when the language specification speaks about "memory safety", it is really unclear what this means, as the language designers seem to think it that it is fine to have undefined behaviour in a section of code that is "verified memory safe".


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