Hi,

we start with the following code snippet, which works.

    import std.algorithm;
    import std.range;
    import std.stdio;

    class A     { int val; }
    class B : A { this() { val = 3; } }
    class C : A { this() { val = 4; } }

    B[] b = [new B(), new B()];
    C[] c = [new C(), new C()];

    void main()
    {
        chain(b, c).each!(a => a.val.writeln);
    }

The output, as expected, is:

    3
    3
    4
    4

Now I change the declarations of B[] b and C[] c to the following, keeping
everything else in the code snippet the same:

    const(B[]) b = [new B(), new B()];
    const(C[]) c = [new C(), new C()];

This makes dmd 2.070 choke: ( http://dpaste.dzfl.pl/eee69fd03dd9 )

    Error: template std.range.chain cannot deduce function from argument
    types !()(const(B[]), const(C[])),
    candidates are: /opt/compilers/dmd2/include/std/range/package.d(804):
    std.range.chain(Ranges...)(Ranges rs) if (Ranges.length > 0 &&
    allSatisfy!(iseputRange, staticMap!(Unqual, Ranges)) && !is(CommonType!(
    staticMap!(ElementType, staticMap!(Unqual, Ranges))) == void))

What's stumping me -- constness doesn't make dmd choke on ranges of
numbers. If I replace the classes B and C with simple 'int' and 'double',
this compiles again:

    const(int[])    b = [1, 2];
    const(double[]) c = [3.3, 4.4];
    void main() { chain(b, c).each!(a => a.writeln); }

Why does it fail for const(array of class)?
Is any template magic about Unqual or staticMap relevant here?

-- Simon

On Sunday, 31 January 2016 at 21:22:06 UTC, SimonN wrote:
> Hi,
>
> we start with the following code snippet, which works.
>
>     import std.algorithm;
>     import std.range;
>     import std.stdio;
>
>     class A     { int val; }
>     class B : A { this() { val = 3; } }
>     class C : A { this() { val = 4; } }
>
>     B[] b = [new B(), new B()];
>     C[] c = [new C(), new C()];
>
>     void main()
>     {
>         chain(b, c).each!(a => a.val.writeln);
>     }
>
> The output, as expected, is:
>
>     3
>     3
>     4
>     4
>
> Now I change the declarations of B[] b and C[] c to the following, keeping
> everything else in the code snippet the same:
>
>     const(B[]) b = [new B(), new B()];
>     const(C[]) c = [new C(), new C()];
>
> This makes dmd 2.070 choke: ( http://dpaste.dzfl.pl/eee69fd03dd9 )
>
>     Error: template std.range.chain cannot deduce function from argument
>     types !()(const(B[]), const(C[])),
>     candidates are: /opt/compilers/dmd2/include/std/range/package.d(804):
>     std.range.chain(Ranges...)(Ranges rs) if (Ranges.length > 0 &&
>     allSatisfy!(iseputRange, staticMap!(Unqual, Ranges)) && !is(CommonType!(
>     staticMap!(ElementType, staticMap!(Unqual, Ranges))) == void))
>
> What's stumping me -- constness doesn't make dmd choke on ranges of
> numbers. If I replace the classes B and C with simple 'int' and 'double',
> this compiles again:
>
>     const(int[])    b = [1, 2];
>     const(double[]) c = [3.3, 4.4];
>     void main() { chain(b, c).each!(a => a.writeln); }
>
> Why does it fail for const(array of class)?
> Is any template magic about Unqual or staticMap relevant here?
>
> -- Simon

 types !()(const(B[]), const(C[])),
>     candidates are: /opt/compilers/dmd2/include/std/range/package.d(804):
>     std.range.chain(Ranges...)(Ranges rs) if (Ranges.length > 0 &&
>     allSatisfy!(iseputRange, staticMap!(Unqual, Ranges)) && !is(CommonType!(
>     staticMap!(ElementType, staticMap!(Unqual, Ranges))) == void))

Unqaul means remove any const or immutable torn the type
StaticMap is like a compile time map

What this error message says is that there is one candidate function that matches what you are attempting to do. and that chain takes a variadic argument and each of those arguments must

1) when unqualified be an input range (Basically you can foreach over it)
2) that the common type of the element type of the unqualified variadic argument types is not void (in this case not arrays of void)

Have you tried changing The declaration of a and b to const(A[])?

