http://d.puremagic.com/issues/show_bug.cgi?id=7878

           Summary: A problem with purity and general templated algorithms
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2012-04-09 10:54:17 PDT ---
This is a spinoff of the closed Issue 7864 , see there for more info.

This is not a bug report and it's not even a real enhancement request, it's just a note to not forget a small but what I think real problem in D+Phobos. At the moment I don't have suggestions to how improve the situation. Ideas are welcome.


This is wrong D2 code:


class C {
    string str;

    this(string str_) {
        this.str = str_;
    }

    override int opCmp(Object o) const {
        import std.string;
        auto rhs = cast(C)o;
        assert(rhs);
        return cmp(str, rhs.str);
    }
}

struct S {
    C c;

    int opCmp()(const auto ref S rhs) const pure {
        if (c < rhs.c) return -1;
        if (c > rhs.c) return 1;
        return 0;
    }
}

void main() {
    import std.algorithm;
    S[] stuff;
    sort(stuff);
}



DMD 2.059beta4 gives:

...\dmd2\src\phobos\std\algorithm.d(6802): Error: static assert  "Invalid
predicate passed to sort: a < b"
test.d(29):        instantiated from here: sort!("a <
b",cast(SwapStrategy)0,S[])


The error message looks correct, but it's really too much general, it doesn't
give enough information to the
programmer, that is left to guess why the code doesn't work. This is quite bad.


To fix this program the first opCmp too must be pure:

    override int opCmp(Object o) const pure {

But in a little more complex program it risks being not easy to find such bugs.

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|DMD                         |Phobos


--- Comment #1 from Kenji Hara <k.hara.pg@gmail.com> 2012-04-10 03:32:39 PDT ---
It is std.functional.binaryFun and std.algorithm.sort problem, not compiler's.

If the instantiation of predicate "a < b" that given to sort template function is failed, they hide actual errors and instead print their own error messages by static assert.

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Jonathan M Davis <jmdavisProg@gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jmdavisProg@gmx.com


--- Comment #2 from Jonathan M Davis <jmdavisProg@gmx.com> 2012-04-10 10:14:26 PDT ---
That may be, but if overloaded operators are wrong (especially opEquals and opCmp), I would expect the compiler to give errors for those types before any instantiation of sort was even attempted, and clearly, that's not happening.

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull


--- Comment #3 from Kenji Hara <k.hara.pg@gmail.com> 2012-04-18 18:00:47 PDT ---
https://github.com/D-Programming-Language/phobos/pull/544

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich@gmail.com


--- Comment #4 from Kenji Hara <k.hara.pg@gmail.com> 2012-04-20 23:47:09 PDT ---
*** Issue 5505 has been marked as a duplicate of this issue. ***

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--- Comment #5 from github-bugzilla@puremagic.com 2012-06-04 01:39:01 PDT ---
Commits pushed to master at https://github.com/D-Programming-Language/phobos

https://github.com/D-Programming-Language/phobos/commit/e2c892fdbfd23f45c2f304edb761b18111857228 fix Issue 7878 - A problem with purity and general templated algorithms

https://github.com/D-Programming-Language/phobos/commit/8e37bf3d01b59f620ed40bab8ee3943fab0289dc Merge pull request #544 from 9rnsr/fix7878

Issue 7878 - A problem with purity and general templated algorithms

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


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