Why doesn't the following code work in D2 (it works in D1)?

void foo (T) (in T[] a, T b)
{
	
}

void main ()
{
	"asd".foo('s');
}

The error I get is:

main.d(10): Error: template main.foo(T) does not match any function template declaration
main.d(10): Error: template main.foo(T) cannot deduce template function from argument types !()(string,char)

It seems to be some problem with the "b" argument, if I change that to "char" it works.

-- 
/Jacob Carlborg

On 06/27/2010 12:18 PM, Jacob Carlborg wrote:
> Why doesn't the following code work in D2 (it works in D1)?
>
> void foo (T) (in T[] a, T b)
> {
>
> }
>
> void main ()
> {
> "asd".foo('s');
> }
>

"asd".foo(cast(immutable) 's');

Jacob Carlborg <doob@me.com> wrote:

> Why doesn't the following code work in D2 (it works in D1)?
>
> void foo (T) (in T[] a, T b)
> {
> 	
> }
>
> void main ()
> {
> 	"asd".foo('s');
> }
>
> The error I get is:
>
> main.d(10): Error: template main.foo(T) does not match any function template declaration
> main.d(10): Error: template main.foo(T) cannot deduce template function from argument types !()(string,char)
>
> It seems to be some problem with the "b" argument, if I change that to "char" it works.

In D2, strings are of type immutable(char)[], so your T would be
immutable(char). However, 's' is a simple, unadorned char.

Ways to fix this would include:

void foo(T)(const T[] a, const T b){
...
}

void foo(T,U)(const T[] a, U b) if (is(Unqual!T == Unqual!U)) {
...
}

-- 
Simen

Simen kjaeraas:
> void foo(T,U)(const T[] a, U b) if (is(Unqual!T == Unqual!U)) {
> ...
> }

Yes, this was my solution.
My first solution was:

import std.traits: Unqual;
void foo(T)(T[] a, Unqual!T b) {}
void main () {
    "asd".foo('s');
}

But it doesn't work, and I don't know if it's supposed to work.

A problem with Unqual is that it strips away too much, so something like this can be better:

void foo(TA, T)(const TA[] a, T b) if (is(Deconst!TA == Deconst!T)) {}

Bye,
bearophile

On 2010-06-27 19:26, Simen kjaeraas wrote:
> Jacob Carlborg <doob@me.com> wrote:
>
>> Why doesn't the following code work in D2 (it works in D1)?
>>
>> void foo (T) (in T[] a, T b)
>> {
>>
>> }
>>
>> void main ()
>> {
>> "asd".foo('s');
>> }
>>
>> The error I get is:
>>
>> main.d(10): Error: template main.foo(T) does not match any function
>> template declaration
>> main.d(10): Error: template main.foo(T) cannot deduce template
>> function from argument types !()(string,char)
>>
>> It seems to be some problem with the "b" argument, if I change that to
>> "char" it works.
>
> In D2, strings are of type immutable(char)[], so your T would be
> immutable(char). However, 's' is a simple, unadorned char.
>
> Ways to fix this would include:
>
> void foo(T)(const T[] a, const T b){
> ...
> }
>
> void foo(T,U)(const T[] a, U b) if (is(Unqual!T == Unqual!U)) {
> ...
> }

That's annoying, specially since "char" is a value type. I would preferably have a solution for both D1 and D2. Can I use a template to cast/alias away the immutable part?


-- 
/Jacob Carlborg

Hello Jacob,

> That's annoying, specially since "char" is a value type. I would
> preferably have a solution for both D1 and D2. Can I use a template to
> cast/alias away the immutable part?

One solution would be to have templates strip off const/immutable from the top level of args.

void F1(T)(T t) { pragam(msg,typeof(t).stringof); }

string s1;
immutable(char[]) s2
char[] s3

F1(s1); // immutable(char)[] // all as normal
F1(s2); // immutable(char)[] // making a mutable copy of a immutable value is OK
F1(s3); // char[] // all as normal

void F2(T)(immutable T t) { pragam(msg,typeof(t).stringof); }

F2(s1); // immutable(char[]) // making an immutable copy of a mutable reference to immutable data is ok
F2(s2); // immutable(char[]) // all as normal
F2(s3); // error, invalid conversion

This solution would match the proposal that popped up a while ago to allow value assignment from const/immutable to mutable. 

-- 
... <IXOYE><

On 2010-06-28 02:28, BCS wrote:
> Hello Jacob,
>
>> That's annoying, specially since "char" is a value type. I would
>> preferably have a solution for both D1 and D2. Can I use a template to
>> cast/alias away the immutable part?
>
> One solution would be to have templates strip off const/immutable from
> the top level of args.
>
> void F1(T)(T t) { pragam(msg,typeof(t).stringof); }
>
> string s1;
> immutable(char[]) s2
> char[] s3
>
> F1(s1); // immutable(char)[] // all as normal
> F1(s2); // immutable(char)[] // making a mutable copy of a immutable
> value is OK
> F1(s3); // char[] // all as normal
>
> void F2(T)(immutable T t) { pragam(msg,typeof(t).stringof); }
>
> F2(s1); // immutable(char[]) // making an immutable copy of a mutable
> reference to immutable data is ok
> F2(s2); // immutable(char[]) // all as normal
> F2(s3); // error, invalid conversion
>
> This solution would match the proposal that popped up a while ago to
> allow value assignment from const/immutable to mutable.

I don't think I understand what you're showing here. How would I strip off the const/immutable with a template ?

