http://d.puremagic.com/issues/show_bug.cgi?id=7838

           Summary: Give some error messages for wrong ranges
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: diagnostic
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2012-04-05 17:55:22 PDT ---
I'd like DMD to give some error messages for wrong definitions of ranges, like in this case:


struct Powers {
    int m;
    BigInt n;
    this(int m_) { this.m = m_; }
    const bool empty = false;
    BigInt front() { return n ^^ m; }
    void popFront() { n += 1; }
}


It looks correct, but it's wrong. ElementType!Powers is void.

The correct code (a @property was missing):

struct Powers {
    int m;
    BigInt n;
    this(int m_) { this.m = m_; }
    const bool empty = false;
    @property BigInt front() { return n ^^ m; }
    void popFront() { n += 1; }
}

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Matt Peterson <revcompgeek@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |revcompgeek@gmail.com


--- Comment #1 from Matt Peterson <revcompgeek@gmail.com> 2012-04-05 19:42:00 PDT ---
My understanding is that you would usually put a static assert with isInputRange, or a more specific template from std.range immediately after the struct. How is DMD supposed to know that that struct is suppose to be a range otherwise?

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--- Comment #2 from bearophile_hugs@eml.cc 2012-04-06 10:09:17 PDT ---
(In reply to comment #1)
> My understanding is that you would usually put a static assert with isInputRange, or a more specific template from std.range immediately after the struct.

This is an example program:


import std.range, std.bigint;
struct Powers {
    int m;
    BigInt n;
    this(int m_) { this.m = m_; }
    const bool empty = false;
    BigInt front() { return n ^^ m; }
    void popFront() { n += 1; }
}
static assert(isInputRange!Powers);
void main() {}


If I compile it with DMD 2.059beta:

...>dmd -property -run temp.d
temp.d(10): Error: static assert  (isInputRange!(Powers)) is false


So it gives me no hint where the problem is. A built-in error message is supposed to be more precise.

As alternative, maybe there is a way to add focused error messages inside a isInputRangeVerify template to be used to verify that  an input range is correct, that uses pragma(msg) or better ctWriteln.


> How is DMD supposed to know that that struct is suppose to be a range otherwise?

I see, it's a problem.
So here we are talking more about a probabilistic compiler tip. If the
class/struct contains a popFront and front and empty methods then the
programmer probably meants it to be a range.

Thank you for your answer.

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--- Comment #3 from Matt Peterson <revcompgeek@gmail.com> 2012-04-06 10:47:21 PDT ---
Yeah, I think the best solution is to add verifyInputRange etc. templates to std.range, where each criteria is checked by a separate static assert. It's just a little ugly because there will end up being nearly duplicate templates for all the different range types.

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--- Comment #4 from bearophile_hugs@eml.cc 2012-04-06 11:06:31 PDT ---
(In reply to comment #3)
> It's just a little ugly because there will end up being nearly duplicate templates for all the different range types.

I think putting such test code inside DMD itself doesn't reduce the overall complexity a lot...

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