Issues with constants, and inout (was Real World usage of D, Today)

[kris]

Andrei Alexandrescu (See Website For Email) wrote:
> kris wrote:
> 
>> Andrei Alexandrescu (See Website For Email) wrote:
>>
>>> That being said, my view on what should be fixed in D is a bit 
>>> different. For example, as I said, one of my main foci is putting a 
>>> lead sarcophagus around "inout" before its radioactive waste affects 
>>> too much code that will become broken later. Also, implicit 
>>> conversion rules will change, and again I want to get that in ASAP so 
>>> not too much code suffers. And so on.
>>
>>
>> Aye, and that is certainly appreciated. Some of the items you've 
>> mentioned recently have long been festering sores for many. It'll 
>> certainly be refreshing to see some attention focused upon those.
>>
>> Regarding implicit conversion rules -- are you including the "issues" 
>> currently surrounding signature-matching when using constants?
> 
> 
> Could you give more detail?
> 
> Andrei


Please pardon the delay, and the length of this post ... here's a test:

extern (C) int printf (char*, ...);

class Foo
{
         void write (int x) {printf("int\n");}
         void write (uint x) {printf("uint\n");}
         void write (long x) {printf("long\n");}
         void write (char x) {printf("char\n");}
         void write (wchar x) {printf("wchar\n");}
         void write (double x) {printf("double\n");}

         void write (char[] x) {printf("char[]\n");}
         void write (wchar[] x) {printf("wchar[]\n");}
}

void main()
{
         auto foo = new Foo;

         foo.write ('c');
         foo.write (1);
         foo.write (1u);
         foo.write (3.14);
         //foo.write ("asa");
}

prints:

char
int
uint
double


DMD has actually become smarter than the last time I tried something 
like this: it manages to select the correct overload for 'c' whereas 
before it couldn't decide whether int or uint was a better match for the 
char instead. This is good.

It seems clear from the above that D /defaults/ the type of character, 
and undecorated integers, to something appropriate? In the above case 
'c' is defaulted to char, rather than wchar, for example. The 
undecorated int constant is defaulted to int, rather than uint or long. 
This is good.

Now for the broken part. When you uncomment the string constant, the 
compiler gets all confused about whether it's a char[] or wchar[]. There 
is no defaulting to one type, as there is for other constants (such as 
char). It /is/ possible to decorate the string constant in a similar 
manner to decorating integer constants:

foo.write ("qwe"c);

And this, of course, compiles. It's a PITA though, and differs from the 
rules for other constants. Things start to go south when using templates 
with string constants. For example, take this template sig:

uint locate(T) (T[] source, T match, uint start=0)

This is intended to handle types of char[], wchar[] and dchar[]. There's 
a uint on the end, as opposed to an int. Suppose I call it like this:

locate ("abc", "ab", 1);

we get a compile error, since the int-constant does not match a uint in 
the sig (IFTI currently needs exact sig matches). In order to get around 
this, we wrap the template with a few functions:

uint locate (char[] source, char[] match, uint start=0)
{
     return locateT!(char) (source, match, start);
}

uint locate (wchar[] source, wchar[] match, uint start=0)
{
     return locateT!(wchar) (source, match, start);
}

and dchar too.

Now we call it:

locate ("abc", "ab", 1);

Well, the int/uint error goes away (since function matching operates 
differently than IFTI matching), but we've now got our old friend back 
again -- the constant char[], wchar[], dchar[] mismatch problem.


How about another type of template? Here's one that does some simply 
text processing:

T[] layout(T) (T[] output, T[] format, T[][] subs...)

This has an output buffer, a format string, and a set of optional args; 
all of the same type. If I call it like so:

char[128] tmp;
char[]    world = "world";
layout (tmp, "hello %1", world);

that compiles ok. If I use wchar[] instead, it doesn't compile:

wchar[128] tmp;
wchar[]    world = "world";
layout (tmp, "hello %1", world);

In this case, the constant string used for formatting remains as a 
char[], so the template match fails (args: wchar[], char[], wchar[])

However, if I change the template signature to this instead:

T[] layout(T) (T[] output, T[][] subs...)

then everything works the way I want it to, but the design is actually 
wrong (the format string is now not required). String constants can be a 
royal PITA.



inout
-----

Since you're working on inout also, I'd like to ask what the plan is 
relating to a couple of examples. Tango uses this style of call quite 
regularly:

get (inout int x);
get (inout char[] x);
etc.

This is a clean way to pass-by-reference, instead of dealing with all 
the pointer syntax. I sure hope D will retain reference semantics like 
this, in some form?


One current problem with inout, which you might not be aware of, is with 
regard to const structs. I need to pass structs by reference, because I 
don't want to pass them by value. Applying inout is the mechanism for 
describing this:

struct Bar {int a, b;}

Bar b = {1, 2};

void parf (inout Bar x) {}

void main()
{
    parf (b);
}

That all works fine. However, when I want to make those structs /const/ 
instead, I cannot use inout since it has mutating semantics: I get a 
compile error to that effect:

const Bar b = {1, 2};

 >> Error: cannot modify const variable 'b'

That is, there's no supported way to pass a const struct by reference. 
The response from Walter in the past has been "just use a pointer 
instead" ... well, yes I could do that. But it appears to be indicative 
of a problem with the language design?

Why do I want to use const? Well, the data held therein is for reference 
only, and (via some future D vendor) I want that reference data placed 
into a ROM segment. I do a lot of work with MCUs, and this sort of thing 
is a common requirement.

Cheers;

- Kris