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digitalmars.D.bugs - [Issue 1951] New: Remove SFINAE

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1951

           Summary: Remove SFINAE
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: jason.james.house gmail.com


I can imagine the request sounding almost crazy, but it really isn't.
(brief background: SFINAE = substitution failure is not an error.  Commonly
used when selecting which templated definition to apply.  Most uses in C++ are
tricks to determine info available in D is expressions)

I've been polling people on the D mailing list and those I know who are deep
into C++, and so far nobody can come up with a real-world use for SFINAE that
can't be done with other mechanisms already in D (is expressions, static if,
etc...)

What I have been able to find out from people is that SFINAE does cause
debugging nightmares with template based code when a specialization fails to
compile as expected.

I think trying no SFINAE is a great thing to try in D 2.x while it's still
considered experimental/alpha.  I expect a lack of SFINAE to make the compiler
simpler and ensure D preserves its fast compile times.

If it does turn out that SFINAE does provide unique functionality (I expect
Andrei may know), I think it may be better to enhance existing generic
programming techniques than to keep SFINAE.  The further SFINAE gets pushed out
of normal use, the more of a corner case it becomes...  biting developers more
often than it helps them.


--
Mar 26 2008
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1951






A good suggestion made on the newsgroups was to first add a switch to D2 for a
while that can be used to either disable SFINAE or to print out a trace message
every time the compiler uses it.


--
Mar 27 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1951




Here's the first example from
http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error
translated into D.

---------------
struct Test {
    alias int foo;
}





void main() {

    f!(int)(10);  // C++ No error (even though there is no int::foo) thanks to
SFINAE.
}
------------
bug.d(5): Error: no property 'foo' for type 'int'
bug.d(5): Error: T.foo is used as a type
------------
Same thing with every other SFINAE example I've been able to find. The
behaviour is the same on D1 and D2.
Does D actually have SFINAE???

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Nov 16 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1951






 ---------------
 struct Test {
     alias int foo;
 }
 

 

 
 void main() {

     f!(int)(10);  // C++ No error (even though there is no int::foo) thanks to
 SFINAE.
 }
 ------------
 bug.d(5): Error: no property 'foo' for type 'int'
 bug.d(5): Error: T.foo is used as a type
 ------------


I think that is not a lack SFINAE, rather the type deduction mechanism issue.
In current, T.foo is not treated as a *partially parameterized* type name.
I recently filed bug 9022 to support such type deduction.

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Nov 16 2012
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=1951


Jason House <jason.james.house gmail.com> changed:

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



05:51:51 PST ---
I believe Don is right. D does not have SFINAE. Many D users simply assume that
was inherited from C++.

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Dec 06 2012