Also have you tried with other reference type (e.g. assoc arrays pointers)?

Nic

Sorry for late reply -- but I got around to test a couple more cases!

On Monday, 1 February 2016 at 00:19:44 UTC, Nicholas Wilson wrote:
> Unqaul means remove any const or immutable torn the type

Okay, that sounds like our 'const' shouldn't matter. 'const' is the outermost qualifier, and stripping that leaves us with B[] and C[], which were chainable earlier.

> StaticMap is like a compile time map
> What this error message says is that there is one candidate function that matches what you are attempting to do. and that chain takes a variadic argument and each of those arguments must
> 1) when unqualified be an input range (Basically you can foreach over it)

Yep, const(B[]) and const(C[]) can be foreached. My workaround has been to replace chain() with several foreaches.

> 2) that the common type of the element type of the unqualified variadic argument types is not void (in this case not arrays of void)
> Have you tried changing The declaration of a and b to const(A[])?

Surprisingly, this compiles and gives the desired output:

    const(B[]) b = [ new B(), new B() ];
    const(A[]) c = [ new C(), new C() ]; // A instead of C

    chain(b, c).each!(a => a.val.writeln);

With two arguments, const(array) has worked iff at least one range is of the base type. Only if none were of the base type, I got the error.

Apparently, the template is smart enough to infer the common base type without 'const', but needs to be fed the basetype in case of 'const'.

My gut feeling is that I should report this as a bug against phobos...

> Also have you tried with other reference type (e.g. assoc arrays pointers)?

    immutable(B[int]) b;
    immutable(C[int]) c;

    chain(b.byValue, c.byValue).each!(a => a.val.writeln);

Error is the same as for the classes:

    template std.range.chain cannot deduce function
    from argument types !()(Result, Result), candidates are: /* snip */

To get this error, again, if at least one range is 'immutable(A[int]).byValue', i.e., using the base class A, the template instantiates with no problems.

-- Simon

The constraint that fails is the one with `CommonType`:

    pragma(msg, CommonType!(const(B), const(C))); // void

`CommonType` uses the `?:` operator to derive the common type:

    writeln(true ? b : c);
    // Error: incompatible types for ((b) : (c)): 'const(B[])' and 'const(C[])'
    writeln(true ? b[0] : c[0]);
    // Error: incompatible types for ((b[0]) : (c[0])): 'const(B)' and 'const(C)'

At the moment I can't see a reason why that shouldn't work.

On Tuesday, 2 February 2016 at 09:51:52 UTC, Marc Schütz wrote:
> The constraint that fails is the one with `CommonType`:
>
>     pragma(msg, CommonType!(const(B), const(C))); // void
>
> `CommonType` uses the `?:` operator to derive the common type:
>
>     writeln(true ? b : c);
>     // Error: incompatible types for ((b) : (c)): 'const(B[])' and 'const(C[])'
>     writeln(true ? b[0] : c[0]);
>     // Error: incompatible types for ((b[0]) : (c[0])): 'const(B)' and 'const(C)'
>
> At the moment I can't see a reason why that shouldn't work.

This change broke it:
https://github.com/D-Programming-Language/dmd/pull/125

I filed a bug report:
https://issues.dlang.org/show_bug.cgi?id=15638

On Tuesday, 2 February 2016 at 10:58:35 UTC, Marc Schütz wrote:
>> The constraint that fails is the one with `CommonType`:
>> `CommonType` uses the `?:` operator to derive the common type:
> I filed a bug report:
> https://issues.dlang.org/show_bug.cgi?id=15638

Interesting reduced case, so it wasn't chain after all. Thanks for filing the issue already; also thanks to Nic for good test cases.

I think Adam D. Ruppe wanted to push more informative template errors -- they'd come in handy. :-)

-- Simon