http://d.puremagic.com/issues/show_bug.cgi?id=9426

           Summary: [enh] polymorphic lambda should be storeable in enum
                    constant
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: code@dawg.eu


--- Comment #0 from Martin Nowak <code@dawg.eu> 2013-01-29 16:02:07 PST ---
I don't see a compelling reason why we should disallow assigning polymorphic
lambdas to manifest constants.
This would allow the following code to work.

enum foo = bar => bar * 2;
foo(10);
foo!uint(10); // probably too

The equivalent template would be.
template foo(T) { auto foo(T bar) { return bar * 2; } }

This is for example very handy to replace C-style macros.

#define MASK(val) ((val) & 0xF)
vs.
enum MASK = val => val & 0xF;

related: Bug 7176, Bug 8452

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |yebblies@gmail.com


--- Comment #1 from yebblies <yebblies@gmail.com> 2013-06-20 19:15:06 EST ---
This would mean 'enum' variable could refer to templates... I don't think this is a good idea, enums are for values.

Alternative:

alias foo = bar => bar * 2;
foo(10);
foo!uint(10); // probably too

This fits much better with the existing lanuage IMO.

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--- Comment #2 from Martin Nowak <code@dawg.eu> 2013-06-20 12:33:49 PDT ---
(In reply to comment #1)
> This fits much better with the existing lanuage IMO.

You are right.

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs@eml.cc


--- Comment #3 from bearophile_hugs@eml.cc 2013-07-31 05:49:42 PDT ---
Now the D language supports enum and alias used as lambdas, so adding template function lambdas seems good:


import std.typecons: Tuple;
void main() {
    alias Pair(T) = Tuple!(T, T); // OK
    auto p = Pair!int(10, 20);
    enum isGood(T) = is(T == int) || is(T == long); // OK
    static assert(isGood!int);
    enum foo(T) = (T x) => x * x; // OK
    static assert(foo!int(5) == 25);
    alias bar = x => x * x; // Error
}

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