-- 
/Jacob Carlborg

On Mon, Jun 28, 2010 at 10:56, Jacob Carlborg <doob@me.com> wrote:

Something to keep in mind: as of 2.04x (.045? maybe), the way UTF-8 / UTF-32
is managed was changed. "asd" is an array of immutable(dchar), not
imutable(char). At least DMD tells me that its element type is 'dchar'.

So your function can be done this way in D2:

void foo(T,U)(in T[] a, U b) if (is(U : Unqual!T)) // that compiles only if
b can be cast to A
{
    writeln(a,b);
}

"asd".foo('s'); // prints "asds".

is(U == Unqual!T) does not work, for U is 'char' while Unqual!T is 'dchar'.

More generally, using ranges and not arrays, the template becomes a bit more heavy:

void foo(Range,Elem)(in Range range, Elem elem)
if (isInputRange!Range && is(Elem : Unqual!(ElementType!Range)))
{
...
}


I don't think I understand what you're showing here. How would I strip off
> the const/immutable with a template ?
>
>
Hmmm...
* plays with is expressions *

This seems to work:

template UnConst(T)
{
    static if (is(T t == const U, U)) // that is:  'if T is a 'const U', for
some U'
        alias U UnConst;                      // then give me the U, (ie, T
without a const)
    else
        alias T UnConst;                      // else give me the (original)
T
}

template UnImmutable(T)
{
    static if (is(T t == immutable U, U)) // 'if T is an 'immutable U', for
some U'
        alias U UnImmutable;
    else
        alias T UnImmutable;
}

test:

void main() {
    alias const int Int;
    writeln(UnConst!Int.stringof);
    writeln(Int.stringof);
    writeln(UnConst!int.stringof);
    writeln(UnConst!(const int).stringof);
    writeln(UnConst!(immutable int).stringof);

    alias immutable int IInt;
    writeln(UnConst!IInt.stringof);
    writeln(IInt.stringof);
    writeln(UnImmutable!int.stringof);
    writeln(UnImmutable!(const int).stringof);
    writeln(UnImmutable!(immutable int).stringof);
}

Philippe

On Mon, 28 Jun 2010 08:14:12 -0400, Philippe Sigaud <philippe.sigaud@gmail.com> wrote:

> On Mon, Jun 28, 2010 at 10:56, Jacob Carlborg <doob@me.com> wrote:
>
> Something to keep in mind: as of 2.04x (.045? maybe), the way UTF-8 / UTF-32
> is managed was changed. "asd" is an array of immutable(dchar), not
> imutable(char). At least DMD tells me that its element type is 'dchar'.

No, that is not true.  It's still an array of immutable(char).  The compiler still sees it as an array of immutable(char).  However, std.range forces the element type of char[] and wchar[] to be bidirectional ranges of dchar.  The tests such as isRandomAccessRange and ElementType are fudged to say string is *not* a random access range, and its element type is dchar.  This was one of Andrei's changes because without such shoehorning, std.algorithm could possible start shearing off strings that weren't valid.

Whether that was the right decision remains to be seen.  I personally would rather have special ranges that do those things.  If I have a string that's always in English, why do I need to generate the dchars based on the characters in that array?

-Steve

On 2010-06-28 14:14, Philippe Sigaud wrote:
> On Mon, Jun 28, 2010 at 10:56, Jacob Carlborg <doob@me.com
> <mailto:doob@me.com>> wrote:
>
> Something to keep in mind: as of 2.04x (.045? maybe), the way UTF-8 /
> UTF-32 is managed was changed. "asd" is an array of immutable(dchar),
> not imutable(char). At least DMD tells me that its element type is 'dchar'.
>
> So your function can be done this way in D2:
>
> void foo(T,U)(in T[] a, U b) if (is(U : Unqual!T)) // that compiles only
> if b can be cast to A
> {
>      writeln(a,b);
> }
>
> "asd".foo('s'); // prints "asds".
>
> is(U == Unqual!T) does not work, for U is 'char' while Unqual!T is 'dchar'.
>
> More generally, using ranges and not arrays, the template becomes a bit
> more heavy:
>
> void foo(Range,Elem)(in Range range, Elem elem)
> if (isInputRange!Range && is(Elem : Unqual!(ElementType!Range)))
> {
> ...
> }
>
>
>     I don't think I understand what you're showing here. How would I
>     strip off the const/immutable with a template ?
>
>
> Hmmm...
> * plays with is expressions *
>
> This seems to work:
>
> template UnConst(T)
> {
>      static if (is(T t == const U, U)) // that is: 'if T is a 'const U',
> for some U'
>          alias U UnConst;                      // then give me the U,
> (ie, T without a const)
>      else
>          alias T UnConst;                      // else give me the
> (original) T
> }
>
> template UnImmutable(T)
> {
>      static if (is(T t == immutable U, U)) // 'if T is an 'immutable U',
> for some U'
>          alias U UnImmutable;
>      else
>          alias T UnImmutable;
> }
>
> test:
>
> void main() {
>      alias const int Int;
>      writeln(UnConst!Int.stringof);
>      writeln(Int.stringof);
>      writeln(UnConst!int.stringof);
>      writeln(UnConst!(const int).stringof);
>      writeln(UnConst!(immutable int).stringof);
>
>      alias immutable int IInt;
>      writeln(UnConst!IInt.stringof);
>      writeln(IInt.stringof);
>      writeln(UnImmutable!int.stringof);
>      writeln(UnImmutable!(const int).stringof);
>      writeln(UnImmutable!(immutable int).stringof);
> }
>
> Philippe

Hmm, now I don't know what I'm doing, I thought you could do something like this:

template Char (T)
{
	alias T Char;
}

void foo (T) (Char!(T) b)
{

}

void main ()
{
	foo('s');
}


-- 
/Jacob Carlborg